rfc9420.original   rfc9420.txt 
Network Working Group R. Barnes Internet Engineering Task Force (IETF) R. Barnes
Internet-Draft Cisco Request for Comments: 9420 Cisco
Intended status: Standards Track B. Beurdouche Category: Standards Track B. Beurdouche
Expires: 28 September 2023 Inria & Mozilla ISSN: 2070-1721 Inria & Mozilla
R. Robert R. Robert
Phoenix R&D Phoenix R&D
J. Millican J. Millican
Meta Platforms Meta Platforms
E. Omara E. Omara
Google
K. Cohn-Gordon K. Cohn-Gordon
University of Oxford University of Oxford
27 March 2023 July 2023
The Messaging Layer Security (MLS) Protocol The Messaging Layer Security (MLS) Protocol
draft-ietf-mls-protocol-20
Abstract Abstract
Messaging applications are increasingly making use of end-to-end Messaging applications are increasingly making use of end-to-end
security mechanisms to ensure that messages are only accessible to security mechanisms to ensure that messages are only accessible to
the communicating endpoints, and not to any servers involved in the communicating endpoints, and not to any servers involved in
delivering messages. Establishing keys to provide such protections delivering messages. Establishing keys to provide such protections
is challenging for group chat settings, in which more than two is challenging for group chat settings, in which more than two
clients need to agree on a key but may not be online at the same clients need to agree on a key but may not be online at the same
time. In this document, we specify a key establishment protocol that time. In this document, we specify a key establishment protocol that
provides efficient asynchronous group key establishment with forward provides efficient asynchronous group key establishment with forward
secrecy and post-compromise security for groups in size ranging from secrecy (FS) and post-compromise security (PCS) for groups in size
two to thousands. ranging from two to thousands.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/mlswg/mls-protocol (https://github.com/mlswg/mls-
protocol).
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79.
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Internet Standards is available in Section 2 of RFC 7841.
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and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9420.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction
1.1. Change Log . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.1. Presentation Language
2.1. Presentation Language . . . . . . . . . . . . . . . . . . 16 2.1.1. Optional Value
2.1.1. Optional Value . . . . . . . . . . . . . . . . . . . 17 2.1.2. Variable-Size Vector Length Headers
2.1.2. Variable-size Vector Length Headers . . . . . . . . . 17 3. Protocol Overview
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 19 3.1. Cryptographic State and Evolution
3.1. Cryptographic State and Evolution . . . . . . . . . . . . 21 3.2. Example Protocol Execution
3.2. Example Protocol Execution . . . . . . . . . . . . . . . 22 3.3. External Joins
3.3. External Joins . . . . . . . . . . . . . . . . . . . . . 26 3.4. Relationships between Epochs
3.4. Relationships Between Epochs . . . . . . . . . . . . . . 27 4. Ratchet Tree Concepts
4. Ratchet Tree Concepts . . . . . . . . . . . . . . . . . . . . 29 4.1. Ratchet Tree Terminology
4.1. Ratchet Tree Terminology . . . . . . . . . . . . . . . . 30 4.1.1. Ratchet Tree Nodes
4.1.1. Ratchet Tree Nodes . . . . . . . . . . . . . . . . . 30 4.1.2. Paths through a Ratchet Tree
4.1.2. Paths through a Ratchet Tree . . . . . . . . . . . . 32 4.2. Views of a Ratchet Tree
4.2. Views of a Ratchet Tree . . . . . . . . . . . . . . . . . 33 5. Cryptographic Objects
5. Cryptographic Objects . . . . . . . . . . . . . . . . . . . . 35 5.1. Cipher Suites
5.1. Ciphersuites . . . . . . . . . . . . . . . . . . . . . . 35 5.1.1. Public Keys
5.1.1. Public Keys . . . . . . . . . . . . . . . . . . . . . 35 5.1.2. Signing
5.1.2. Signing . . . . . . . . . . . . . . . . . . . . . . . 36 5.1.3. Public Key Encryption
5.1.3. Public-Key Encryption . . . . . . . . . . . . . . . . 37 5.2. Hash-Based Identifiers
5.2. Hash-Based Identifiers . . . . . . . . . . . . . . . . . 37 5.3. Credentials
5.3. Credentials . . . . . . . . . . . . . . . . . . . . . . . 38 5.3.1. Credential Validation
5.3.1. Credential Validation . . . . . . . . . . . . . . . . 39 5.3.2. Credential Expiry and Revocation
5.3.2. Credential Expiry and Revocation . . . . . . . . . . 40 5.3.3. Uniquely Identifying Clients
5.3.3. Uniquely Identifying Clients . . . . . . . . . . . . 41 6. Message Framing
6. Message Framing . . . . . . . . . . . . . . . . . . . . . . . 42 6.1. Content Authentication
6.1. Content Authentication . . . . . . . . . . . . . . . . . 45 6.2. Encoding and Decoding a Public Message
6.2. Encoding and Decoding a Public Message . . . . . . . . . 47 6.3. Encoding and Decoding a Private Message
6.3. Encoding and Decoding a Private Message . . . . . . . . . 48 6.3.1. Content Encryption
6.3.1. Content Encryption . . . . . . . . . . . . . . . . . 48 6.3.2. Sender Data Encryption
6.3.2. Sender Data Encryption . . . . . . . . . . . . . . . 50 7. Ratchet Tree Operations
7. Ratchet Tree Operations . . . . . . . . . . . . . . . . . . . 51 7.1. Parent Node Contents
7.1. Parent Node Contents . . . . . . . . . . . . . . . . . . 52 7.2. Leaf Node Contents
7.2. Leaf Node Contents . . . . . . . . . . . . . . . . . . . 52 7.3. Leaf Node Validation
7.3. Leaf Node Validation . . . . . . . . . . . . . . . . . . 55 7.4. Ratchet Tree Evolution
7.4. Ratchet Tree Evolution . . . . . . . . . . . . . . . . . 57 7.5. Synchronizing Views of the Tree
7.5. Synchronizing Views of the Tree . . . . . . . . . . . . . 59 7.6. Update Paths
7.6. Update Paths . . . . . . . . . . . . . . . . . . . . . . 62 7.7. Adding and Removing Leaves
7.7. Adding and Removing Leaves . . . . . . . . . . . . . . . 63 7.8. Tree Hashes
7.8. Tree Hashes . . . . . . . . . . . . . . . . . . . . . . . 64 7.9. Parent Hashes
7.9. Parent Hashes . . . . . . . . . . . . . . . . . . . . . . 65 7.9.1. Using Parent Hashes
7.9.1. Using Parent Hashes . . . . . . . . . . . . . . . . . 68 7.9.2. Verifying Parent Hashes
7.9.2. Verifying Parent Hashes . . . . . . . . . . . . . . . 69 8. Key Schedule
8. Key Schedule . . . . . . . . . . . . . . . . . . . . . . . . 70 8.1. Group Context
8.1. Group Context . . . . . . . . . . . . . . . . . . . . . . 72 8.2. Transcript Hashes
8.2. Transcript Hashes . . . . . . . . . . . . . . . . . . . . 74 8.3. External Initialization
8.3. External Initialization . . . . . . . . . . . . . . . . . 77 8.4. Pre-Shared Keys
8.4. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . . . 77 8.5. Exporters
8.5. Exporters . . . . . . . . . . . . . . . . . . . . . . . . 80 8.6. Resumption PSK
8.6. Resumption PSK . . . . . . . . . . . . . . . . . . . . . 81 8.7. Epoch Authenticators
8.7. Epoch Authenticators . . . . . . . . . . . . . . . . . . 81 9. Secret Tree
9. Secret Tree . . . . . . . . . . . . . . . . . . . . . . . . . 82 9.1. Encryption Keys
9.1. Encryption Keys . . . . . . . . . . . . . . . . . . . . . 83 9.2. Deletion Schedule
9.2. Deletion Schedule . . . . . . . . . . . . . . . . . . . . 84 10. Key Packages
10. Key Packages . . . . . . . . . . . . . . . . . . . . . . . . 86 10.1. KeyPackage Validation
10.1. KeyPackage Validation . . . . . . . . . . . . . . . . . 87 11. Group Creation
11. Group Creation . . . . . . . . . . . . . . . . . . . . . . . 88 11.1. Required Capabilities
11.1. Required Capabilities . . . . . . . . . . . . . . . . . 90 11.2. Reinitialization
11.2. Reinitialization . . . . . . . . . . . . . . . . . . . . 90 11.3. Subgroup Branching
11.3. Subgroup Branching . . . . . . . . . . . . . . . . . . . 91 12. Group Evolution
12. Group Evolution . . . . . . . . . . . . . . . . . . . . . . . 92 12.1. Proposals
12.1. Proposals . . . . . . . . . . . . . . . . . . . . . . . 92 12.1.1. Add
12.1.1. Add . . . . . . . . . . . . . . . . . . . . . . . . 93 12.1.2. Update
12.1.2. Update . . . . . . . . . . . . . . . . . . . . . . . 94 12.1.3. Remove
12.1.3. Remove . . . . . . . . . . . . . . . . . . . . . . . 94 12.1.4. PreSharedKey
12.1.4. PreSharedKey . . . . . . . . . . . . . . . . . . . . 95 12.1.5. ReInit
12.1.5. ReInit . . . . . . . . . . . . . . . . . . . . . . . 95 12.1.6. ExternalInit
12.1.6. ExternalInit . . . . . . . . . . . . . . . . . . . . 96 12.1.7. GroupContextExtensions
12.1.7. GroupContextExtensions . . . . . . . . . . . . . . . 96 12.1.8. External Proposals
12.1.8. External Proposals . . . . . . . . . . . . . . . . . 97 12.2. Proposal List Validation
12.2. Proposal List Validation . . . . . . . . . . . . . . . . 98 12.3. Applying a Proposal List
12.3. Applying a Proposal List . . . . . . . . . . . . . . . . 100 12.4. Commit
12.4. Commit . . . . . . . . . . . . . . . . . . . . . . . . . 100 12.4.1. Creating a Commit
12.4.1. Creating a Commit . . . . . . . . . . . . . . . . . 103 12.4.2. Processing a Commit
12.4.2. Processing a Commit . . . . . . . . . . . . . . . . 106 12.4.3. Adding Members to the Group
12.4.3. Adding Members to the Group . . . . . . . . . . . . 109 13. Extensibility
13. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 118 13.1. Additional Cipher Suites
13.1. Additional Ciphersuites . . . . . . . . . . . . . . . . 119 13.2. Proposals
13.2. Proposals . . . . . . . . . . . . . . . . . . . . . . . 119 13.3. Credential Extensibility
13.3. Credential Extensibility . . . . . . . . . . . . . . . . 119 13.4. Extensions
13.4. Extensions . . . . . . . . . . . . . . . . . . . . . . . 120 13.5. GREASE
13.5. GREASE . . . . . . . . . . . . . . . . . . . . . . . . . 122 14. Sequencing of State Changes
14. Sequencing of State Changes . . . . . . . . . . . . . . . . . 123 15. Application Messages
15. Application Messages . . . . . . . . . . . . . . . . . . . . 124 15.1. Padding
15.1. Padding . . . . . . . . . . . . . . . . . . . . . . . . 124 15.2. Restrictions
15.2. Restrictions . . . . . . . . . . . . . . . . . . . . . . 125 15.3. Delayed and Reordered Application Messages
15.3. Delayed and Reordered Application messages . . . . . . . 125 16. Security Considerations
16. Security Considerations . . . . . . . . . . . . . . . . . . . 126 16.1. Transport Security
16.1. Transport Security . . . . . . . . . . . . . . . . . . . 126 16.2. Confidentiality of Group Secrets
16.2. Confidentiality of the Group Secrets . . . . . . . . . . 126 16.3. Confidentiality of Sender Data
16.3. Confidentiality of Sender Data . . . . . . . . . . . . . 126 16.4. Confidentiality of Group Metadata
16.4. Confidentiality of Group Metadata . . . . . . . . . . . 127 16.4.1. GroupID, Epoch, and Message Frequency
16.4.1. GroupID, Epoch, and Message Frequency . . . . . . . 128 16.4.2. Group Extensions
16.4.2. Group Extensions . . . . . . . . . . . . . . . . . . 129 16.4.3. Group Membership
16.4.3. Group Membership . . . . . . . . . . . . . . . . . . 129 16.5. Authentication
16.5. Authentication . . . . . . . . . . . . . . . . . . . . . 130 16.6. Forward Secrecy and Post-Compromise Security
16.6. Forward Secrecy and Post-Compromise Security . . . . . . 130 16.7. Uniqueness of Ratchet Tree Key Pairs
16.7. Uniqueness of Ratchet Tree Key Pairs . . . . . . . . . . 132 16.8. KeyPackage Reuse
16.8. KeyPackage Reuse . . . . . . . . . . . . . . . . . . . . 132 16.9. Delivery Service Compromise
16.9. Delivery Service Compromise . . . . . . . . . . . . . . 132 16.10. Authentication Service Compromise
16.10. Authentication Service Compromise . . . . . . . . . . . 133 16.11. Additional Policy Enforcement
16.11. Additional Policy Enforcement . . . . . . . . . . . . . 133 16.12. Group Fragmentation by Malicious Insiders
16.12. Group Fragmentation by Malicious Insiders . . . . . . . 133 17. IANA Considerations
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 134 17.1. MLS Cipher Suites
17.1. MLS Ciphersuites . . . . . . . . . . . . . . . . . . . . 135 17.2. MLS Wire Formats
17.2. MLS Wire Formats . . . . . . . . . . . . . . . . . . . . 140 17.3. MLS Extension Types
17.3. MLS Extension Types . . . . . . . . . . . . . . . . . . 140 17.4. MLS Proposal Types
17.4. MLS Proposal Types . . . . . . . . . . . . . . . . . . . 142 17.5. MLS Credential Types
17.5. MLS Credential Types . . . . . . . . . . . . . . . . . . 144 17.6. MLS Signature Labels
17.6. MLS Signature Labels . . . . . . . . . . . . . . . . . . 146 17.7. MLS Public Key Encryption Labels
17.7. MLS Public Key Encryption Labels . . . . . . . . . . . . 146 17.8. MLS Exporter Labels
17.8. MLS Exporter Labels . . . . . . . . . . . . . . . . . . 147 17.9. MLS Designated Expert Pool
17.9. MLS Designated Expert Pool . . . . . . . . . . . . . . . 148 17.10. The "message/mls" Media Type
17.10. The "message/mls" MIME Type . . . . . . . . . . . . . . 148 18. References
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 150 18.1. Normative References
18.1. Normative References . . . . . . . . . . . . . . . . . . 150 18.2. Informative References
18.2. Informative References . . . . . . . . . . . . . . . . . 150 Appendix A. Protocol Origins of Example Trees
Appendix A. Protocol Origins of Example Trees . . . . . . . . . 153 Appendix B. Evolution of Parent Hashes
Appendix B. Evolution of Parent Hashes . . . . . . . . . . . . . 154 Appendix C. Array-Based Trees
Appendix C. Array-Based Trees . . . . . . . . . . . . . . . . . 156 Appendix D. Link-Based Trees
Appendix D. Link-Based Trees . . . . . . . . . . . . . . . . . . 160 Contributors
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 162 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 163
1. Introduction 1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for this
draft is maintained in GitHub. Suggested changes should be submitted
as pull requests at https://github.com/mlswg/mls-protocol.
Instructions are on that page as well. Editorial changes can be
managed in GitHub, but any substantive change should be discussed on
the MLS mailing list.
A group of users who want to send each other encrypted messages needs A group of users who want to send each other encrypted messages needs
a way to derive shared symmetric encryption keys. For two parties, a way to derive shared symmetric encryption keys. For two parties,
this problem has been studied thoroughly, with the Double Ratchet this problem has been studied thoroughly, with the Double Ratchet
emerging as a common solution [DoubleRatchet] [Signal]. Channels emerging as a common solution [DoubleRatchet] [Signal]. Channels
implementing the Double Ratchet enjoy fine-grained forward secrecy as implementing the Double Ratchet enjoy fine-grained forward secrecy as
well as post-compromise security, but are nonetheless efficient well as post-compromise security, but are nonetheless efficient
enough for heavy use over low-bandwidth networks. enough for heavy use over low-bandwidth networks.
For a group of size greater than two, a common strategy is to For a group of size greater than two, a common strategy is to
distribute symmetric "sender keys" over existing 1:1 secure channels, distribute symmetric "sender keys" over existing 1:1 secure channels,
skipping to change at page 5, line 42 skipping to change at line 213
compromise security with sender keys, requiring a number of key compromise security with sender keys, requiring a number of key
update messages that scales as the square of the group size. An update messages that scales as the square of the group size. An
adversary who learns a sender key can often indefinitely and adversary who learns a sender key can often indefinitely and
passively eavesdrop on that member's messages. Generating and passively eavesdrop on that member's messages. Generating and
distributing a new sender key provides a form of post-compromise distributing a new sender key provides a form of post-compromise
security with regard to that sender. However, it requires security with regard to that sender. However, it requires
computation and communications resources that scale linearly with the computation and communications resources that scale linearly with the
size of the group. size of the group.
In this document, we describe a protocol based on tree structures In this document, we describe a protocol based on tree structures
that enable asynchronous group keying with forward secrecy and post- that enables asynchronous group keying with forward secrecy and post-
compromise security. Based on earlier work on "asynchronous compromise security. Based on earlier work on "asynchronous
ratcheting trees" [ART], the protocol presented here uses an ratcheting trees" [ART], the protocol presented here uses an
asynchronous key-encapsulation mechanism for tree structures. This asynchronous key-encapsulation mechanism for tree structures. This
mechanism allows the members of the group to derive and update shared mechanism allows the members of the group to derive and update shared
keys with costs that scale as the log of the group size. keys with costs that scale as the log of the group size.
1.1. Change Log
RFC EDITOR PLEASE DELETE THIS SECTION.
draft-18
* Make the document standards track
* Make the ratchet tree non-malleable (*)
* Use ExpandWithLabel to derive welcome key (*)
* Change MLS-Exporter label from "exporter" to "exported" (*)
* Loosen chain requirements (*)
* Clarify transcript hash initialization
* GREASE for MLS registries
* Move pseudocode out of KDFLabel definition.
* Rename PrivateContentTBE to PrivateMessageContent
* Fix DecryptWithLabel argument order for Welcome
* Responses to IESG reviews
* Describe varint length check more clearly
draft-17
* Rename MLSCiphertext and MLSPlaintext to PrivateMessage and
PublicMesssage respectively (*)
* Add label and context to public-key encryption (*)
* Include leaf index in LeafNodeTBS for better parent-hash
guarantees (*)
* Make ProtocolVersion two bytes (*)
* Clarify group creation (*)
* Validate additional properties of unmerged leaves (*)
* Clarify that the AS needs to vet the signature key
* Remove "MLS" prefix on structs
* Credentials should be replaced before expiring
* Add a section discussing the security of the sender data
protection
* Minor fixes in presentation language.
* Allow multiple welcomes per commit
* Remove reference to BasicCredential.
* Client aware of its own removal in group
* Create IANA registries for signature and export labels
* Complete IANA media type registration
* Make more vendor code points available
* Update Recommended column definition to match 8447bis
* Responses to early ARTART review
* Responses to early OPSDIR review
* Responses to early TSV-ART review
draft-16
* Fix GroupInfoTBS (*)
* Make reference to h2 informative
draft-15
* Include ciphersuite in group context (*)
* Add new new_proposal_member SenderType (*)
* Always use a full tree (*)
* Change KeyPackage identifier extension to be LeafNode identifier
(*)
* Use new tree for context in path secret encryption (*)
* Use a hash function for hash identifiers (*)
* Add a marker byte to tree hash input structs (*)
* Recommend that group ids are generated randomly (*)
* Update external senders extension to have SignaturePublicKey and
Credential (*)
* Replace LeafNodeRef with leaf index (*)
* Remove AppAck proposal (*)
* Make padding arbitrary-size and all-zero (*)
* Require that unmerged_leaves be ordered
* Derive the commit secret from the end of the UpdatePath, not the
root
* Specify the precise points in the protocol where credential
validation must be done
* Make PSK provisions more uniform, e.g., always generating a fresh
random nonce
* Improve parent hash guarantees with stricter checks on tree
correctness
* Streamline some structs, e.g., folding GroupContext into GroupInfo
* Provide clearer rules for validating and applying commits
* Clarify tree hash and parent hash, and correct examples
* Clean up struct names and references to outdated structs
* Cite AEAD limits draft
draft-14
* Ensure that a signature public key is always intelligible (*)
* Clean up terminology of derived secrets/keys
* Fix parent hash (*)
* Specify compatibility behavior around new credentials
* Add Path Required to Proposal Type template
* Sub-group branching requires fresh key packages for each member
* Use aasvg and typed code blocks
* Require init key and leaf key to be different
* Preconfigured senders extension and removal of signature key
indirection
draft-13
* TLS syntax updates (including variable-header-length vectors) (*)
* Stop generating redundant PKE key pairs. (*)
* Move validation of identity change to the AS
* Add message/mls MIME type registration
* Split LeafNode from KeyPackage (*)
* Remove endpoint_id (*)
* Reorganize to make section layout more sane
* Forbid proposals by reference in external commits (*)
* Domain separation for KeyPackage and Proposal references (*)
* Downgrade MUST to SHOULD for commit senders including all valid
commits
* Stronger parent hashes for authenticated identities (*)
* Move wire_format to a separate tagged-union structure MLSMessage
* Generalize tree extend/truncate algorithms
* Add algorithms for link-based trees
* Forbid self-Update entirely (*)
* Consolidate resumption PSK cases (*)
* 384 Ciphersuite Addition
* Remove explicit version pin on HPKE (*)
* Remove the requirement for Add in external commit (*)
* Use smaller, fixed-size hash-based identifiers (*)
* Be explicit that Credentials can attest to multiple identities (*)
draft-12
* Use the GroupContext to derive the joiner_secret (*)
* Make PreSharedKeys non optional in GroupSecrets (*)
* Update name for this particular key (*)
* Truncate tree size on removal (*)
* Use HPKE draft-08 (*)
* Clarify requirements around identity in MLS groups (*)
* Signal the intended wire format for MLS messages (*)
* Inject GroupContext as HPKE info instead of AAD (*)
* Clarify extension handling and make extension updatable (*)
* Improve extensibility of Proposals (*)
* Constrain proposal in External Commit (*)
* Remove the notion of a 'leaf index' (*)
* Add group_context_extensions proposal ID (*)
* Add RequiredCapabilities extension (*)
* Use cascaded KDF instead of concatenation to consolidate PSKs (*)
* Use key package hash to index clients in message structs (*)
* Don't require PublicGroupState for external init (*)
* Make ratchet tree section clearer.
* Handle non-member sender cases in MLSPlaintextTBS
* Clarify encoding of signatures with NIST curves
* Remove OPEN ISSUEs and TODOs
* Normalize the description of the zero vector
draft-11
* Include subtree keys in parent hash (*)
* Pin HPKE to draft-07 (*)
* Move joiner secret to the end of the first key schedule epoch (*)
* Add an AppAck proposal
* Make initializations of transcript hashes consistent
draft-10
* Allow new members to join via an external Commit (*)
* Enable proposals to be sent inline in a Commit (*)
* Re-enable constant-time Add (*)
* Change expiration extension to lifetime extension (*)
* Make the tree in the Welcome optional (*)
* PSK injection, re-init, sub-group branching (*)
* Require the initial init_secret to be a random value (*)
* Remove explicit sender data nonce (*)
* Do not encrypt to joiners in UpdatePath generation (*)
* Move MLSPlaintext signature under the confirmation tag (*)
* Explicitly authenticate group membership with MLSPLaintext (*)
* Clarify X509Credential structure (*)
* Remove unneeded interim transcript hash from GroupInfo (*)
* IANA considerations
* Derive an authentication secret
* Use Extract/Expand from HPKE KDF
* Clarify that application messages MUST be encrypted
draft-09
* Remove blanking of nodes on Add (*)
* Change epoch numbers to uint64 (*)
* Add PSK inputs (*)
* Add key schedule exporter (*)
* Sign the updated direct path on Commit, using "parent hashes" and
one signature per leaf (*)
* Use structured types for external senders (*)
* Redesign Welcome to include confirmation and use derived keys (*)
* Remove ignored proposals (*)
* Always include an Update with a Commit (*)
* Add per-message entropy to guard against nonce reuse (*)
* Use the same hash ratchet construct for both application and
handshake keys (*)
* Add more ciphersuites
* Use HKDF to derive key pairs (*)
* Mandate expiration of ClientInitKeys (*)
* Add extensions to GroupContext and flesh out the extensibility
story (*)
* Rename ClientInitKey to KeyPackage
draft-08
* Change ClientInitKeys so that they only refer to one ciphersuite
(*)
* Decompose group operations into Proposals and Commits (*)
* Enable Add and Remove proposals from outside the group (*)
* Replace Init messages with multi-recipient Welcome message (*)
* Add extensions to ClientInitKeys for expiration and downgrade
resistance (*)
* Allow multiple Proposals and a single Commit in one MLSPlaintext
(*)
draft-07
* Initial version of the Tree based Application Key Schedule (*)
* Initial definition of the Init message for group creation (*)
* Fix issue with the transcript used for newcomers (*)
* Clarifications on message framing and HPKE contexts (*)
draft-06
* Reorder blanking and update in the Remove operation (*)
* Rename the GroupState structure to GroupContext (*)
* Rename UserInitKey to ClientInitKey
* Resolve the circular dependency that draft-05 introduced in the
confirmation MAC calculation (*)
* Cover the entire MLSPlaintext in the transcript hash (*)
draft-05
* Common framing for handshake and application messages (*)
* Handshake message encryption (*)
* Convert from literal state to a commitment via the "tree hash" (*)
* Add credentials to the tree and remove the "roster" concept (*)
* Remove the secret field from tree node values
draft-04
* Updating the language to be similar to the Architecture document
* ECIES is now renamed in favor of HPKE (*)
* Using a KDF instead of a Hash in TreeKEM (*)
draft-03
* Added ciphersuites and signature schemes (*)
* Re-ordered fields in UserInitKey to make parsing easier (*)
* Fixed inconsistencies between Welcome and GroupState (*)
* Added encryption of the Welcome message (*)
draft-02
* Removed ART (*)
* Allowed partial trees to avoid double-joins (*)
* Added explicit key confirmation (*)
draft-01
* Initial description of the Message Protection mechanism. (*)
* Initial specification proposal for the Application Key Schedule
using the per-participant chaining of the Application Secret
design. (*)
* Initial specification proposal for an encryption mechanism to
protect Application Messages using an AEAD scheme. (*)
* Initial specification proposal for an authentication mechanism of
Application Messages using signatures. (*)
* Initial specification proposal for a padding mechanism to
improving protection of Application Messages against traffic
analysis. (*)
* Inversion of the Group Init Add and Application Secret derivations
in the Handshake Key Schedule to be ease chaining in case we
switch design. (*)
* Removal of the UserAdd construct and split of GroupAdd into Add
and Welcome messages (*)
* Initial proposal for authenticating handshake messages by signing
over group state and including group state in the key schedule (*)
* Added an appendix with example code for tree math
* Changed the ECIES mechanism used by TreeKEM so that it uses nonces
generated from the shared secret
draft-00
* Initial adoption of draft-barnes-mls-protocol-01 as a WG item.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Client: An agent that uses this protocol to establish shared Client: An agent that uses this protocol to establish shared
cryptographic state with other clients. A client is defined by cryptographic state with other clients. A client is defined by
the cryptographic keys it holds. the cryptographic keys it holds.
Group: A group represents a logical collection of clients that share Group: A group represents a logical collection of clients that share
a common secret value at any given time. Its state is represented a common secret value at any given time. Its state is represented
as a linear sequence of epochs in which each epoch depends on its as a linear sequence of epochs in which each epoch depends on its
predecessor. predecessor.
Epoch: A state of a group in which a specific set of authenticated Epoch: A state of a group in which a specific set of authenticated
clients hold shared cryptographic state. clients hold shared cryptographic state.
Member: A client that is included in the shared state of a group, Member: A client that is included in the shared state of a group and
hence has access to the group's secrets. hence has access to the group's secrets.
Key Package: A signed object describing a client's identity and Key Package: A signed object describing a client's identity and
capabilities, and including a hybrid public-key encryption (HPKE capabilities, including a hybrid public key encryption (HPKE)
[RFC9180]) public key that can be used to encrypt to that client, [RFC9180] public key that can be used to encrypt to that client.
and which other clients can use to introduce the client to a new Other clients can use a client's KeyPackage to introduce the
group. client to a new group.
Group Context: An object that summarizes the shared, public state of Group Context: An object that summarizes the shared, public state of
the group. The group context is typically distributed in a signed the group. The group context is typically distributed in a signed
GroupInfo message, which is provided to new members to help them GroupInfo message, which is provided to new members to help them
join a group. join a group.
Signature Key: A signing key pair used to authenticate the sender of Signature Key: A signing key pair used to authenticate the sender of
a message. a message.
Proposal: A message that proposes a change to the group, e.g., Proposal: A message that proposes a change to the group, e.g.,
adding or removing a member. adding or removing a member.
Commit: A message that implements the changes to the group proposed Commit: A message that implements the changes to the group proposed
in a set of Proposals. in a set of Proposals.
PublicMessage: An MLS protocol message that is signed by its sender PublicMessage: An MLS protocol message that is signed by its sender
and authenticated as coming from a member of the group in a and authenticated as coming from a member of the group in a
particular epoch, but not encrypted. particular epoch, but not encrypted.
PrivateMessage: An MLS protocol message that is both signed by its PrivateMessage: An MLS protocol message that is signed by its
sender, authenticated as coming from a member of the group in a sender, authenticated as coming from a member of the group in a
particular epoch, and encrypted so that it is confidential to the particular epoch, and encrypted so that it is confidential to the
members of the group in that epoch. members of the group in that epoch.
Handshake Message: A PublicMessage or PrivateMessage carrying an MLS Handshake Message: A PublicMessage or PrivateMessage carrying an MLS
Proposal or Commit object, as opposed to application data. Proposal or Commit object, as opposed to application data.
Application Message: A PrivateMessage carrying application data. Application Message: A PrivateMessage carrying application data.
Terminology specific to tree computations is described in Terminology specific to tree computations is described in
Section 4.1. Section 4.1.
In general, symmetric values are referred to as "keys" or "secrets" In general, symmetric values are referred to as "keys" or "secrets"
interchangeably. Either term denotes a value that MUST be kept interchangeably. Either term denotes a value that MUST be kept
confidential to a Client. When labeling individual values, we confidential to a client. When labeling individual values, we
typically use "secret" to refer to a value that is used derive typically use "secret" to refer to a value that is used to derive
further secret values, and "key" to refer to a value that is used further secret values and "key" to refer to a value that is used with
with an algorithm such as HMAC or an AEAD algorithm. an algorithm such as Hashed Message Authentication Code (HMAC) or an
Authenticated Encryption with Associated Data (AEAD) algorithm.
The PublicMessage and PrivateMessage formats are defined in The PublicMessage and PrivateMessage formats are defined in
Section 6. Security notions such as forward secrecy and post- Section 6. Security notions such as forward secrecy and post-
compromise security are defined in Section 16. compromise security are defined in Section 16.
As detailed in Section 13.5, MLS uses the "Generate Random Extensions As detailed in Section 13.5, MLS uses the "Generate Random Extensions
And Sustain Extensibility" (GREASE) approach to maintaining And Sustain Extensibility" (GREASE) approach to maintaining
extensibility, where senders insert random values into fields in extensibility, where senders insert random values into fields in
which receivers are required to ignore unknown values. Specific which receivers are required to ignore unknown values. Specific
"GREASE values" for this purpose are registered in the appropriate "GREASE values" for this purpose are registered in the appropriate
IANA registries. IANA registries.
2.1. Presentation Language 2.1. Presentation Language
We use the TLS presentation language [RFC8446] to describe the We use the TLS presentation language [RFC8446] to describe the
structure of protocol messages. In addition to the base syntax, we structure of protocol messages. In addition to the base syntax, we
add two additional features, the ability for fields to be optional add two additional features: the ability for fields to be optional
and the ability for vectors to have variable-size length headers. and the ability for vectors to have variable-size length headers.
2.1.1. Optional Value 2.1.1. Optional Value
An optional value is encoded with a presence-signaling octet, An optional value is encoded with a presence-signaling octet,
followed by the value itself if present. When decoding, a presence followed by the value itself if present. When decoding, a presence
octet with a value other than 0 or 1 MUST be rejected as malformed. octet with a value other than 0 or 1 MUST be rejected as malformed.
struct { struct {
uint8 present; uint8 present;
select (present) { select (present) {
case 0: struct{}; case 0: struct{};
case 1: T value; case 1: T value;
}; };
} optional<T>; } optional<T>;
2.1.2. Variable-size Vector Length Headers 2.1.2. Variable-Size Vector Length Headers
In the TLS presentation language, vectors are encoded as a sequence In the TLS presentation language, vectors are encoded as a sequence
of encoded elements prefixed with a length. The length field has a of encoded elements prefixed with a length. The length field has a
fixed size set by specifying the minimum and maximum lengths of the fixed size set by specifying the minimum and maximum lengths of the
encoded sequence of elements. encoded sequence of elements.
In MLS, there are several vectors whose sizes vary over significant In MLS, there are several vectors whose sizes vary over significant
ranges. So instead of using a fixed-size length field, we use a ranges. So instead of using a fixed-size length field, we use a
variable-size length using a variable-length integer encoding based variable-size length using a variable-length integer encoding based
on the one in Section 16 of [RFC9000]. They differ only in that the on the one described in Section 16 of [RFC9000]. They differ only in
one here requires a minimum-size encoding. Instead of presenting min that the one here requires a minimum-size encoding. Instead of
and max values, the vector description simply includes a V. For presenting min and max values, the vector description simply includes
example: a V. For example:
struct { struct {
uint32 fixed<0..255>; uint32 fixed<0..255>;
opaque variable<V>; opaque variable<V>;
} StructWithVectors; } StructWithVectors;
Such a vector can represent values with length from 0 bytes to 2^30 Such a vector can represent values with length from 0 bytes to 2^30
bytes. The variable-length integer encoding reserves the two most bytes. The variable-length integer encoding reserves the two most
significant bits of the first byte to encode the base 2 logarithm of significant bits of the first byte to encode the base 2 logarithm of
the integer encoding length in bytes. The integer value is encoded the integer encoding length in bytes. The integer value is encoded
on the remaining bits, so that the overall value is in network byte on the remaining bits, so that the overall value is in network byte
order. The encoded value MUST use the smallest number of bits order. The encoded value MUST use the smallest number of bits
required to represent the value. When decoding, values using more required to represent the value. When decoding, values using more
bits than necessary MUST be treated as malformed. bits than necessary MUST be treated as malformed.
This means that integers are encoded on 1, 2, or 4 bytes and can This means that integers are encoded in 1, 2, or 4 bytes and can
encode 6-, 14-, or 30-bit values respectively. encode 6-, 14-, or 30-bit values, respectively.
+========+=========+=============+=======+============+ +========+=========+=============+=======+============+
| Prefix | Length | Usable Bits | Min | Max | | Prefix | Length | Usable Bits | Min | Max |
+========+=========+=============+=======+============+ +========+=========+=============+=======+============+
| 00 | 1 | 6 | 0 | 63 | | 00 | 1 | 6 | 0 | 63 |
+--------+---------+-------------+-------+------------+ +--------+---------+-------------+-------+------------+
| 01 | 2 | 14 | 64 | 16383 | | 01 | 2 | 14 | 64 | 16383 |
+--------+---------+-------------+-------+------------+ +--------+---------+-------------+-------+------------+
| 10 | 4 | 30 | 16384 | 1073741823 | | 10 | 4 | 30 | 16384 | 1073741823 |
+--------+---------+-------------+-------+------------+ +--------+---------+-------------+-------+------------+
| 11 | invalid | - | - | - | | 11 | invalid | - | - | - |
+--------+---------+-------------+-------+------------+ +--------+---------+-------------+-------+------------+
Table 1: Summary of Integer Encodings Table 1: Summary of Integer Encodings
Vectors that start with "11" prefix are invalid and MUST be rejected. Vectors that start with the prefix "11" are invalid and MUST be
rejected.
For example: For example:
* The four byte length value 0x9d7f3e7d decodes to 494878333. * The four-byte length value 0x9d7f3e7d decodes to 494878333.
* The two byte length value 0x7bbd decodes to 15293. * The two-byte length value 0x7bbd decodes to 15293.
* The single byte length value 0x25 decodes to 37. * The single-byte length value 0x25 decodes to 37.
The following figure adapts the pseudocode provided in [RFC9000] to The following figure adapts the pseudocode provided in [RFC9000] to
add a check for minimum-length encoding: add a check for minimum-length encoding:
ReadVarint(data): ReadVarint(data):
// The length of variable-length integers is encoded in the // The length of variable-length integers is encoded in the
// first two bits of the first byte. // first two bits of the first byte.
v = data.next_byte() v = data.next_byte()
prefix = v >> 6 prefix = v >> 6
if prefix == 3: if prefix == 3:
skipping to change at page 19, line 14 skipping to change at line 411
The use of variable-size integers for vector lengths allows vectors The use of variable-size integers for vector lengths allows vectors
to grow very large, up to 2^30 bytes. Implementations should take to grow very large, up to 2^30 bytes. Implementations should take
care not to allow vectors to overflow available storage. To care not to allow vectors to overflow available storage. To
facilitate debugging of potential interoperability problems, facilitate debugging of potential interoperability problems,
implementations SHOULD provide a clear error when such an overflow implementations SHOULD provide a clear error when such an overflow
condition occurs. condition occurs.
3. Protocol Overview 3. Protocol Overview
MLS is designed to operate in the context described in MLS is designed to operate in the context described in [MLS-ARCH].
[I-D.ietf-mls-architecture]. In particular, we assume that the In particular, we assume that the following services are provided:
following services are provided:
* An Authentication Service (AS) that enables group members to * An Authentication Service (AS) that enables group members to
authenticate the credentials presented by other group members. authenticate the credentials presented by other group members.
* A Delivery Service (DS) that routes MLS messages among the * A Delivery Service (DS) that routes MLS messages among the
participants in the protocol. participants in the protocol.
MLS assumes a trusted AS but a largely untrusted DS. Section 16.10 MLS assumes a trusted AS but a largely untrusted DS. Section 16.10
describes the impact of compromise or misbehavior of an AS. MLS is describes the impact of compromise or misbehavior of an AS. MLS is
designed to protect the confidentiality and integrity of the group designed to protect the confidentiality and integrity of the group
data even in the face of a compromised DS; in general, the DS is just data even in the face of a compromised DS; in general, the DS is only
expected to reliably deliver messages. Section 16.9 describes the expected to reliably deliver messages. Section 16.9 describes the
impact of compromise or misbehavior of a DS. impact of compromise or misbehavior of a DS.
The core functionality of MLS is continuous group authenticated key The core functionality of MLS is continuous group authenticated key
exchange (AKE). As with other authenticated key exchange protocols exchange (AKE). As with other authenticated key exchange protocols
(such as TLS), the participants in the protocol agree on a common (such as TLS), the participants in the protocol agree on a common
secret value, and each participant can verify the identity of the secret value, and each participant can verify the identity of the
other participants. That secret can then be used to protect messages other participants. That secret can then be used to protect messages
sent from one participant in the group to the other participants sent from one participant in the group to the other participants
using the MLS framing layer or can be exported for use with other using the MLS framing layer or can be exported for use with other
skipping to change at page 20, line 26 skipping to change at line 460
The creator of an MLS group creates the group's first epoch The creator of an MLS group creates the group's first epoch
unilaterally, with no protocol interactions. Thereafter, the members unilaterally, with no protocol interactions. Thereafter, the members
of the group advance their shared cryptographic state from one epoch of the group advance their shared cryptographic state from one epoch
to another by exchanging MLS messages. to another by exchanging MLS messages.
* A _KeyPackage_ object describes a client's capabilities and * A _KeyPackage_ object describes a client's capabilities and
provides keys that can be used to add the client to a group. provides keys that can be used to add the client to a group.
* A _Proposal_ message proposes a change to be made in the next * A _Proposal_ message proposes a change to be made in the next
epoch, such as adding or removing a member epoch, such as adding or removing a member.
* A _Commit_ message initiates a new epoch by instructing members of * A _Commit_ message initiates a new epoch by instructing members of
the group to implement a collection of proposals the group to implement a collection of proposals.
* A _Welcome_ message provides a new member to the group with the * A _Welcome_ message provides a new member to the group with the
information to initialize their state for the epoch in which they information to initialize their state for the epoch in which they
were added or in which they want to add themselves to the group were added or in which they want to add themselves to the group.
KeyPackage and Welcome messages are used to initiate a group or KeyPackage and Welcome messages are used to initiate a group or
introduce new members, so they are exchanged between group members introduce new members, so they are exchanged between group members
and clients not yet in the group. A client publishes a KeyPackage and clients not yet in the group. A client publishes a KeyPackage
via the DS, thus enabling other clients to add it to groups. When a via the DS, thus enabling other clients to add it to groups. When a
group member wants to add a new member to a group it uses the new group member wants to add a new member to a group, it uses the new
member's KeyPackage to add the new member to the group and construct member's KeyPackage to add them and constructs a Welcome message with
a Welcome message with which the new member can initialize its local which the new member can initialize their local state.
state.
Proposal and Commit messages are sent from one member of a group to Proposal and Commit messages are sent from one member of a group to
the others. MLS provides a common framing layer for sending messages the others. MLS provides a common framing layer for sending messages
within a group: A _PublicMessage_ provides sender authentication for within a group: A _PublicMessage_ provides sender authentication for
unencrypted Proposal and Commit messages. A _PrivateMessage_ unencrypted Proposal and Commit messages. A _PrivateMessage_
provides encryption and authentication for both Proposal/Commit provides encryption and authentication for both Proposal/Commit
messages as well as any application data. messages as well as any application data.
3.1. Cryptographic State and Evolution 3.1. Cryptographic State and Evolution
skipping to change at page 21, line 34 skipping to change at line 513
| / | | | \ | | / | | | \ |
|/ | | | \| |/ | | | \|
' | V | ' ' | V | '
| epoch_secret | | epoch_secret |
| | | | | |
| | | | | |
| V | | V |
| | | |
'- ... -' '- ... -'
Figure 1: Overview of MLS group evolution Figure 1: Overview of MLS Group Evolution
* A _ratchet tree_ that represents the membership of the group, * A _ratchet tree_ that represents the membership of the group,
providing group members a way to authenticate each other and providing group members a way to authenticate each other and
efficiently encrypt messages to subsets of the group. Each epoch efficiently encrypt messages to subsets of the group. Each epoch
has a distinct ratchet tree. It seeds the _key schedule_. has a distinct ratchet tree. It seeds the _key schedule_.
* A _key schedule_ that describes the chain of key derivations used * A _key schedule_ that describes the chain of key derivations used
to progress from epoch to epoch (mainly using the _init_secret_ to progress from epoch to epoch (mainly using the _init_secret_
and _epoch_secret_), as well as the derivation of a variety of and _epoch_secret_), as well as the derivation of a variety of
other secrets (see Table 4), for example: other secrets (see Table 4). For example:
- An _encryption secret_ that is used to initialize the secret - An _encryption secret_ that is used to initialize the secret
tree for the epoch. tree for the epoch.
- An _exporter secret_ that allows other protocols to leverage - An _exporter secret_ that allows other protocols to leverage
MLS as a generic authenticated group key exchange. MLS as a generic authenticated group key exchange.
- A _resumption secret_ that members can use to prove their - A _resumption secret_ that members can use to prove their
membership in the group, e.g., when creating a subgroup or a membership in the group, e.g., when creating a subgroup or a
successor group. successor group.
skipping to change at page 22, line 35 skipping to change at line 561
For each Commit that adds one or more members to the group, there are For each Commit that adds one or more members to the group, there are
one or more corresponding Welcome messages. Each Welcome message one or more corresponding Welcome messages. Each Welcome message
provides new members with the information they need to initialize provides new members with the information they need to initialize
their views of the key schedule and ratchet tree, so that these views their views of the key schedule and ratchet tree, so that these views
align with the views held by other members of the group in this align with the views held by other members of the group in this
epoch. epoch.
3.2. Example Protocol Execution 3.2. Example Protocol Execution
There are three major operations in the lifecycle of a group: There are three major operations in the life of a group:
* Adding a member, initiated by a current member; * Adding a member, initiated by a current member;
* Updating the keys that represent a member in the tree; * Updating the keys that represent a member in the tree; and
* Removing a member. * Removing a member.
Each of these operations is "proposed" by sending a message of the Each of these operations is "proposed" by sending a message of the
corresponding type (Add / Update / Remove). The state of the group corresponding type (Add / Update / Remove). The state of the group
is not changed, however, until a Commit message is sent to provide is not changed, however, until a Commit message is sent to provide
the group with fresh entropy. In this section, we show each proposal the group with fresh entropy. In this section, we show each proposal
being committed immediately, but in more advanced deployment cases an being committed immediately, but in more advanced deployment cases,
application might gather several proposals before committing them all an application might gather several proposals before committing them
at once. In the illustrations below, we show the Proposal and Commit all at once. In the illustrations below, we show the Proposal and
messages directly, while in reality they would be sent encapsulated Commit messages directly, while in reality they would be sent
in PublicMessage or PrivateMessage objects. encapsulated in PublicMessage or PrivateMessage objects.
Before the initialization of a group, clients publish KeyPackages to Before the initialization of a group, clients publish KeyPackages to
a directory provided by the DS (see Figure 2). a directory provided by the DS (see Figure 2).
Delivery Service Delivery Service
| |
.--------' '--------. .--------' '--------.
| | | |
Group Group
A B C Directory Channel A B C Directory Channel
skipping to change at page 23, line 27 skipping to change at line 601
| | | | | | | | | |
| | KeyPackageB | | | | | KeyPackageB | | |
| +-------------------------------->| | | +-------------------------------->| |
| | | | | | | | | |
| | | KeyPackageC | | | | | KeyPackageC | |
| | +--------------->| | | | +--------------->| |
| | | | | | | | | |
Figure 2: Clients A, B, and C publish KeyPackages to the directory Figure 2: Clients A, B, and C publish KeyPackages to the directory
Figure 3 shows how these prepublished KeyPackages are used to create Figure 3 shows how these pre-published KeyPackages are used to create
a group. When a client A wants to establish a group with B and C, it a group. When client A wants to establish a group with clients B and
first initializes a group state containing only itself and downloads C, it first initializes a group state containing only itself and
KeyPackages for B and C. For each member, A generates an Add and downloads KeyPackages for B and C. For each member, A generates an
Commit message adding that member, and broadcasts them to the group. Add proposal and a Commit message to add that member and then
It also generates a Welcome message and sends this directly to the broadcasts the two messages to the group. Client A also generates a
new member (there's no need to send it to the group). Only after A Welcome message and sends it directly to the new member (there's no
has received its Commit message back from the Delivery Service does need to send it to the group). Only after A has received its Commit
it update its state to reflect the new member's addition. message back from the Delivery Service does it update its state to
reflect the new member's addition.
Once A has updated its state, the new member has processed the Once A has updated its state, the new member has processed the
Welcome, and any other group members have processed the Commit, they Welcome, and any other group members have processed the Commit, they
will all have consistent representations of the group state, will all have consistent representations of the group state,
including a group secret that is known only to the members the group. including a group secret that is known only to the members the group.
The new member will be able to read and send new messages to the The new member will be able to read and send new messages to the
group, but messages sent before they were added to the group will not group, but messages sent before they were added to the group will not
be accessible. be accessible.
Group Group
skipping to change at page 24, line 39 skipping to change at line 654
| | | | | | | | | |
| | | | Add(AB->ABC) | | | | | Add(AB->ABC) |
| | | | Commit(Add) | | | | | Commit(Add) |
|<---------------------------------------------------------------+ |<---------------------------------------------------------------+
| |<------------------------------------------------+ | |<------------------------------------------------+
| | | | | | | | | |
Figure 3: Client A creates a group with clients B and C Figure 3: Client A creates a group with clients B and C
Subsequent additions of group members proceed in the same way. Any Subsequent additions of group members proceed in the same way. Any
member of the group can download a KeyPackage for a new client and member of the group can download a KeyPackage for a new client,
broadcast Add and Commit messages that the current group will use to broadcast Add and Commit messages that the current group will use to
update their state, and a Welcome message that the new client can use update their state, and send a Welcome message that the new client
to initialize its state and join the group. can use to initialize its state and join the group.
To enforce the forward secrecy and post-compromise security of To enforce the forward secrecy and post-compromise security of
messages, each member periodically updates the keys that represent messages, each member periodically updates the keys that represent
them to the group. A member does this by sending a Commit (possibly them to the group. A member does this by sending a Commit (possibly
with no proposals), or by sending an Update message that is committed with no proposals) or by sending an Update message that is committed
by another member (see Figure 4). Once the other members of the by another member (see Figure 4). Once the other members of the
group have processed these messages, the group's secrets will be group have processed these messages, the group's secrets will be
unknown to an attacker that had compromised the secrets corresponding unknown to an attacker that had compromised the secrets corresponding
to the sender's leaf in the tree. to the sender's leaf in the tree. At the end of the scenario shown
in Figure 4, the group has post-compromise security with respect to
both A and B.
Update messages SHOULD be sent at regular intervals of time as long Update messages SHOULD be sent at regular intervals of time as long
as the group is active, and members that don't update SHOULD as the group is active, and members that don't update SHOULD
eventually be removed from the group. It's left to the application eventually be removed from the group. It's left to the application
to determine an appropriate amount of time between Updates. Since to determine an appropriate amount of time between Updates. Since
the purpose of sending an Update is to proactively constrain a the purpose of sending an Update is to proactively constrain a
compromise window, the right frequency is usually on the order of compromise window, the right frequency is usually on the order of
hours or days, not milliseconds. For example, an application might hours or days, not milliseconds. For example, an application might
send an Update each time a member sends an application message after send an Update each time a member sends an application message after
receiving from other members, or daily if no application messages are receiving any message from another member, or daily if no application
sent. messages are sent.
The MLS architecture recommends that MLS be operated over a secure The MLS architecture recommends that MLS be operated over a secure
transport (see Section 7.1 of [I-D.ietf-mls-architecture]). Such transport (see Section 7.1 of [MLS-ARCH]). Such transport protocols
transport protocols will typically provide functions such as will typically provide functions such as congestion control that
congestion control that manage the impact of an MLS-using application manage the impact of an MLS-using application on other applications
on other applications sharing the same network. Applications should sharing the same network. Applications should take care that they do
take care that they do not send MLS messages at a rate that will not send MLS messages at a rate that will cause problems such as
cause problems such as network congestion, especially if they are not network congestion, especially if they are not following the above
following the above recommendation (e.g., sending MLS directly over recommendation (e.g., sending MLS directly over UDP instead).
UDP instead).
Group Group
A B ... Z Directory Channel A B ... Z Directory Channel
| | | | | | | | | |
| | Update(B) | | | | | Update(B) | | |
| +------------------------------------------->| | +------------------------------------------->|
| | | | Update(B) | | | | | Update(B) |
|<----------------------------------------------------------+ |<----------------------------------------------------------+
| |<-------------------------------------------+ | |<-------------------------------------------+
| | |<----------------------------+ | | |<----------------------------+
| | | | | | | | | |
| Commit(Upd) | | | | | Commit(Upd) | | | |
+---------------------------------------------------------->| +---------------------------------------------------------->|
| | | | Commit(Upd) | | | | | Commit(Upd) |
|<----------------------------------------------------------+ |<----------------------------------------------------------+
| |<-------------------------------------------+ | |<-------------------------------------------+
| | |<----------------------------+ | | |<----------------------------+
| | | | | | | | | |
Figure 4: Client B proposes to update its key, and client A Figure 4: Client B proposes to update its key, and client A
commits the proposal. As a result, the keys for both B and A commits the proposal
updated, so the group has post-compromise security with respect
to both of them.
Members are removed from the group in a similar way, as shown in Members are removed from the group in a similar way, as shown in
Figure 5. Any member of the group can send a Remove proposal Figure 5. Any member of the group can send a Remove proposal
followed by a Commit message. The Commit message provides new followed by a Commit message. The Commit message provides new
entropy to all members of the group except the removed member. This entropy to all members of the group except the removed member. This
new entropy is added to the epoch secret for the new epoch so that it new entropy is added to the epoch secret for the new epoch so that it
is not known to the removed member. Note that this does not is not known to the removed member. Note that this does not
necessarily imply that any member is actually allowed to evict other necessarily imply that any member is actually allowed to evict other
members; groups can enforce access control policies on top of these members; groups can enforce access control policies on top of these
basic mechanisms. basic mechanisms.
skipping to change at page 26, line 36 skipping to change at line 747
Note that the flows in this section are examples; applications can Note that the flows in this section are examples; applications can
arrange message flows in other ways. For example: arrange message flows in other ways. For example:
* Welcome messages don't necessarily need to be sent directly to new * Welcome messages don't necessarily need to be sent directly to new
joiners. Since they are encrypted to new joiners, they could be joiners. Since they are encrypted to new joiners, they could be
distributed more broadly, say if the application only had access distributed more broadly, say if the application only had access
to a broadcast channel for the group. to a broadcast channel for the group.
* Proposal messages don't need to be immediately sent to all group * Proposal messages don't need to be immediately sent to all group
members. They need to be available to the committer before members. They need to be available to the committer before
generating a commit, and to other members before processing the generating a Commit, and to other members before processing the
commit. Commit.
* The sender of a Commit doesn't necessarily have to wait to receive * The sender of a Commit doesn't necessarily have to wait to receive
its own Commit back before advancing its state. It only needs to its own Commit back before advancing its state. It only needs to
know that its Commit will be the next one applied by the group, know that its Commit will be the next one applied by the group,
say based on a promise from an orchestration server. say based on a promise from an orchestration server.
3.3. External Joins 3.3. External Joins
In addition to the Welcome-based flow for adding a new member to the In addition to the Welcome-based flow for adding a new member to the
group, it is also possible for a new member to join by means of an group, it is also possible for a new member to join by means of an
skipping to change at page 27, line 32 skipping to change at line 791
| | |<-------------+ | | | |<-------------+ |
| | | | | | | | | |
| | | Commit(ExtZ) | | | | | Commit(ExtZ) | |
| | +---------------------------->| | | +---------------------------->|
| | | | Commit(ExtZ) | | | | | Commit(ExtZ) |
|<----------------------------------------------------------+ |<----------------------------------------------------------+
| |<-------------------------------------------+ | |<-------------------------------------------+
| | |<----------------------------+ | | |<----------------------------+
| | | | | | | | | |
Figure 6: Client A publishes a GroupInfo object and Client Z uses Figure 6: Client A publishes a GroupInfo object, and Client Z
it to join the group uses it to join the group
3.4. Relationships Between Epochs 3.4. Relationships between Epochs
A group has a single linear sequence of epochs. Groups and epochs A group has a single linear sequence of epochs. Groups and epochs
are generally independent of one another. However, it can sometimes are generally independent of one another. However, it can sometimes
be useful to link epochs cryptographically, either within a group or be useful to link epochs cryptographically, either within a group or
across groups. MLS derives a resumption pre-shared key (PSK) from across groups. MLS derives a resumption pre-shared key (PSK) from
each epoch to allow entropy extracted from one epoch to be injected each epoch to allow entropy extracted from one epoch to be injected
into a future epoch. A group member that wishes to inject a PSK into a future epoch. A group member that wishes to inject a PSK
issues a PreSharedKey proposal (Section 12.1.4) describing the PSK to issues a PreSharedKey proposal (Section 12.1.4) describing the PSK to
be injected. When this proposal is committed, the corresponding PSK be injected. When this proposal is committed, the corresponding PSK
will be incorporated into the key schedule as described in will be incorporated into the key schedule as described in
Section 8.4. Section 8.4.
Linking epochs in this way guarantees that members entering the new Linking epochs in this way guarantees that members entering the new
epoch agree on a key if and only if they were members of the group epoch agree on a key if and only if they were members of the group
during the epoch from which the resumption key was extracted. during the epoch from which the resumption key was extracted.
MLS supports two ways to tie a new group to an existing group, MLS supports two ways to tie a new group to an existing group, which
illustrated in Figure 7 and Figure 8. Reinitialization closes one are illustrated in Figures 7 and 8. Reinitialization closes one
group and creates a new group comprising the same members with group and creates a new group comprising the same members with
different parameters. Branching starts a new group with a subset of different parameters. Branching starts a new group with a subset of
the original group's participants (with no effect on the original the original group's participants (with no effect on the original
group). In both cases, the new group is linked to the old group via group). In both cases, the new group is linked to the old group via
a resumption PSK. a resumption PSK.
epoch_A_[n-1] epoch_A_[n-1]
| |
| |
|<-- ReInit |<-- ReInit
| |
V V
epoch_A_[n] epoch_B_[0] epoch_A_[n] epoch_B_[0]
. | . |
. PSK(usage=reinit) | . PSK(usage=reinit) |
.....................>| .....................>|
| |
V V
epoch_B_[1] epoch_B_[1]
Figure 7: Reinitializing a group Figure 7: Reinitializing a Group
epoch_A_[n] epoch_B_[0] epoch_A_[n] epoch_B_[0]
| | | |
| PSK(usage=branch) | | PSK(usage=branch) |
|....................>| |....................>|
| | | |
V V V V
epoch_A_[n+1] epoch_B_[1] epoch_A_[n+1] epoch_B_[1]
Figure 8: Branching a group Figure 8: Branching a Group
Applications may also choose to use resumption PSKs to link epochs in Applications may also choose to use resumption PSKs to link epochs in
other ways. For example, Figure 9 shows a case where a resumption other ways. For example, Figure 9 shows a case where a resumption
PSK from epoch n is injected into epoch n+k. This demonstrates that PSK from epoch n is injected into epoch n+k. This demonstrates that
the members of the group at epoch n+k were also members at epoch n, the members of the group at epoch n+k were also members at epoch n,
irrespective of any changes to these members' keys due to Updates or irrespective of any changes to these members' keys due to Updates or
Commits. Commits.
epoch_A_[n] epoch_A_[n]
| |
skipping to change at page 29, line 23 skipping to change at line 870
| . | .
V . V .
epoch_A_[n+k-1] . epoch_A_[n+k-1] .
| . | .
| . | .
|<.................... |<....................
| |
V V
epoch_A_[n+k] epoch_A_[n+k]
Figure 9: Reinjecting entropy from an earlier epoch Figure 9: Reinjecting Entropy from an Earlier Epoch
4. Ratchet Tree Concepts 4. Ratchet Tree Concepts
The protocol uses "ratchet trees" for deriving shared secrets among a The protocol uses "ratchet trees" for deriving shared secrets among a
group of clients. A ratchet tree is an arrangement of secrets and group of clients. A ratchet tree is an arrangement of secrets and
key pairs among the members of a group in a way that allows for key pairs among the members of a group in a way that allows for
secrets to be efficiently updated to reflect changes in the group. secrets to be efficiently updated to reflect changes in the group.
Ratchet trees allow a group to efficiently remove any member by Ratchet trees allow a group to efficiently remove any member by
encrypting new entropy to a subset of the group. A ratchet tree encrypting new entropy to a subset of the group. A ratchet tree
skipping to change at page 30, line 7 skipping to change at line 895
is the number of members in the group). is the number of members in the group).
This remove operation allows MLS to efficiently achieve post- This remove operation allows MLS to efficiently achieve post-
compromise security. In an Update proposal or a full Commit message, compromise security. In an Update proposal or a full Commit message,
an old (possibly compromised) representation of a member is an old (possibly compromised) representation of a member is
efficiently removed from the group and replaced with a freshly efficiently removed from the group and replaced with a freshly
generated instance. generated instance.
4.1. Ratchet Tree Terminology 4.1. Ratchet Tree Terminology
Trees consist of _nodes_. A node is a _leaf_ if it has no children, Trees consist of _nodes_. A node is a _leaf_ if it has no children;
and a _parent_ otherwise; note that all parents in our trees have otherwise, it is a _parent_. All parents in our trees have precisely
precisely two children, a _left_ child and a _right_ child. A node two children, a _left_ child and a _right_ child. A node is the
is the _root_ of a tree if it has no parents, and _intermediate_ if _root_ of a tree if it has no parent, and _intermediate_ if it has
it has both children and parents. The _descendants_ of a node are both children and a parent. The _descendants_ of a node are that
that node's children, and the descendants of its children, and we say node's children, and the descendants of its children. We say a tree
a tree _contains_ a node if that node is a descendant of the root of _contains_ a node if that node is a descendant of the root of the
the tree, or if the node itself is the root of the tree. Nodes are tree, or if the node itself is the root of the tree. Nodes are
_siblings_ if they share the same parent. _siblings_ if they share the same parent.
A _subtree_ of a tree is the tree given by any node (the _head_ of A _subtree_ of a tree is the tree given by any node (the _head_ of
the subtree) and its descendants. The _size_ of a tree or subtree is the subtree) and its descendants. The _size_ of a tree or subtree is
the number of leaf nodes it contains. For a given parent node, its the number of leaf nodes it contains. For a given parent node, its
_left subtree_ is the subtree with its left child as head _left subtree_ is the subtree with its left child as head and its
(respectively _right subtree_). _right subtree_ is the subtree with its right child as head.
Every tree used in this protocol is a perfect binary tree, that is, a Every tree used in this protocol is a perfect binary tree, that is, a
complete balanced binary tree with 2^d leaves all at the same depth complete balanced binary tree with 2^d leaves all at the same depth
d. This structure is unique for a given depth d. d. This structure is unique for a given depth d.
There are multiple ways that an implementation might represent a There are multiple ways that an implementation might represent a
ratchet tree in memory. A convenient property of left-balanced ratchet tree in memory. A convenient property of left-balanced
binary trees (including the complete trees used here) is that they binary trees (including the complete trees used here) is that they
can be represented as an array of nodes, with node relationships can be represented as an array of nodes, with node relationships
computed based on the nodes' indices in the array. A more computed based on the nodes' indices in the array. A more
traditional representation based on linked node objects may also be traditional representation based on linked node objects may also be
used. Appendix C and Appendix D provide some details on how to used. Appendices C and D provide some details on how to implement
implement the tree operations required for MLS in these the tree operations required for MLS in these representations. MLS
representations. MLS places no requirements on implementations' places no requirements on implementations' internal representations
internal representations of ratchet trees. An implementation may use of ratchet trees. An implementation may use any tree representation
any tree representation and associated algorithms, as long as they and associated algorithms, as long as they produce correct protocol
produce correct protocol messages. messages.
4.1.1. Ratchet Tree Nodes 4.1.1. Ratchet Tree Nodes
Each leaf node in a ratchet tree is given an _index_ (or _leaf Each leaf node in a ratchet tree is given an _index_ (or _leaf
index_), starting at 0 from the left to 2^d - 1 at the right (for a index_), starting at 0 from the left to 2^d - 1 at the right (for a
tree with 2^d leaves). A tree with 2^d leaves has 2^(d+1) - 1 nodes, tree with 2^d leaves). A tree with 2^d leaves has 2^(d+1) - 1 nodes,
including parent nodes. including parent nodes.
Each node in a ratchet tree is either _blank_ (containing no value) Each node in a ratchet tree is either _blank_ (containing no value)
or it holds an HPKE public key with some associated data: or it holds an HPKE public key with some associated data:
* A public key (for the HPKE scheme in use, see Section 5.1) * A public key (for the HPKE scheme in use; see Section 5.1)
* A credential (only for leaf nodes; see Section 5.3)
* A credential (only for leaf nodes, see Section 5.3)
* An ordered list of "unmerged" leaves (see Section 4.2) * An ordered list of "unmerged" leaves (see Section 4.2)
* A hash of certain information about the node's parent, as of the * A hash of certain information about the node's parent, as of the
last time the node was changed (see Section 7.9). last time the node was changed (see Section 7.9).
As described in Section 4.2, different members know different subsets As described in Section 4.2, different members know different subsets
of the set of private keys corresponding to the public keys in nodes of the set of private keys corresponding to the public keys in nodes
in the tree. The private key corresponding to a parent node is known in the tree. The private key corresponding to a parent node is known
only to members at leaf nodes that are descedants of that node. The only to members at leaf nodes that are descendants of that node. The
private key corresponding to a leaf node is known only to the member private key corresponding to a leaf node is known only to the member
at that leaf node. A leaf node is _unmerged_ relative to one of its at that leaf node. A leaf node is _unmerged_ relative to one of its
ancestor nodes if the member at the leaf node does not know the ancestor nodes if the member at the leaf node does not know the
private key corresponding to the ancestor node. private key corresponding to the ancestor node.
Every node, regardless of whether the node is blank or populated, has Every node, regardless of whether the node is blank or populated, has
a corresponding _hash_ that summarizes the contents of the subtree a corresponding _hash_ that summarizes the contents of the subtree
below that node. The rules for computing these hashes are described below that node. The rules for computing these hashes are described
in Section 7.8. in Section 7.8.
The _resolution_ of a node is an ordered list of non-blank nodes that The _resolution_ of a node is an ordered list of non-blank nodes that
collectively cover all non-blank descendants of the node. The collectively cover all non-blank descendants of the node. The
resolution of the root contains the set of keys which are resolution of the root contains the set of keys that are collectively
collectively necessary to encrypt to every node in the group. The necessary to encrypt to every node in the group. The resolution of a
resolution of a node is effectively a depth-first, left-first node is effectively a depth-first, left-first enumeration of the
enumeration of the nearest non-blank nodes below the node: nearest non-blank nodes below the node:
* The resolution of a non-blank node comprises the node itself, * The resolution of a non-blank node comprises the node itself,
followed by its list of unmerged leaves, if any followed by its list of unmerged leaves, if any.
* The resolution of a blank leaf node is the empty list * The resolution of a blank leaf node is the empty list.
* The resolution of a blank intermediate node is the result of * The resolution of a blank intermediate node is the result of
concatenating the resolution of its left child with the resolution concatenating the resolution of its left child with the resolution
of its right child, in that order of its right child, in that order.
For example, consider the following subtree, where the _ character For example, consider the following subtree, where the _ character
represents a blank node and unmerged leaves are indicated in square represents a blank node and unmerged leaves are indicated in square
brackets: brackets:
... ...
/ /
_ _
______|______ ______|______
/ \ / \
X[B] _ X[B] _
__|__ __|__ __|__ __|__
/ \ / \ / \ / \
_ _ Y _ _ _ Y _
/ \ / \ / \ / \ / \ / \ / \ / \
A B _ D E F _ H A B _ D E F _ H
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Figure 10: A tree with blanks and unmerged leaves Figure 10: A Tree with Blanks and Unmerged Leaves
In this tree, we can see all of the above rules in play: In this tree, we can see all of the above rules in play:
* The resolution of node X is the list [X, B] * The resolution of node X is the list [X, B].
* The resolution of leaf 2 or leaf 6 is the empty list [] * The resolution of leaf 2 or leaf 6 is the empty list [].
* The resolution of top node is the list [X, B, Y, H] * The resolution of top node is the list [X, B, Y, H].
4.1.2. Paths through a Ratchet Tree 4.1.2. Paths through a Ratchet Tree
The _direct path_ of a root is the empty list, and of any other node The _direct path_ of a root is the empty list. The direct path of
is the concatenation of that node's parent along with the parent's any other node is the concatenation of that node's parent along with
direct path. the parent's direct path.
The _copath_ of a node is the node's sibling concatenated with the The _copath_ of a node is the node's sibling concatenated with the
list of siblings of all the nodes in its direct path, excluding the list of siblings of all the nodes in its direct path, excluding the
root. root.
The _filtered direct path_ of a leaf node L is the node's direct The _filtered direct path_ of a leaf node L is the node's direct
path, with any node removed whose child on the copath of L has an path, with any node removed whose child on the copath of L has an
empty resolution (keeping in mind that any unmerged leaves of the empty resolution (keeping in mind that any unmerged leaves of the
copath child count toward its resolution). The removed nodes do not copath child count toward its resolution). The removed nodes do not
need their own key pairs because encrypting to the node's key pair need their own key pairs because encrypting to the node's key pair
skipping to change at page 33, line 19 skipping to change at line 1039
_=U Y _=U Y
| | | |
.-+-. .-+-. .-+-. .-+-.
/ \ / \ / \ / \
T _=V X _=Z T _=V X _=Z
/ \ / \ / \ / \ / \ / \ / \ / \
A B _ _ E F G _=H A B _ _ E F G _=H
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Figure 11: A complete tree with five members, with labels for Figure 11: A Complete Tree with Five Members, with Labels for
blank parent nodes Blank Parent Nodes
In this tree, the direct paths, copaths, and filtered direct paths In this tree, the direct paths, copaths, and filtered direct paths
for the leaf nodes are as follows: for the leaf nodes are as follows:
+======+=============+=========+======================+ +======+=============+=========+======================+
| Node | Direct path | Copath | Filtered Direct Path | | Node | Direct path | Copath | Filtered Direct Path |
+======+=============+=========+======================+ +======+=============+=========+======================+
| A | T, U, W | B, V, Y | T, W | | A | T, U, W | B, V, Y | T, W |
+------+-------------+---------+----------------------+ +------+-------------+---------+----------------------+
| B | T, U, W | A, V, Y | T, W | | B | T, U, W | A, V, Y | T, W |
skipping to change at page 34, line 8 skipping to change at line 1076
associated with the leaf nodes. associated with the leaf nodes.
No participant in an MLS group knows the private key associated with No participant in an MLS group knows the private key associated with
every node in the tree. Instead, each member is assigned to a leaf every node in the tree. Instead, each member is assigned to a leaf
of the tree, which determines the subset of private keys it knows. of the tree, which determines the subset of private keys it knows.
The credential stored at that leaf is one provided by the member. The credential stored at that leaf is one provided by the member.
In particular, MLS maintains the members' views of the tree in such a In particular, MLS maintains the members' views of the tree in such a
way as to maintain the _tree invariant_: way as to maintain the _tree invariant_:
The private key for a node in the tree is known to a member of | The private key for a node in the tree is known to a member of the
the group only if the node's subtree contains that member's leaf. | group only if the node's subtree contains that member's leaf.
In other words, if a node is not blank, then it holds a public key. In other words, if a node is not blank, then it holds a public key.
The corresponding private key is known only to members occupying The corresponding private key is known only to members occupying
leaves below that node. leaves below that node.
The reverse implication is not true: A member may not know the The reverse implication is not true: A member may not know the
private key of an intermediate node above them. Such a member has an private key of an intermediate node above them. Such a member has an
_unmerged_ leaf at the intermediate node. Encrypting to an _unmerged_ leaf at the intermediate node. Encrypting to an
intermediate node requires encrypting to the node's public key, as intermediate node requires encrypting to the node's public key, as
well as the public keys of all the unmerged leaves below it. A leaf well as the public keys of all the unmerged leaves below it. A leaf
skipping to change at page 35, line 4 skipping to change at line 1117
/ \ / \ / \ / \
pk(A) pk(B) pk(C) pk(D) pk(A) pk(B) pk(C) pk(D)
Private @ A Private @ B Private @ C Private @ D Private @ A Private @ B Private @ C Private @ D
============= ============= ============= ============= ============= ============= ============= =============
ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD
/ \ / \ / \ / \ / \ / \ / \ / \
AB _ AB _ ? _ ? _ AB _ AB _ ? _ ? _
/ \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \
A ? ? ? ? B ? ? ? ? C ? ? ? ? D A ? ? ? ? B ? ? ? ? C ? ? ? ? D
Note how the tree invariant applies: Each member knows only their own Note how the tree invariant applies: Each member knows only their own
leaf, the private key AB is known only to A and B, and the private leaf, the private key AB is known only to A and B, and the private
key ABCD is known to all four members. This also illustrates another key ABCD is known to all four members. This also illustrates another
important point: it is possible for there to be "holes" on the path important point: it is possible for there to be "holes" on the path
from a member's leaf to the root in which the member knows the key from a member's leaf to the root in which the member knows the key
both above and below a given node, but not for that node, as in the both above and below a given node, but not for that node, as in the
case with D. case with D.
5. Cryptographic Objects 5. Cryptographic Objects
5.1. Ciphersuites 5.1. Cipher Suites
Each MLS session uses a single ciphersuite that specifies the Each MLS session uses a single cipher suite that specifies the
following primitives to be used in group key computations: following primitives to be used in group key computations:
* HPKE parameters: * HPKE parameters:
- A Key Encapsulation Mechanism (KEM) - A Key Encapsulation Mechanism (KEM)
- A Key Derivation Function (KDF) - A Key Derivation Function (KDF)
- An Authenticated Encryption with Associated Data (AEAD) - An Authenticated Encryption with Associated Data (AEAD)
encryption algorithm encryption algorithm
* A hash algorithm * A hash algorithm
* A MAC algorithm * A Message Authentication Code (MAC) algorithm
* A signature algorithm * A signature algorithm
MLS uses HPKE for public-key encryption [RFC9180]. The DeriveKeyPair MLS uses HPKE for public key encryption [RFC9180]. The DeriveKeyPair
function associated to the KEM for the ciphersuite maps octet strings function associated to the KEM for the cipher suite maps octet
to HPKE key pairs. As in HPKE, MLS assumes that an AEAD algorithm strings to HPKE key pairs. As in HPKE, MLS assumes that an AEAD
produces a single ciphertext output from AEAD encryption (aligning algorithm produces a single ciphertext output from AEAD encryption
with [RFC5116]), as opposed to a separate ciphertext and tag. (aligning with [RFC5116]), as opposed to a separate ciphertext and
tag.
Ciphersuites are represented with the CipherSuite type. The Cipher suites are represented with the CipherSuite type. The cipher
ciphersuites are defined in Section 17.1. suites are defined in Section 17.1.
5.1.1. Public Keys 5.1.1. Public Keys
HPKE public keys are opaque values in a format defined by the HPKE public keys are opaque values in a format defined by the
underlying protocol (see Section 4 of [RFC9180] for more underlying protocol (see Section 4 of [RFC9180] for more
information). information).
opaque HPKEPublicKey<V>; opaque HPKEPublicKey<V>;
Signature public keys are likewise represented as opaque values in a Signature public keys are likewise represented as opaque values in a
format defined by the ciphersuite's signature scheme. format defined by the cipher suite's signature scheme.
opaque SignaturePublicKey<V>; opaque SignaturePublicKey<V>;
For ciphersuites using Ed25519 or Ed448 signature schemes, the public For cipher suites using the Edwards-curve Digital Signature Algorithm
key is in the format specified in [RFC8032]. For ciphersuites using (EdDSA) signature schemes (Ed25519 or Ed448), the public key is in
ECDSA with the NIST curves (P-256, P-384, or P-521), the public key the format specified in [RFC8032].
is represented as an encoded UncompressedPointRepresentation struct,
as defined in [RFC8446]. For cipher suites using the Elliptic Curve Digital Signature
Algorithm (ECDSA) with the NIST curves (P-256, P-384, or P-521), the
public key is represented as an encoded
UncompressedPointRepresentation struct, as defined in [RFC8446].
5.1.2. Signing 5.1.2. Signing
The signature algorithm specified in a group's ciphersuite is the The signature algorithm specified in a group's cipher suite is the
mandatory algorithm to be used for signing messages within the group. mandatory algorithm to be used for signing messages within the group.
It MUST be the same as the signature algorithm specified in the It MUST be the same as the signature algorithm specified in the
credentials in the leaves of the tree (including the leaf node credentials in the leaves of the tree (including the leaf node
information in KeyPackages used to add new members). information in KeyPackages used to add new members).
The signatures used in this document are encoded as specified in The signatures used in this document are encoded as specified in
[RFC8446]. In particular, ECDSA signatures are DER-encoded and EdDSA [RFC8446]. In particular, ECDSA signatures are DER encoded, and
signatures are defined as the concatenation of r and s as specified EdDSA signatures are defined as the concatenation of R and S, as
in [RFC8032]. specified in [RFC8032].
To disambiguate different signatures used in MLS, each signed value To disambiguate different signatures used in MLS, each signed value
is prefixed by a label as shown below: is prefixed by a label as shown below:
SignWithLabel(SignatureKey, Label, Content) = SignWithLabel(SignatureKey, Label, Content) =
Signature.Sign(SignatureKey, SignContent) Signature.Sign(SignatureKey, SignContent)
VerifyWithLabel(VerificationKey, Label, Content, SignatureValue) = VerifyWithLabel(VerificationKey, Label, Content, SignatureValue) =
Signature.Verify(VerificationKey, SignContent, SignatureValue) Signature.Verify(VerificationKey, SignContent, SignatureValue)
Where SignContent is specified as: Where SignContent is specified as:
struct { struct {
opaque label<V>; opaque label<V>;
opaque content<V>; opaque content<V>;
} SignContent; } SignContent;
And its fields set to: And its fields are set to:
label = "MLS 1.0 " + Label; label = "MLS 1.0 " + Label;
content = Content; content = Content;
Here, the functions Signature.Sign and Signature.Verify are defined
by the signature algorithm. If MLS extensions require signatures by The functions Signature.Sign and Signature.Verify are defined by the
group members, they should re-use the SignWithLabel construction, signature algorithm. If MLS extensions require signatures by group
using a distinct label. To avoid collisions in these labels, an IANA members, they should reuse the SignWithLabel construction, using a
distinct label. To avoid collisions in these labels, an IANA
registry is defined in Section 17.6. registry is defined in Section 17.6.
5.1.3. Public-Key Encryption 5.1.3. Public Key Encryption
As with signing, MLS includes a label and context in encryption As with signing, MLS includes a label and context in encryption
operations to avoid confusion between ciphertexts produced for operations to avoid confusion between ciphertexts produced for
different purposes. Encryption and decryption including this label different purposes. Encryption and decryption including this label
and context are done as follows: and context are done as follows:
EncryptWithLabel(PublicKey, Label, Context, Plaintext) = EncryptWithLabel(PublicKey, Label, Context, Plaintext) =
SealBase(PublicKey, EncryptContext, "", Plaintext) SealBase(PublicKey, EncryptContext, "", Plaintext)
DecryptWithLabel(PrivateKey, Label, Context, KEMOutput, Ciphertext) = DecryptWithLabel(PrivateKey, Label, Context, KEMOutput, Ciphertext) =
OpenBase(KEMOutput, PrivateKey, EncryptContext, "", Ciphertext) OpenBase(KEMOutput, PrivateKey, EncryptContext, "", Ciphertext)
Where EncryptContext is specified as: Where EncryptContext is specified as:
struct { struct {
opaque label<V>; opaque label<V>;
opaque context<V>; opaque context<V>;
} EncryptContext; } EncryptContext;
And its fields set to: And its fields are set to:
label = "MLS 1.0 " + Label; label = "MLS 1.0 " + Label;
context = Context; context = Context;
Here, the functions SealBase and OpenBase are defined [RFC9180], The functions SealBase and OpenBase are defined in Section 6.1 of
using the HPKE algorithms specified by the group's ciphersuite. If [RFC9180] (with "Base" as the MODE), using the HPKE algorithms
MLS extensions require HPKE encryption operations, they should re-use specified by the group's cipher suite. If MLS extensions require
the EncryptWithLabel construction, using a distinct label. To avoid HPKE encryption operations, they should reuse the EncryptWithLabel
collisions in these labels, an IANA registry is defined in construction, using a distinct label. To avoid collisions in these
Section 17.7. labels, an IANA registry is defined in Section 17.7.
5.2. Hash-Based Identifiers 5.2. Hash-Based Identifiers
Some MLS messages refer to other MLS objects by hash. For example, Some MLS messages refer to other MLS objects by hash. For example,
Welcome messages refer to KeyPackages for the members being welcomed, Welcome messages refer to KeyPackages for the members being welcomed,
and Commits refer to Proposals they cover. These identifiers are and Commits refer to Proposals they cover. These identifiers are
computed as follows: computed as follows:
opaque HashReference<V>; opaque HashReference<V>;
HashReference KeyPackageRef; HashReference KeyPackageRef;
HashReference ProposalRef; HashReference ProposalRef;
MakeKeyPackageRef(value) = RefHash("MLS 1.0 KeyPackage Reference", value) MakeKeyPackageRef(value)
MakeProposalRef(value) = RefHash("MLS 1.0 Proposal Reference", value) = RefHash("MLS 1.0 KeyPackage Reference", value)
MakeProposalRef(value)
= RefHash("MLS 1.0 Proposal Reference", value)
RefHash(label, value) = Hash(RefHashInput) RefHash(label, value) = Hash(RefHashInput)
Where RefHashInput is defined as: Where RefHashInput is defined as:
struct { struct {
opaque label<V>; opaque label<V>;
opaque value<V>; opaque value<V>;
} RefHashInput; } RefHashInput;
And its fields set to: And its fields are set to:
label = label; label = label;
value = value; value = value;
For a KeyPackageRef, the value input is the encoded KeyPackage, and For a KeyPackageRef, the value input is the encoded KeyPackage, and
the ciphersuite specified in the KeyPackage determines the KDF used. the cipher suite specified in the KeyPackage determines the KDF used.
For a ProposalRef, the value input is the AuthenticatedContent For a ProposalRef, the value input is the AuthenticatedContent
carrying the proposal. In the latter two cases, the KDF is carrying the Proposal. In the latter two cases, the KDF is
determined by the group's ciphersuite. determined by the group's cipher suite.
5.3. Credentials 5.3. Credentials
Each member of a group presents a credential that provides one or Each member of a group presents a credential that provides one or
more identities for the member, and associates them with the member's more identities for the member and associates them with the member's
signing key. The identities and signing key are verified by the signing key. The identities and signing key are verified by the
Authentication Service in use for a group. Authentication Service in use for a group.
It is up to the application to decide which identifier or identifiers It is up to the application to decide which identifiers to use at the
to use at the application level. For example, a certificate in an application level. For example, a certificate in an X509Credential
X509Credential may attest to several domain names or email addresses may attest to several domain names or email addresses in its
in its subjectAltName extension. An application may decide to subjectAltName extension. An application may decide to present all
present all of these to a user, or if it knows a "desired" domain of these to a user, or if it knows a "desired" domain name or email
name or email address, it can check that the desired identifier is address, it can check that the desired identifier is among those
among those attested. Using the terminology from [RFC6125], a attested. Using the terminology from [RFC6125], a credential
Credential provides "presented identifiers", and it is up to the provides "presented identifiers", and it is up to the application to
application to supply a "reference identifier" for the authenticated supply a "reference identifier" for the authenticated client, if any.
client, if any.
// See IANA registry for registered values // See the "MLS Credential Types" IANA registry for values
uint16 CredentialType; uint16 CredentialType;
struct { struct {
opaque cert_data<V>; opaque cert_data<V>;
} Certificate; } Certificate;
struct { struct {
CredentialType credential_type; CredentialType credential_type;
select (Credential.credential_type) { select (Credential.credential_type) {
case basic: case basic:
skipping to change at page 40, line 6 skipping to change at line 1351
words, following the model that [RFC6125] describes for TLS, the words, following the model that [RFC6125] describes for TLS, the
application maintains a list of "reference identifiers" for the application maintains a list of "reference identifiers" for the
members of a group, and the credentials provide "presented members of a group, and the credentials provide "presented
identifiers". A member of a group is authenticated by first identifiers". A member of a group is authenticated by first
validating that the member's credential legitimately represents some validating that the member's credential legitimately represents some
presented identifiers, and then ensuring that the reference presented identifiers, and then ensuring that the reference
identifiers for the member are authenticated by those presented identifiers for the member are authenticated by those presented
identifiers. identifiers.
The parts of the system that perform these functions are collectively The parts of the system that perform these functions are collectively
referred to as the Authentication Service (AS) referred to as the Authentication Service (AS) [MLS-ARCH]. A
[I-D.ietf-mls-architecture]. A member's credential is said to be member's credential is said to be _validated with the AS_ when the AS
_validated with the AS_ when the AS verifies that the credential's verifies that the credential's presented identifiers are correctly
presented identifiers are correctly associated with the signature_key associated with the signature_key field in the member's LeafNode, and
field in the member's LeafNode, and verifies that those identifiers that those identifiers match the reference identifiers for the
match the reference identifiers for the member. member.
Whenever a new credential is introduced in the group, it MUST be Whenever a new credential is introduced in the group, it MUST be
validated with the AS. In particular, at the following events in the validated with the AS. In particular, at the following events in the
protocol: protocol:
* When a member receives a KeyPackage that it will use in an Add * When a member receives a KeyPackage that it will use in an Add
proposal to add a new member to the group. proposal to add a new member to the group
* When a member receives a GroupInfo object that it will use to join * When a member receives a GroupInfo object that it will use to join
a group, either via a Welcome or via an External Commit a group, either via a Welcome or via an external Commit
* When a member receives an Add proposal adding a member to the * When a member receives an Add proposal adding a member to the
group. group
* When a member receives an Update proposal whose LeafNode has a new * When a member receives an Update proposal whose LeafNode has a new
credential for the member. credential for the member
* When a member receives a Commit with an UpdatePath whose LeafNode * When a member receives a Commit with an UpdatePath whose LeafNode
has a new credential for the committer. has a new credential for the committer
* When an external_senders extension is added to the group, or an * When an external_senders extension is added to the group
existing external_senders extension is updated.
In cases where a member's credential is being replaced, such as * When an existing external_senders extension is updated
In cases where a member's credential is being replaced, such as the
Update and Commit cases above, the AS MUST also verify that the set Update and Commit cases above, the AS MUST also verify that the set
of presented identifiers in the new credential is valid as a of presented identifiers in the new credential is valid as a
successor to the set of presented identifiers in the old credential, successor to the set of presented identifiers in the old credential,
according to the application's policy. according to the application's policy.
5.3.2. Credential Expiry and Revocation 5.3.2. Credential Expiry and Revocation
In some credential schemes, a valid credential can "expire", or In some credential schemes, a valid credential can "expire" or become
become invalid after a certain point in time. For example, each invalid after a certain point in time. For example, each X.509
X.509 certificate has a notAfter field, expressing a time after which certificate has a notAfter field, expressing a time after which the
the certificate is not valid. certificate is not valid.
Expired credentials can cause operational problems in light of the Expired credentials can cause operational problems in light of the
validation requirements of Section 5.3.1. Applications can apply validation requirements of Section 5.3.1. Applications can apply
some operational practices and adaptations to Authentication Service some operational practices and adaptations to Authentication Service
policies to moderate these impacts. policies to moderate these impacts.
In general, to avoid operational problems such as new joiners In general, to avoid operational problems such as new joiners
rejecting expired credentials in a group, applications that use such rejecting expired credentials in a group, applications that use such
credentials should ensure to the extent practical that all of the credentials should ensure to the extent practical that all of the
credentials in use in a group are valid at all times. credentials in use in a group are valid at all times.
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If a member finds that another member's credential has expired, they If a member finds that another member's credential has expired, they
may issue a Remove that removes that member. For example, an may issue a Remove that removes that member. For example, an
application could require a member preparing to issue a Commit to application could require a member preparing to issue a Commit to
check the tree for expired credentials and include Remove proposals check the tree for expired credentials and include Remove proposals
for those members in its Commit. In situations where the group tree for those members in its Commit. In situations where the group tree
is known to the DS, the DS could also monitor the tree for expired is known to the DS, the DS could also monitor the tree for expired
credentials and issue external Remove proposals. credentials and issue external Remove proposals.
Some credential schemes also allow credentials to be revoked. Some credential schemes also allow credentials to be revoked.
Revocation is similar to expiry, in that a previously valid Revocation is similar to expiry in that a previously valid credential
credential becomes invalid. As such, most of the considerations becomes invalid. As such, most of the considerations above also
above also apply to revoked credentials. However, applications may apply to revoked credentials. However, applications may want to
want to treat revoked credentials differently, e.g., removing members treat revoked credentials differently, e.g., by removing members with
with revoked credentials while allowing members with expired revoked credentials while allowing members with expired credentials
credentials time to update. time to update.
5.3.3. Uniquely Identifying Clients 5.3.3. Uniquely Identifying Clients
MLS implementations will presumably provide applications with a way MLS implementations will presumably provide applications with a way
to request protocol operations with regard to other clients (e.g., to request protocol operations with regard to other clients (e.g.,
removing clients). Such functions will need to refer to the other removing clients). Such functions will need to refer to the other
clients using some identifier. MLS clients have a few types of clients using some identifier. MLS clients have a few types of
identifiers, with different operational properties. identifiers, with different operational properties.
Internally to the protocol, group members are uniquely identified by Internally to the protocol, group members are uniquely identified by
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Handshake and application messages use a common framing structure. Handshake and application messages use a common framing structure.
This framing provides encryption to ensure confidentiality within the This framing provides encryption to ensure confidentiality within the
group, as well as signing to authenticate the sender. group, as well as signing to authenticate the sender.
In most of the protocol, messages are handled in the form of In most of the protocol, messages are handled in the form of
AuthenticatedContent objects. These structures contain the content AuthenticatedContent objects. These structures contain the content
of the message itself as well as information to authenticate the of the message itself as well as information to authenticate the
sender (see Section 6.1). The additional protections required to sender (see Section 6.1). The additional protections required to
transmit these messages over an untrusted channel (group membership transmit these messages over an untrusted channel (group membership
authentication or AEAD encryption) are added by encoding the authentication or AEAD encryption) are added by encoding the
AuthenticatedContent as an PublicMessage or PrivateMessage message, AuthenticatedContent as a PublicMessage or PrivateMessage message,
which can then be sent as an MLSMessage. Likewise, these protections which can then be sent as an MLSMessage. Likewise, these protections
are enforced (via membership verification or AEAD decryption) when are enforced (via membership verification or AEAD decryption) when
decoding an PublicMessage or PrivateMessage into an decoding a PublicMessage or PrivateMessage into an
AuthenticatedContent object. AuthenticatedContent object.
PrivateMessage represents a signed and encrypted message, with PrivateMessage represents a signed and encrypted message, with
protections for both the content of the message and related metadata. protections for both the content of the message and related metadata.
PublicMessage represents a message that is only signed, and not PublicMessage represents a message that is only signed, and not
encrypted. Applications MUST use PrivateMessage to encrypt encrypted. Applications MUST use PrivateMessage to encrypt
application messages and SHOULD use PrivateMessage to encode application messages and SHOULD use PrivateMessage to encode
handshake messages, but MAY transmit handshake messages encoded as handshake messages, but they MAY transmit handshake messages encoded
PublicMessage objects in cases where it is necessary for the Delivery as PublicMessage objects in cases where it is necessary for the
Service to examine such messages. Delivery Service to examine such messages.
enum { enum {
reserved(0), reserved(0),
mls10(1), mls10(1),
(65535) (65535)
} ProtocolVersion; } ProtocolVersion;
enum { enum {
reserved(0), reserved(0),
application(1), application(1),
proposal(2), proposal(2),
commit(3), commit(3),
(255) (255)
} ContentType; } ContentType;
enum { enum {
reserved(0), reserved(0),
skipping to change at page 43, line 34 skipping to change at line 1525
case member: case member:
uint32 leaf_index; uint32 leaf_index;
case external: case external:
uint32 sender_index; uint32 sender_index;
case new_member_commit: case new_member_commit:
case new_member_proposal: case new_member_proposal:
struct{}; struct{};
}; };
} Sender; } Sender;
// See IANA registry for registered values // See the "MLS Wire Formats" IANA registry for values
uint16 WireFormat; uint16 WireFormat;
struct { struct {
opaque group_id<V>; opaque group_id<V>;
uint64 epoch; uint64 epoch;
Sender sender; Sender sender;
opaque authenticated_data<V>; opaque authenticated_data<V>;
ContentType content_type; ContentType content_type;
select (FramedContent.content_type) { select (FramedContent.content_type) {
skipping to change at page 44, line 4 skipping to change at line 1543
ContentType content_type; ContentType content_type;
select (FramedContent.content_type) { select (FramedContent.content_type) {
case application: case application:
opaque application_data<V>; opaque application_data<V>;
case proposal: case proposal:
Proposal proposal; Proposal proposal;
case commit: case commit:
Commit commit; Commit commit;
}; };
} FramedContent; } FramedContent;
struct { struct {
ProtocolVersion version = mls10; ProtocolVersion version = mls10;
WireFormat wire_format; WireFormat wire_format;
select (MLSMessage.wire_format) { select (MLSMessage.wire_format) {
case mls_public_message: case mls_public_message:
PublicMessage public_message; PublicMessage public_message;
case mls_private_message: case mls_private_message:
PrivateMessage private_message; PrivateMessage private_message;
case mls_welcome: case mls_welcome:
Welcome welcome; Welcome welcome;
case mls_group_info: case mls_group_info:
GroupInfo group_info; GroupInfo group_info;
case mls_key_package: case mls_key_package:
KeyPackage key_package; KeyPackage key_package;
}; };
} MLSMessage; } MLSMessage;
Messages from senders that aren't in the group are sent as Messages from senders that aren't in the group are sent as
PublicMessage. See Section 12.1.8 and Section 12.4.3.2 for more PublicMessage. See Sections 12.1.8 and 12.4.3.2 for more details.
details.
The following structure is used to fully describe the data The following structure is used to fully describe the data
transmitted in plaintexts or ciphertexts. transmitted in plaintexts or ciphertexts.
struct { struct {
WireFormat wire_format; WireFormat wire_format;
FramedContent content; FramedContent content;
FramedContentAuthData auth; FramedContentAuthData auth;
} AuthenticatedContent; } AuthenticatedContent;
The following figure illustrates how the various structures described The following figure illustrates how the various structures described
in this section relate to each other, and the high-level operations in this section relate to each other, and the high-level operations
used to produce and consume them: used to produce and consume them:
Proposal Commit Application Data Proposal Commit Application Data
| | | | | |
+--------------+--------------+ +--------------+--------------+
| |
V V
FramedContent FramedContent
| | -. | | -.
| | | +--------+ | |
+--------+ | | | | |
| | | V | +-- Asymmetric
V | +-- Asymmetric FramedContentAuthData | | Sign / Verify
FramedContentAuthData | | Sign / Verify | | |
| | | +--------+ | |
+--------+ | | | | |
| | | V V -'
V V -' AuthenticatedContent
AuthenticatedContent | -.
| -. +--------+--------+ |
| | | | +-- Symmetric
| | V V | Protect / Unprotect
+--------+--------+ +-- Symmetric PublicMessage PrivateMessage -'
| | | Protect / Unprotect | |
V V | | | Welcome KeyPackage GroupInfo
Welcome KeyPackage GroupInfo PublicMessage PrivateMessage -' | | | | |
| | | | | +-----------------+-----+----------+----------+
| | | | | |
+----------+----------+----+--------+-----------------+ V
| MLSMessage
V
MLSMessage
Figure 12: Relationships among MLS objects Figure 12: Relationships among MLS Objects
6.1. Content Authentication 6.1. Content Authentication
FramedContent is authenticated using the FramedContentAuthData FramedContent is authenticated using the FramedContentAuthData
structure. structure.
struct { struct {
ProtocolVersion version = mls10; ProtocolVersion version = mls10;
WireFormat wire_format; WireFormat wire_format;
FramedContent content; FramedContent content;
skipping to change at page 47, line 16 skipping to change at line 1670
in the external_senders group context extension (see in the external_senders group context extension (see
Section 12.1.8.1). The content_type of the message MUST be Section 12.1.8.1). The content_type of the message MUST be
proposal and the proposal_type MUST be a value that is allowed for proposal and the proposal_type MUST be a value that is allowed for
external senders. external senders.
* new_member_commit: The signature key in the LeafNode in the * new_member_commit: The signature key in the LeafNode in the
Commit's path (see Section 12.4.3.2). The content_type of the Commit's path (see Section 12.4.3.2). The content_type of the
message MUST be commit. message MUST be commit.
* new_member_proposal: The signature key in the LeafNode in the * new_member_proposal: The signature key in the LeafNode in the
KeyPackage embedded in an External Add Proposal. The content_type KeyPackage embedded in an external Add proposal. The content_type
of the message MUST be proposal and the proposal_type of the of the message MUST be proposal and the proposal_type of the
Proposal MUST be add. Proposal MUST be add.
Recipients of an MLSMessage MUST verify the signature with the key Recipients of an MLSMessage MUST verify the signature with the key
depending on the sender_type of the sender as described above. depending on the sender_type of the sender as described above.
The confirmation tag value confirms that the members of the group The confirmation tag value confirms that the members of the group
have arrived at the same state of the group. A FramedContentAuthData have arrived at the same state of the group. A FramedContentAuthData
is said to be valid when both the signature and confirmation_tag is said to be valid when both the signature and confirmation_tag
fields are valid. fields are valid.
skipping to change at page 48, line 12 skipping to change at line 1711
the sender's membership in the group. For messages sent by members, the sender's membership in the group. For messages sent by members,
it MUST be set to the following value: it MUST be set to the following value:
struct { struct {
FramedContentTBS content_tbs; FramedContentTBS content_tbs;
FramedContentAuthData auth; FramedContentAuthData auth;
} AuthenticatedContentTBM; } AuthenticatedContentTBM;
membership_tag = MAC(membership_key, AuthenticatedContentTBM) membership_tag = MAC(membership_key, AuthenticatedContentTBM)
When decoding an PublicMessage into an AuthenticatedContent, the When decoding a PublicMessage into an AuthenticatedContent, the
application MUST check membership_tag and MUST check that the application MUST check membership_tag and MUST check that the
FramedContentAuthData is valid. FramedContentAuthData is valid.
6.3. Encoding and Decoding a Private Message 6.3. Encoding and Decoding a Private Message
Authenticated and encrypted messages are encoded using the Authenticated and encrypted messages are encoded using the
PrivateMessage structure. PrivateMessage structure.
struct { struct {
opaque group_id<V>; opaque group_id<V>;
uint64 epoch; uint64 epoch;
ContentType content_type; ContentType content_type;
opaque authenticated_data<V>; opaque authenticated_data<V>;
opaque encrypted_sender_data<V>; opaque encrypted_sender_data<V>;
opaque ciphertext<V>; opaque ciphertext<V>;
} PrivateMessage; } PrivateMessage;
encrypted_sender_data and ciphertext are encrypted using the AEAD encrypted_sender_data and ciphertext are encrypted using the AEAD
function specified by the ciphersuite in use, using as input the function specified by the cipher suite in use, using the SenderData
structures SenderData and PrivateMessageContent. and PrivateMessageContent structures as input.
6.3.1. Content Encryption 6.3.1. Content Encryption
Content to be encrypted is encoded in a PrivateMessageContent Content to be encrypted is encoded in a PrivateMessageContent
structure. structure.
struct { struct {
select (PrivateMessage.content_type) { select (PrivateMessage.content_type) {
case application: case application:
opaque application_data<V>; opaque application_data<V>;
skipping to change at page 49, line 22 skipping to change at line 1755
case commit: case commit:
Commit commit; Commit commit;
}; };
FramedContentAuthData auth; FramedContentAuthData auth;
opaque padding[length_of_padding]; opaque padding[length_of_padding];
} PrivateMessageContent; } PrivateMessageContent;
The padding field is set by the sender, by first encoding the content The padding field is set by the sender, by first encoding the content
(via the select) and the auth field, then appending the chosen number (via the select) and the auth field, and then appending the chosen
of zero bytes. A receiver identifies the padding field in a number of zero bytes. A receiver identifies the padding field in a
plaintext decoded from PrivateMessage.ciphertext by first decoding plaintext decoded from PrivateMessage.ciphertext by first decoding
the content and the auth field; then the padding field comprises any the content and the auth field; then the padding field comprises any
remaining octets of plaintext. The padding field MUST be filled with remaining octets of plaintext. The padding field MUST be filled with
all zero bytes. A receiver MUST verify that there are no non-zero all zero bytes. A receiver MUST verify that there are no non-zero
bytes in the padding field, and if this check fails, the enclosing bytes in the padding field, and if this check fails, the enclosing
PrivateMessage MUST be rejected as malformed. This check ensures PrivateMessage MUST be rejected as malformed. This check ensures
that the padding process is deterministic, so that, for example, that the padding process is deterministic, so that, for example,
padding cannot be used as a covert channel. padding cannot be used as a covert channel.
In the MLS key schedule, the sender creates two distinct key ratchets In the MLS key schedule, the sender creates two distinct key ratchets
for handshake and application messages for each member of the group. for handshake and application messages for each member of the group.
When encrypting a message, the sender looks at the ratchets it When encrypting a message, the sender looks at the ratchets it
derived for its own member and chooses an unused generation from derived for its own member and chooses an unused generation from
either the handshake or application ratchet depending on the content either the handshake ratchet or the application ratchet, depending on
type of the message. This generation of the ratchet is used to the content type of the message. This generation of the ratchet is
derive a provisional nonce and key. used to derive a provisional nonce and key.
Before use in the encryption operation, the nonce is XORed with a Before use in the encryption operation, the nonce is XORed with a
fresh random value to guard against reuse. Because the key schedule fresh random value to guard against reuse. Because the key schedule
generates nonces deterministically, a client MUST keep persistent generates nonces deterministically, a client MUST keep persistent
state as to where in the key schedule it is; if this persistent state state as to where in the key schedule it is; if this persistent state
is lost or corrupted, a client might reuse a generation that has is lost or corrupted, a client might reuse a generation that has
already been used, causing reuse of a key/nonce pair. already been used, causing reuse of a key/nonce pair.
To avoid this situation, the sender of a message MUST generate a To avoid this situation, the sender of a message MUST generate a
fresh random four-byte "reuse guard" value and XOR it with the first fresh random four-byte "reuse guard" value and XOR it with the first
skipping to change at page 50, line 42 skipping to change at line 1818
ContentType content_type; ContentType content_type;
opaque authenticated_data<V>; opaque authenticated_data<V>;
} PrivateContentAAD; } PrivateContentAAD;
When decoding a PrivateMessageContent, the application MUST check When decoding a PrivateMessageContent, the application MUST check
that the FramedContentAuthData is valid. that the FramedContentAuthData is valid.
It is up to the application to decide what authenticated_data to It is up to the application to decide what authenticated_data to
provide and how much padding to add to a given message (if any). The provide and how much padding to add to a given message (if any). The
overall size of the AAD and ciphertext MUST fit within the limits overall size of the AAD and ciphertext MUST fit within the limits
established for the group's AEAD algorithm in [!I-D.irtf-cfrg-aead- established for the group's AEAD algorithm in [CFRG-AEAD-LIMITS].
limits].
6.3.2. Sender Data Encryption 6.3.2. Sender Data Encryption
The "sender data" used to look up the key for content encryption is The "sender data" used to look up the key for content encryption is
encrypted with the ciphersuite's AEAD with a key and nonce derived encrypted with the cipher suite's AEAD with a key and nonce derived
from both the sender_data_secret and a sample of the encrypted from both the sender_data_secret and a sample of the encrypted
content. Before being encrypted, the sender data is encoded as an content. Before being encrypted, the sender data is encoded as an
object of the following form: object of the following form:
struct { struct {
uint32 leaf_index; uint32 leaf_index;
uint32 generation; uint32 generation;
opaque reuse_guard[4]; opaque reuse_guard[4];
} SenderData; } SenderData;
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whole ciphertext is used. In pseudocode, the key and nonce are whole ciphertext is used. In pseudocode, the key and nonce are
derived as: derived as:
ciphertext_sample = ciphertext[0..KDF.Nh-1] ciphertext_sample = ciphertext[0..KDF.Nh-1]
sender_data_key = ExpandWithLabel(sender_data_secret, "key", sender_data_key = ExpandWithLabel(sender_data_secret, "key",
ciphertext_sample, AEAD.Nk) ciphertext_sample, AEAD.Nk)
sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce", sender_data_nonce = ExpandWithLabel(sender_data_secret, "nonce",
ciphertext_sample, AEAD.Nn) ciphertext_sample, AEAD.Nn)
The Additional Authenticated Data (AAD) for the SenderData ciphertext The AAD for the SenderData ciphertext is the first three fields of
is the first three fields of PrivateMessage: PrivateMessage:
struct { struct {
opaque group_id<V>; opaque group_id<V>;
uint64 epoch; uint64 epoch;
ContentType content_type; ContentType content_type;
} SenderDataAAD; } SenderDataAAD;
When parsing a SenderData struct as part of message decryption, the When parsing a SenderData struct as part of message decryption, the
recipient MUST verify that the leaf index indicated in the leaf_index recipient MUST verify that the leaf index indicated in the leaf_index
field identifies a non-blank node. field identifies a non-blank node.
7. Ratchet Tree Operations 7. Ratchet Tree Operations
The ratchet tree for an epoch describes the membership of a group in The ratchet tree for an epoch describes the membership of a group in
that epoch, providing public-key encryption (HPKE) keys that can be that epoch, providing public key encryption (HPKE) keys that can be
used to encrypt to subsets of the group as well as information to used to encrypt to subsets of the group as well as information to
authenticate the members. In order to reflect changes to the authenticate the members. In order to reflect changes to the
membership of the group from one epoch to the next, corresponding membership of the group from one epoch to the next, corresponding
changes are made to the ratchet tree. In this section, we describe changes are made to the ratchet tree. In this section, we describe
the content of the tree and the required operations. the content of the tree and the required operations.
7.1. Parent Node Contents 7.1. Parent Node Contents
As discussed in Section 4.1.1, the nodes of a ratchet tree contain As discussed in Section 4.1.1, the nodes of a ratchet tree contain
several types of data describing individual members (for leaf nodes) several types of data describing individual members (for leaf nodes)
skipping to change at page 52, line 44 skipping to change at line 1916
enum { enum {
reserved(0), reserved(0),
key_package(1), key_package(1),
update(2), update(2),
commit(3), commit(3),
(255) (255)
} LeafNodeSource; } LeafNodeSource;
struct { struct {
ProtocolVersion versions<V>; ProtocolVersion versions<V>;
CipherSuite ciphersuites<V>; CipherSuite cipher_suites<V>;
ExtensionType extensions<V>; ExtensionType extensions<V>;
ProposalType proposals<V>; ProposalType proposals<V>;
CredentialType credentials<V>; CredentialType credentials<V>;
} Capabilities; } Capabilities;
struct { struct {
uint64 not_before; uint64 not_before;
uint64 not_after; uint64 not_after;
} Lifetime; } Lifetime;
// See IANA registry for registered values // See the "MLS Extension Types" IANA registry for values
uint16 ExtensionType; uint16 ExtensionType;
struct { struct {
ExtensionType extension_type; ExtensionType extension_type;
opaque extension_data<V>; opaque extension_data<V>;
} Extension; } Extension;
struct { struct {
HPKEPublicKey encryption_key; HPKEPublicKey encryption_key;
SignaturePublicKey signature_key; SignaturePublicKey signature_key;
skipping to change at page 54, line 33 skipping to change at line 2000
} LeafNodeTBS; } LeafNodeTBS;
The encryption_key field contains an HPKE public key whose private The encryption_key field contains an HPKE public key whose private
key is held only by the member occupying this leaf (or in the case of key is held only by the member occupying this leaf (or in the case of
a LeafNode in a KeyPackage object, the issuer of the KeyPackage). a LeafNode in a KeyPackage object, the issuer of the KeyPackage).
The signature_key field contains the member's public signing key. The signature_key field contains the member's public signing key.
The credential field contains information authenticating both the The credential field contains information authenticating both the
member's identity and the provided signing key, as described in member's identity and the provided signing key, as described in
Section 5.3. Section 5.3.
The capabilities field indicates what protocol versions, The capabilities field indicates the protocol features that the
ciphersuites, extensions, credential types, and non-default proposal client supports, including protocol versions, cipher suites,
types are supported by a client. Proposal and extension types credential types, non-default proposal types, and non-default
defined in this document are considered "default" and thus need not extension types. The following proposal and extension types are
be listed, while any credential types the application wishes to use considered "default" and MUST NOT be listed:
MUST be listed. Extensions that appear in the extensions field of a
LeafNode MUST be included in the extensions field of the capabilities * Proposal types:
field, and the credential type used in the LeafNode MUST be included
in the credentials field of the capabilities field. As discussed in - 0x0001 - add
Section 13, unknown values in capabilities MUST be ignored, and the
creator of a capabilities field SHOULD include some random GREASE - 0x0002 - update
values to help ensure that other clients correctly ignore unknown
values. - 0x0003 - remove
- 0x0004 - psk
- 0x0005 - reinit
- 0x0006 - external_init
- 0x0007 - group_context_extensions
* Extension types:
- 0x0001 - application_id
- 0x0002 - ratchet_tree
- 0x0003 - required_capabilities
- 0x0004 - external_pub
- 0x0005 - external_senders
There are no default values for the other fields of a capabilities
object. The client MUST list all values for the respective
parameters that it supports.
The types of any non-default extensions that appear in the extensions
field of a LeafNode MUST be included in the extensions field of the
capabilities field, and the credential type used in the LeafNode MUST
be included in the credentials field of the capabilities field.
As discussed in Section 13, unknown values in capabilities MUST be
ignored, and the creator of a capabilities field SHOULD include some
random GREASE values to help ensure that other clients correctly
ignore unknown values.
The leaf_node_source field indicates how this LeafNode came to be The leaf_node_source field indicates how this LeafNode came to be
added to the tree. This signal tells other members of the group added to the tree. This signal tells other members of the group
whether the leaf node is required to have a lifetime or parent_hash, whether the leaf node is required to have a lifetime or parent_hash,
and whether the group_id is added as context to the signature. and whether the group_id is added as context to the signature. These
Whether these fields can be computed by the client represented by the fields are included selectively because the client creating a
LeafNode depends on when the LeafNode was created. For example, a LeafNode is not always able to compute all of them. For example, a
KeyPackage is created before the client knows which group it will be KeyPackage is created before the client knows which group it will be
used with, so its signature can't bind to a group_id. used with, so its signature can't bind to a group_id.
In the case where the leaf was added to the tree based on a pre- In the case where the leaf was added to the tree based on a pre-
published KeyPackage, the lifetime field represents the times between published KeyPackage, the lifetime field represents the times between
which clients will consider a LeafNode valid. These times are which clients will consider a LeafNode valid. These times are
represented as absolute times, measured in seconds since the Unix represented as absolute times, measured in seconds since the Unix
epoch (1970-01-01T00:00:00Z). Applications MUST define a maximum epoch (1970-01-01T00:00:00Z). Applications MUST define a maximum
total lifetime that is acceptable for a LeafNode, and reject any total lifetime that is acceptable for a LeafNode, and reject any
LeafNode where the total lifetime is longer than this duration. In LeafNode where the total lifetime is longer than this duration. In
skipping to change at page 55, line 42 skipping to change at line 2085
group is added as context to the signature. group is added as context to the signature.
LeafNode objects stored in the group's ratchet tree are updated LeafNode objects stored in the group's ratchet tree are updated
according to the evolution of the tree. Each modification of according to the evolution of the tree. Each modification of
LeafNode content MUST be reflected by a change in its signature. LeafNode content MUST be reflected by a change in its signature.
This allows other members to verify the validity of the LeafNode at This allows other members to verify the validity of the LeafNode at
any time, particularly in the case of a newcomer joining the group. any time, particularly in the case of a newcomer joining the group.
7.3. Leaf Node Validation 7.3. Leaf Node Validation
The validity of a LeafNode needs to be verified at a few stages: The validity of a LeafNode needs to be verified at the following
stages:
* When a LeafNode is downloaded in a KeyPackage, before it is used * When a LeafNode is downloaded in a KeyPackage, before it is used
to add the client to the group to add the client to the group
* When a LeafNode is received by a group member in an Add, Update, * When a LeafNode is received by a group member in an Add, Update,
or Commit message or Commit message
* When a client validates a ratchet tree, e.g., when joining a group * When a client validates a ratchet tree, e.g., when joining a group
or after processing a commit or after processing a Commit
The client verifies the validity of a LeafNode using the following The client verifies the validity of a LeafNode using the following
steps: steps:
* Verify that the credential in the LeafNode is valid as described * Verify that the credential in the LeafNode is valid, as described
in Section 5.3.1. in Section 5.3.1.
* Verify that the signature on the LeafNode is valid using * Verify that the signature on the LeafNode is valid using
signature_key. signature_key.
* Verify that the LeafNode is compatible with the group's * Verify that the LeafNode is compatible with the group's
parameters. If the GroupContext has a required_capabilities parameters. If the GroupContext has a required_capabilities
extension, then the required extensions, proposals, and credential extension, then the required extensions, proposals, and credential
types MUST be listed in the LeafNode's capabilities field. types MUST be listed in the LeafNode's capabilities field.
* Verify that the credential type is supported by all members of the * Verify that the credential type is supported by all members of the
group, as specified by the capabilities field of each member's group, as specified by the capabilities field of each member's
LeafNode, and that the capabilities field of this LeafNode LeafNode, and that the capabilities field of this LeafNode
indicates support for all the credential types currently in use by indicates support for all the credential types currently in use by
other members. other members.
* Verify the lifetime field: * Verify the lifetime field:
- If the LeafNode appears in a message being sent by the client, - If the LeafNode appears in a message being sent by the client,
e.g., a proposal or a commit, then the client MUST verify that e.g., a Proposal or a Commit, then the client MUST verify that
the current time is within the range of the lifetime field. the current time is within the range of the lifetime field.
- If instead the LeafNode appears in a message being received by - If instead the LeafNode appears in a message being received by
the client, e.g., a proposal, a commit, or a ratchet tree of the client, e.g., a Proposal, a Commit, or a ratchet tree of
the group the client is joining, it is RECOMMENDED that the the group the client is joining, it is RECOMMENDED that the
client verifies that the current time is within the range of client verifies that the current time is within the range of
the lifetime field. (This check is not mandatory because the the lifetime field. (This check is not mandatory because the
LeafNode might have expired in the time between when the LeafNode might have expired in the time between when the
message was sent and when it was received.) message was sent and when it was received.)
* Verify that the extensions in the LeafNode are supported by * Verify that the extensions in the LeafNode are supported by
checking that the ID for each extension in the extensions field is checking that the ID for each extension in the extensions field is
listed in the capabilities.extensions field of the LeafNode. listed in the capabilities.extensions field of the LeafNode.
skipping to change at page 57, line 50 skipping to change at line 2190
path_secret[n] = DeriveSecret(path_secret[n-1], "path") path_secret[n] = DeriveSecret(path_secret[n-1], "path")
* Compute the sequence of HPKE key pairs (node_priv,node_pub), one * Compute the sequence of HPKE key pairs (node_priv,node_pub), one
for each node on the leaf's direct path, as follows. for each node on the leaf's direct path, as follows.
node_secret[n] = DeriveSecret(path_secret[n], "node") node_secret[n] = DeriveSecret(path_secret[n], "node")
node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n]) node_priv[n], node_pub[n] = KEM.DeriveKeyPair(node_secret[n])
The node secret is derived as a temporary intermediate secret so that The node secret is derived as a temporary intermediate secret so that
each secret is only used with one algorithm: The path secret is used each secret is only used with one algorithm: The path secret is used
as an input to DeriveSecret and the node secret is used as an input as an input to DeriveSecret, and the node secret is used as an input
to DeriveKeyPair. to DeriveKeyPair.
For example, suppose there is a group with four members, with C an For example, suppose there is a group with four members, with C an
unmerged leaf at Z: unmerged leaf at Z:
Y Y
| |
.-+-. .-+-.
/ \ / \
X Z[C] X Z[C]
/ \ / \ / \ / \
A B C D A B C D
0 1 2 3 0 1 2 3
Figure 13: A full tree with one unmerged leaf Figure 13: A Full Tree with One Unmerged Leaf
If member B subsequently generates an UpdatePath based on a secret If member B subsequently generates an UpdatePath based on a secret
"leaf_secret", then it would generate the following sequence of path "leaf_secret", then it would generate the following sequence of path
secrets: secrets:
path_secret[1] ---> node_secret[1] -------> node_priv[1], node_pub[1] path_secret[1] ---> node_secret[1] -------> node_priv[1], node_pub[1]
^ ^
| |
| |
skipping to change at page 58, line 40 skipping to change at line 2228
^ ^
| |
| |
leaf_secret ------> leaf_node_secret --+--> leaf_priv, leaf_pub leaf_secret ------> leaf_node_secret --+--> leaf_priv, leaf_pub
| | | |
'-------. .-------' '-------. .-------'
| |
leaf_node leaf_node
Figure 14: Derivation of ratchet tree keys along a direct path Figure 14: Derivation of Ratchet Tree Keys along a Direct Path
After applying the UpdatePath, the tree will have the following After applying the UpdatePath, the tree will have the following
structure: structure:
node_priv[1] --------> Y' node_priv[1] --------> Y'
| |
.-+-. .-+-.
/ \ / \
node_priv[0] ----> X' Z[C] node_priv[0] ----> X' Z[C]
/ \ / \ / \ / \
A B C D A B C D
^ ^
leaf_priv -----------+ leaf_priv -----------+
0 1 2 3 0 1 2 3
Figure 15: Placement of keys in a ratchet tree Figure 15: Placement of Keys in a Ratchet Tree
7.5. Synchronizing Views of the Tree 7.5. Synchronizing Views of the Tree
After generating fresh key material and applying it to ratchet After generating fresh key material and applying it to update their
forward their local tree state as described in the Section 7.4, the local tree state as described in Section 7.4, the generator
generator broadcasts this update to other members of the group in a broadcasts this update to other members of the group in a Commit
Commit message, who apply it to keep their local views of the tree in message, who apply it to keep their local views of the tree in sync
sync with the sender's. More specifically, when a member commits a with the sender's. More specifically, when a member commits a change
change to the tree (e.g., to add or remove a member), it transmits an to the tree (e.g., to add or remove a member), it transmits an
UpdatePath containing a set of public keys and encrypted path secrets UpdatePath containing a set of public keys and encrypted path secrets
for intermediate nodes in the filtered direct path of its leaf. The for intermediate nodes in the filtered direct path of its leaf. The
other members of the group use these values to update their view of other members of the group use these values to update their view of
the tree, aligning their copy of the tree to the sender's. the tree, aligning their copy of the tree to the sender's.
An UpdatePath contains the following information for each node in the An UpdatePath contains the following information for each node in the
filtered direct path of the sender's leaf, including the root: filtered direct path of the sender's leaf, including the root:
* The public key for the node * The public key for the node
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the node the node
The path secret value for a given node is encrypted to the subtree The path secret value for a given node is encrypted to the subtree
rooted at the parent's non-updated child, i.e., the child on the rooted at the parent's non-updated child, i.e., the child on the
copath of the sender's leaf node. There is one encryption of the copath of the sender's leaf node. There is one encryption of the
path secret to each public key in the resolution of the non-updated path secret to each public key in the resolution of the non-updated
child. child.
A member of the group _updates their direct path_ by computing new A member of the group _updates their direct path_ by computing new
values for their leaf node and the nodes along their filtered direct values for their leaf node and the nodes along their filtered direct
path: path as follows:
1. Blank all nodes along the direct path of the sender's leaf. 1. Blank all nodes along the direct path of the sender's leaf.
2. Compute updated path secrets and public keys for the nodes on the 2. Compute updated path secrets and public keys for the nodes on the
sender's filtered direct path. sender's filtered direct path.
* Generate a sequence of path secrets of the same length as the * Generate a sequence of path secrets of the same length as the
filtered direct path, as defined in Section 7.4 filtered direct path, as defined in Section 7.4.
* For each node in the filtered direct path, replace the node's * For each node in the filtered direct path, replace the node's
public key with the node_pub[n] value derived from the public key with the node_pub[n] value derived from the
corresponding path secret path_secret[n]. corresponding path secret path_secret[n].
3. Compute the new parent hashes for the nodes along the filtered 3. Compute the new parent hashes for the nodes along the filtered
direct path and the sender's leaf node. direct path and the sender's leaf node.
4. Update the leaf node for the sender. 4. Update the leaf node for the sender.
* Set the leaf_node_source to commit. * Set the leaf_node_source to commit.
* Set the encryption_key to the public key of a freshly sampled * Set the encryption_key to the public key of a freshly sampled
key pair key pair.
* Set the parent hash to the parent hash for the leaf. * Set the parent hash to the parent hash for the leaf.
* Re-sign the leaf node with its new contents * Re-sign the leaf node with its new contents.
Since the new leaf node effectively updates an existing leaf node in Since the new leaf node effectively updates an existing leaf node in
the group, it MUST adhere to the same restrictions as LeafNodes used the group, it MUST adhere to the same restrictions as LeafNodes used
in Update proposals (aside from leaf_node_source). The application in Update proposals (aside from leaf_node_source). The application
MAY specify other changes to the leaf node, e.g., providing a new MAY specify other changes to the leaf node, e.g., providing a new
signature key, updated capabilities, or different extensions. signature key, updated capabilities, or different extensions.
The member then _encrypts path secrets to the group_. For each node The member then _encrypts path secrets to the group_. For each node
in the member's filtered direct path, the member takes the following in the member's filtered direct path, the member takes the following
steps: steps:
1. Compute the resolution of the node's child that is on the copath 1. Compute the resolution of the node's child that is on the copath
of the sender (the child that is not in the direct path of the of the sender (the child that is not in the direct path of the
sender). Any new member (from an Add proposal) added in the same sender). Any new member (from an Add proposal) added in the same
Commit MUST be excluded from this resolution. Commit MUST be excluded from this resolution.
2. For each node in the resolution, encrypt the path secret for the 2. For each node in the resolution, encrypt the path secret for the
direct path node using the public key of the resolution node, as direct path node using the public key of the resolution node, as
defined in Section 7.6 defined in Section 7.6.
The recipient of an UpdatePath performs the corresponding steps. The recipient of an UpdatePath performs the corresponding steps.
First, the recipient _merges UpdatePath into the tree_: First, the recipient _merges UpdatePath into the tree_:
1. Blank all nodes on the direct path of the sender's leaf. 1. Blank all nodes on the direct path of the sender's leaf.
2. For all nodes on the filtered direct path of the sender's leaf, 2. For all nodes on the filtered direct path of the sender's leaf,
* Set the public key to the public key in the UpdatePath. * Set the public key to the public key in the UpdatePath.
skipping to change at page 62, line 7 skipping to change at line 2380
+=============+====================================================+ +=============+====================================================+
| node_pub[1] | E(pk(Z), path_secret[1]), E(pk(C), path_secret[1]) | | node_pub[1] | E(pk(Z), path_secret[1]), E(pk(C), path_secret[1]) |
+-------------+----------------------------------------------------+ +-------------+----------------------------------------------------+
| node_pub[0] | E(pk(A), path_secret[0]) | | node_pub[0] | E(pk(A), path_secret[0]) |
+-------------+----------------------------------------------------+ +-------------+----------------------------------------------------+
Table 3 Table 3
In this table, the value node_pub[i] represents the public key In this table, the value node_pub[i] represents the public key
derived from node_secret[i], pk(X) represents the current public key derived from node_secret[i], pk(X) represents the current public key
of node X, and E(K, S) represents the public-key encryption of the of node X, and E(K, S) represents the public key encryption of the
path secret S to the public key K (using HPKE). path secret S to the public key K (using HPKE).
A recipient at node A would decrypt E(pk(A), path_secret\[0\]) to A recipient at node A would decrypt E(pk(A), path_secret\[0\]) to
obtain path_secret\[0\], then use it to derive path_secret[1] and the obtain path_secret\[0\], then use it to derive path_secret[1] and the
resulting node secrets and key pairs. Thus, A would have the private resulting node secrets and key pairs. Thus, A would have the private
keys to nodes X' and Y', in accordance with the tree invariant. keys to nodes X' and Y', in accordance with the tree invariant.
Similarly, a recipient at node D would decrypt E(pk(Z), Similarly, a recipient at node D would decrypt E(pk(Z),
path_secret[1]) to obtain path_secret[1], then use it to derive the path_secret[1]) to obtain path_secret[1], then use it to derive the
node secret and key pair for the node Y'. As required to maintain node secret and key pair for the node Y'. As required to maintain
skipping to change at page 63, line 30 skipping to change at line 2451
Here node_public_key is the public key of the node for which the path Here node_public_key is the public key of the node for which the path
secret is encrypted, group_context is the provisional GroupContext secret is encrypted, group_context is the provisional GroupContext
object for the group, and the EncryptWithLabel function is as defined object for the group, and the EncryptWithLabel function is as defined
in Section 5.1.3. in Section 5.1.3.
7.7. Adding and Removing Leaves 7.7. Adding and Removing Leaves
In addition to the path-based updates to the tree described above, it In addition to the path-based updates to the tree described above, it
is also necessary to add and remove leaves of the tree in order to is also necessary to add and remove leaves of the tree in order to
reflect changes to the membership of the group (see Section 12.1.1 reflect changes to the membership of the group (see Sections 12.1.1
and Section 12.1.3). Since the tree is always full, adding or and 12.1.3). Since the tree is always full, adding or removing
removing leaves corresponds to increasing or decreasing the depth of leaves corresponds to increasing or decreasing the depth of the tree,
the tree, resulting in the number of leaves being doubled or halved. resulting in the number of leaves being doubled or halved. These
These operations are also known as _extending_ and _truncating_ the operations are also known as _extending_ and _truncating_ the tree.
tree.
Leaves are always added and removed at the right edge of the tree. Leaves are always added and removed at the right edge of the tree.
When the size of the tree needs to be increased, a new blank root When the size of the tree needs to be increased, a new blank root
node is added, whose left subtree is the existing tree and right node is added, whose left subtree is the existing tree and right
subtree is a new all-blank subtree. This operation is typically done subtree is a new all-blank subtree. This operation is typically done
when adding a member to the group. when adding a member to the group.
_ <-- new blank root _ _ <-- new blank root _
__|__ __|__ __|__ __|__
/ \ / \ / \ / \
X ===> X _ <-- new blank subtree ===> X _ X ===> X _ <-- new blank subtree ===> X _
/ \ / \ / \ / \ / \ / \ / \ / \ / \ / \
A B A B _ _ A B C _ A B A B _ _ A B C _
^ ^
| |
+-- new member new member --+
Figure 16: Extending the tree to make room for a third member Figure 16: Extending the Tree to Make Room for a Third Member
When the right subtree of the tree no longer has any non-blank nodes, When the right subtree of the tree no longer has any non-blank nodes,
it can be safely removed. The root of the tree and the right subtree it can be safely removed. The root of the tree and the right subtree
are discarded (whether or not the root node is blank). The left are discarded (whether or not the root node is blank). The left
child of the root becomes the new root node, and the left subtree child of the root becomes the new root node, and the left subtree
becomes the new tree. This operation is typically done after becomes the new tree. This operation is typically done after
removing a member from the group. removing a member from the group.
Y Y Y Y
__|__ __|__ __|__ __|__
/ \ / \ / \ / \
X _ ===> X _ ==> X <-- new root X _ ===> X _ ==> X <-- new root
/ \ / \ / \ / \ / \ / \ / \ / \ / \ / \
A B C _ A B _ _ A B A B C _ A B _ _ A B
^ ^
| |
removed member --+ removed member --+
Figure 17: Cleaning up after removing member C Figure 17: Cleaning Up after Removing Member C
Concrete algorithms for these operations on array-based and link- Concrete algorithms for these operations on array-based and link-
based trees are provided in Appendix C and Appendix D. The concrete based trees are provided in Appendices C and D. The concrete
algorithms are non-normative. An implementation may use any algorithms are non-normative. An implementation may use any
algorithm that produces the correct tree in its internal algorithm that produces the correct tree in its internal
representation. representation.
7.8. Tree Hashes 7.8. Tree Hashes
MLS hashes the contents of the tree in two ways to authenticate MLS hashes the contents of the tree in two ways to authenticate
different properties of the tree. _Tree hashes_ are defined in this different properties of the tree. _Tree hashes_ are defined in this
section, and _parent hashes_ are defined in Section 7.9. section, and _parent hashes_ are defined in Section 7.9.
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subtree below that node. The tree hash of the root is used in the subtree below that node. The tree hash of the root is used in the
GroupContext to confirm that the group agrees on the whole tree. GroupContext to confirm that the group agrees on the whole tree.
Tree hashes are computed recursively from the leaves up to the root. Tree hashes are computed recursively from the leaves up to the root.
P --> th(P) P --> th(P)
^ ^ ^ ^
/ \ / \
/ \ / \
th(L) th(R) th(L) th(R)
Figure 18: Composition of the tree hash Figure 18: Composition of the Tree Hash
The tree hash of an individual node is the hash of the node's The tree hash of an individual node is the hash of the node's
TreeHashInput object, which may contain either a LeafNodeHashInput or TreeHashInput object, which may contain either a LeafNodeHashInput or
a ParentNodeHashInput depending on the type of node. a ParentNodeHashInput depending on the type of node.
LeafNodeHashInput objects contain the leaf_index and the LeafNode (if LeafNodeHashInput objects contain the leaf_index and the LeafNode (if
any). ParentNodeHashInput objects contain the ParentNode (if any) any). ParentNodeHashInput objects contain the ParentNode (if any)
and the tree hash of the node's left and right children. For both and the tree hash of the node's left and right children. For both
parent and leaf nodes, the optional node value MUST be absent if the parent and leaf nodes, the optional node value MUST be absent if the
node is blank and present if the node contains a value. node is blank and present if the node contains a value.
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The tree hash of an entire tree corresponds to the tree hash of the The tree hash of an entire tree corresponds to the tree hash of the
root node, which is computed recursively by starting at the leaf root node, which is computed recursively by starting at the leaf
nodes and building up. nodes and building up.
7.9. Parent Hashes 7.9. Parent Hashes
While tree hashes summarize the state of a tree at point in time, While tree hashes summarize the state of a tree at point in time,
parent hashes capture information about how keys in the tree were parent hashes capture information about how keys in the tree were
populated. populated.
When a client sends a commit to change a group, it can include an When a client sends a Commit to change a group, it can include an
UpdatePath to assign new keys to the nodes along its filtered direct UpdatePath to assign new keys to the nodes along its filtered direct
path. When a client computes an UpdatePath (as defined in path. When a client computes an UpdatePath (as defined in
Section 7.5), it computes and signs a parent hash that summarizes the Section 7.5), it computes and signs a parent hash that summarizes the
state of the tree after the UpdatePath has been applied. These state of the tree after the UpdatePath has been applied. These
summaries are constructed in a chain from the root to the member's summaries are constructed in a chain from the root to the member's
leaf so that the part of the chain closer to the root can be leaf so that the part of the chain closer to the root can be
overwritten as nodes set in one UpdatePath are reset by a later overwritten as nodes set in one UpdatePath are reset by a later
UpdatePath. UpdatePath.
ph(Q) ph(Q)
/ /
/ /
V V
P.public_key --> ph(P) P.public_key --> ph(P)
/ ^ / ^
/ \ / \
V \ V \
N.parent_hash th(S) N.parent_hash th(S)
Figure 19: Inputs to a parent hash Figure 19: Inputs to a Parent Hash
As a result, the signature over the parent hash in each member's leaf As a result, the signature over the parent hash in each member's leaf
effectively signs the subtree of the tree that hasn't been changed effectively signs the subtree of the tree that hasn't been changed
since that leaf was last changed in an UpdatePath. A new member since that leaf was last changed in an UpdatePath. A new member
joining the group uses these parent hashes to verify that the parent joining the group uses these parent hashes to verify that the parent
nodes in the tree were set by members of the group, not chosen by an nodes in the tree were set by members of the group, not chosen by an
external attacker. For an example of how this works, see Appendix B. external attacker. For an example of how this works, see Appendix B.
Consider a ratchet tree with a non-blank parent node P and children D Consider a ratchet tree with a non-blank parent node P and children D
and S (for "parent", "direct path", and "sibling"), with D and P in and S (for "parent", "direct path", and "sibling"), with D and P in
skipping to change at page 66, line 45 skipping to change at line 2608
/ /
P P
__|__ __|__
/ \ / \
D S D S
/ \ / \ / \ / \
... ... ... ... ... ... ... ...
/ /
L L
Figure 20: Nodes involved in a parent hash computation Figure 20: Nodes Involved in a Parent Hash Computation
The parent hash of P changes whenever an UpdatePath object is applied The parent hash of P changes whenever an UpdatePath object is applied
to the ratchet tree along a path from a leaf L traversing node D (and to the ratchet tree along a path from a leaf L traversing node D (and
hence also P). The new "Parent hash of P (with copath child S)" is hence also P). The new "Parent hash of P (with copath child S)" is
obtained by hashing P's ParentHashInput struct. obtained by hashing P's ParentHashInput struct.
struct { struct {
HPKEPublicKey encryption_key; HPKEPublicKey encryption_key;
opaque parent_hash<V>; opaque parent_hash<V>;
opaque original_sibling_tree_hash<V>; opaque original_sibling_tree_hash<V>;
} ParentHashInput; } ParentHashInput;
The field encryption_key contains the HPKE public key of P. If P is The field encryption_key contains the HPKE public key of P. If P is
the root, then the parent_hash field is set to a zero-length octet the root, then the parent_hash field is set to a zero-length octet
string. Otherwise, parent_hash is the Parent Hash of the next node string. Otherwise, parent_hash is the parent hash of the next node
after P on the filtered direct path of the leaf L. This way, P's after P on the filtered direct path of the leaf L. This way, P's
Parent Hash fixes the new HPKE public key of each non-blank node on parent hash fixes the new HPKE public key of each non-blank node on
the path from P to the root. Note that the path from P to the root the path from P to the root. Note that the path from P to the root
may contain some blank nodes that are not fixed by P's Parent Hash. may contain some blank nodes that are not fixed by P's parent hash.
However, for each node that has an HPKE key, this key is fixed by P's However, for each node that has an HPKE key, this key is fixed by P's
Parent Hash. parent hash.
Finally, original_sibling_tree_hash is the tree hash of S in the Finally, original_sibling_tree_hash is the tree hash of S in the
ratchet tree modified as follows: For each leaf L in ratchet tree modified as follows: For each leaf L in
P.unmerged_leaves, blank L and remove it from the unmerged_leaves P.unmerged_leaves, blank L and remove it from the unmerged_leaves
sets of all parent nodes. sets of all parent nodes.
Observe that original_sibling_tree_hash does not change between Observe that original_sibling_tree_hash does not change between
updates of P. This property is crucial for the correctness of the updates of P. This property is crucial for the correctness of the
protocol. protocol.
skipping to change at page 67, line 49 skipping to change at line 2659
W [F] W [F]
______|_____ ______|_____
/ \ / \
U Y [F] U Y [F]
__|__ __|__ __|__ __|__
/ \ / \ / \ / \
T _ _ _ T _ _ _
/ \ / \ / \ / \ / \ / \ / \ / \
A B C D E F G _ A B C D E F G _
Figure 21: A tree illustrating parent hash computations. Figure 21: A Tree Illustrating Parent Hash Computations
With P = W and S = Y, original_sibling_tree_hash is the tree hash of With P = W and S = Y, original_sibling_tree_hash is the tree hash of
the following tree: the following tree:
Y Y
__|__ __|__
/ \ / \
_ _ _ _
/ \ / \ / \ / \
E _ G _ E _ G _
Because W.unmerged_leaves includes F, F is blanked and removed from Because W.unmerged_leaves includes F, F is blanked and removed from
Y.unmerged_leaves. Y.unmerged_leaves.
Note that no recomputation is needed if the tree hash of S is Note that no recomputation is needed if the tree hash of S is
unchanged since the last time P was updated. This is the case for unchanged since the last time P was updated. This is the case for
computing or processing a Commit whose UpdatePath traverses P, since computing or processing a Commit whose UpdatePath traverses P, since
the Commit itself resets P. (In other words, it is only necessary to the Commit itself resets P. (In other words, it is only necessary to
recompute the original sibling tree hash when validating a group's recompute the original sibling tree hash when validating a group's
tree on joining.) More generally, if none of the entries in tree on joining.) More generally, if none of the entries in
P.unmerged_leaves is in the subtree under S (and thus no leaves were P.unmerged_leaves are in the subtree under S (and thus no leaves were
blanked), then the original tree hash at S is the tree hash of S in blanked), then the original tree hash at S is the tree hash of S in
the current tree. the current tree.
If it is necessary to recompute the original tree hash of a node, the If it is necessary to recompute the original tree hash of a node, the
efficiency of recomputation can be improved by caching intermediate efficiency of recomputation can be improved by caching intermediate
tree hashes, to avoid recomputing over the subtree when the subtree tree hashes, to avoid recomputing over the subtree when the subtree
is included in multiple parent hashes. A subtree hash can be reused is included in multiple parent hashes. A subtree hash can be reused
as long as the intersection of the parent's unmerged leaves with the as long as the intersection of the parent's unmerged leaves with the
subtree is the same as in the earlier computation. subtree is the same as in the earlier computation.
skipping to change at page 68, line 43 skipping to change at line 2702
In ParentNode objects and LeafNode objects with leaf_node_source set In ParentNode objects and LeafNode objects with leaf_node_source set
to commit, the value of the parent_hash field is the parent hash of to commit, the value of the parent_hash field is the parent hash of
the next non-blank parent node above the node in question (the next the next non-blank parent node above the node in question (the next
node in the filtered direct path). Using the node labels in node in the filtered direct path). Using the node labels in
Figure 20, the parent_hash field of D is equal to the parent hash of Figure 20, the parent_hash field of D is equal to the parent hash of
P with copath child S. This is the case even when the node D is a P with copath child S. This is the case even when the node D is a
leaf node. leaf node.
The parent_hash field of a LeafNode is signed by the member. The The parent_hash field of a LeafNode is signed by the member. The
signature of such a LeafNode thus also attests to which keys the signature of such a LeafNode thus attests to which keys the group
group member introduced into the ratchet tree and to whom the member introduced into the ratchet tree and to whom the corresponding
corresponding secret keys were sent. This prevents malicious secret keys were sent, in addition to the other contents of the
insiders from constructing artificial ratchet trees with a node D LeafNode. This prevents malicious insiders from constructing
whose HPKE secret key is known to the insider yet where the insider artificial ratchet trees with a node D whose HPKE secret key is known
isn't assigned a leaf in the subtree rooted at D. Indeed, such a to the insider, yet where the insider isn't assigned a leaf in the
ratchet tree would violate the tree invariant. subtree rooted at D. Indeed, such a ratchet tree would violate the
tree invariant.
7.9.2. Verifying Parent Hashes 7.9.2. Verifying Parent Hashes
Parent hashes are verified at two points in the protocol: When Parent hashes are verified at two points in the protocol: When
joining a group and when processing a Commit. joining a group and when processing a Commit.
The parent hash in a node D is valid with respect to a parent node P The parent hash in a node D is valid with respect to a parent node P
if the following criteria hold. Here C and S are the children of P if the following criteria hold. Here C and S are the children of P
(for "child" and "sibling"), with C being the child that is on the (for "child" and "sibling"), with C being the child that is on the
direct path of D (possibly D itself) and S the other child: direct path of D (possibly D itself) and S being the other child:
* D is a descendant of P in the tree. * D is a descendant of P in the tree.
* The parent_hash field of D is equal to the parent hash of P with * The parent_hash field of D is equal to the parent hash of P with
copath child S. copath child S.
* D is in the resolution of C, and the intersection of P's * D is in the resolution of C, and the intersection of P's
unmerged_leaves with the subtree under C is equal to the unmerged_leaves with the subtree under C is equal to the
resolution of C with D removed. resolution of C with D removed.
These checks verify that D and P were updated at the same time (in These checks verify that D and P were updated at the same time (in
the same UpdatePath), and that they were neighbors in the UpdatePath the same UpdatePath), and that they were neighbors in the UpdatePath
because the nodes in between them would have omitted from the because the nodes in between them would have omitted from the
filtered direct path. filtered direct path.
A parent node P is "parent-hash valid" if it can be chained back to a A parent node P is "parent-hash valid" if it can be chained back to a
leaf node in this way. That is, if there is leaf node L and a leaf node in this way. That is, if there is leaf node L and a
sequence of parent nodes P_1, ..., P_N such that P_N = P and each sequence of parent nodes P_1, ..., P_N such that P_N = P and each
step in the chain is authenticated by a parent hash: L's parent hash step in the chain is authenticated by a parent hash, then L's parent
is valid with respect to P_1, P_1's parent hash is valid with respect hash is valid with respect to P_1, P_1's parent hash is valid with
to P_2, and so on. respect to P_2, and so on.
When joining a group, the new member MUST authenticate that each non- When joining a group, the new member MUST authenticate that each non-
blank parent node P is parent-hash valid. This can be done "bottom blank parent node P is parent-hash valid. This can be done "bottom
up" by building chains up from leaves and verifying that all non- up" by building chains up from leaves and verifying that all non-
blank parent nodes are covered by exactly one such chain, or "top blank parent nodes are covered by exactly one such chain, or "top
down" by verifying that there is exactly one descendant of each non- down" by verifying that there is exactly one descendant of each non-
blank parent node for which the parent node is parent-hash valid. blank parent node for which the parent node is parent-hash valid.
When processing a Commit message that includes an UpdatePath, clients When processing a Commit message that includes an UpdatePath, clients
MUST recompute the expected value of parent_hash for the committer's MUST recompute the expected value of parent_hash for the committer's
new leaf and verify that it matches the parent_hash value in the new leaf and verify that it matches the parent_hash value in the
supplied leaf_node. After being merged into the tree, the nodes in supplied leaf_node. After being merged into the tree, the nodes in
the UpdatePath form a parent-hash chain from the committer's leaf to the UpdatePath form a parent-hash chain from the committer's leaf to
the root. the root.
8. Key Schedule 8. Key Schedule
Group keys are derived using the Extract and Expand functions from Group keys are derived using the Extract and Expand functions from
the KDF for the group's ciphersuite, as well as the functions defined the KDF for the group's cipher suite, as well as the functions
below: defined below:
ExpandWithLabel(Secret, Label, Context, Length) = ExpandWithLabel(Secret, Label, Context, Length) =
KDF.Expand(Secret, KDFLabel, Length) KDF.Expand(Secret, KDFLabel, Length)
DeriveSecret(Secret, Label) = DeriveSecret(Secret, Label) =
ExpandWithLabel(Secret, Label, "", KDF.Nh) ExpandWithLabel(Secret, Label, "", KDF.Nh)
Where KDFLabel is specified as: Where KDFLabel is specified as:
struct { struct {
uint16 length; uint16 length;
opaque label<V>; opaque label<V>;
opaque context<V>; opaque context<V>;
} KDFLabel; } KDFLabel;
And its fields set to: And its fields are set to:
length = Length; length = Length;
label = "MLS 1.0 " + Label; label = "MLS 1.0 " + Label;
context = Context; context = Context;
The value KDF.Nh is the size of an output from KDF.Extract, in bytes. The value KDF.Nh is the size of an output from KDF.Extract, in bytes.
In the below diagram: In the below diagram:
* KDF.Extract takes its salt argument from the top and its Input Key * KDF.Extract takes its salt argument from the top and its Input
Material (IKM) argument from the left Keying Material (IKM) argument from the left.
* DeriveSecret takes its Secret argument from the incoming arrow * DeriveSecret takes its Secret argument from the incoming arrow.
* 0 represents an all-zero byte string of length KDF.Nh. * 0 represents an all-zero byte string of length KDF.Nh.
When processing a handshake message, a client combines the following When processing a handshake message, a client combines the following
information to derive new epoch secrets: information to derive new epoch secrets:
* The init secret from the previous epoch * The init secret from the previous epoch
* The commit secret for the current epoch * The commit secret for the current epoch
skipping to change at page 71, line 45 skipping to change at line 2844
+--> DeriveSecret(., <label>) +--> DeriveSecret(., <label>)
| = <secret> | = <secret>
| |
V V
DeriveSecret(., "init") DeriveSecret(., "init")
| |
| |
V V
init_secret_[n] init_secret_[n]
Figure 22: The MLS key schedule Figure 22: The MLS Key Schedule
A number of values are derived from the epoch secret for different A number of values are derived from the epoch secret for different
purposes: purposes:
+==================+=====================+=======================+ +==================+=====================+=======================+
| Label | Secret | Purpose | | Label | Secret | Purpose |
+==================+=====================+=======================+ +==================+=====================+=======================+
| "sender data" | sender_data_secret | Deriving keys to | | "sender data" | sender_data_secret | Deriving keys to |
| | | encrypt sender data | | | | encrypt sender data |
+------------------+---------------------+-----------------------+ +------------------+---------------------+-----------------------+
skipping to change at page 72, line 26 skipping to change at line 2870
| | | secrets | | | | secrets |
+------------------+---------------------+-----------------------+ +------------------+---------------------+-----------------------+
| "external" | external_secret | Deriving the external | | "external" | external_secret | Deriving the external |
| | | init key | | | | init key |
+------------------+---------------------+-----------------------+ +------------------+---------------------+-----------------------+
| "confirm" | confirmation_key | Computing the | | "confirm" | confirmation_key | Computing the |
| | | confirmation MAC for | | | | confirmation MAC for |
| | | an epoch | | | | an epoch |
+------------------+---------------------+-----------------------+ +------------------+---------------------+-----------------------+
| "membership" | membership_key | Computing the | | "membership" | membership_key | Computing the |
| | | membership MAC for an | | | | membership MAC for a |
| | | PublicMessage | | | | PublicMessage |
+------------------+---------------------+-----------------------+ +------------------+---------------------+-----------------------+
| "resumption" | resumption_psk | Proving membership in | | "resumption" | resumption_psk | Proving membership in |
| | | this epoch (via a PSK | | | | this epoch (via a PSK |
| | | injected later) | | | | injected later) |
+------------------+---------------------+-----------------------+ +------------------+---------------------+-----------------------+
| "authentication" | epoch_authenticator | Confirming that two | | "authentication" | epoch_authenticator | Confirming that two |
| | | clients have the same | | | | clients have the same |
| | | view of the group | | | | view of the group |
+------------------+---------------------+-----------------------+ +------------------+---------------------+-----------------------+
Table 4: Epoch-derived secrets Table 4: Epoch-Derived Secrets
The external_secret is used to derive an HPKE key pair whose private The external_secret is used to derive an HPKE key pair whose private
key is held by the entire group: key is held by the entire group:
external_priv, external_pub = KEM.DeriveKeyPair(external_secret) external_priv, external_pub = KEM.DeriveKeyPair(external_secret)
The public key external_pub can be published as part of the GroupInfo The public key external_pub can be published as part of the GroupInfo
struct in order to allow non-members to join the group using an struct in order to allow non-members to join the group using an
external commit. external Commit.
8.1. Group Context 8.1. Group Context
Each member of the group maintains a GroupContext object that Each member of the group maintains a GroupContext object that
summarizes the state of the group: summarizes the state of the group:
struct { struct {
ProtocolVersion version = mls10; ProtocolVersion version = mls10;
CipherSuite cipher_suite; CipherSuite cipher_suite;
opaque group_id<V>; opaque group_id<V>;
skipping to change at page 74, line 6 skipping to change at line 2945
* The group_id field is constant. * The group_id field is constant.
* The epoch field increments by one for each Commit message that is * The epoch field increments by one for each Commit message that is
processed. processed.
* The tree_hash is updated to represent the current tree and * The tree_hash is updated to represent the current tree and
credentials. credentials.
* The confirmed_transcript_hash field is updated with the data for * The confirmed_transcript_hash field is updated with the data for
an AuthenticatedContent encoding a Commit message as described an AuthenticatedContent encoding a Commit message, as described
below. below.
* The extensions field changes when a GroupContextExtensions * The extensions field changes when a GroupContextExtensions
proposal is committed. proposal is committed.
8.2. Transcript Hashes 8.2. Transcript Hashes
The transcript hashes computed in MLS represent a running hash over The transcript hashes computed in MLS represent a running hash over
all Proposal and Commit messages that have ever been sent in a group. all Proposal and Commit messages that have ever been sent in a group.
Commit messages are included directly. Proposal messages are Commit messages are included directly. Proposal messages are
indirectly included via the Commit that applied them. Both types of indirectly included via the Commit that applied them. Messages of
message are included by hashing the AuthenticatedContent object in both types are included by hashing the AuthenticatedContent object in
which they were sent. which they were sent.
The transcript hash comprises two individual hashes: The transcript hash comprises two individual hashes:
* A confirmed_transcript_hash that represents a transcript over the * A confirmed_transcript_hash that represents a transcript over the
whole history of Commit messages, up to and including the whole history of Commit messages, up to and including the
signature of the most recent Commit. signature of the most recent Commit.
* An interim_transcript_hash that covers the confirmed transcript * An interim_transcript_hash that covers the confirmed transcript
hash plus the confirmation_tag of the most recent Commit. hash plus the confirmation_tag of the most recent Commit.
New members compute the interim transcript hash using the New members compute the interim transcript hash using the
confirmation_tag field of the GroupInfo struct, while existing confirmation_tag field of the GroupInfo struct, while existing
members can compute it directly. members can compute it directly.
Each Commit message updates these hashes by way of its enclosing Each Commit message updates these hashes by way of its enclosing
AuthenticatedContent. The AuthenticatedContent struct is split into AuthenticatedContent. The AuthenticatedContent struct is split into
ConfirmedTranscriptHashInput and InterimTranscriptHashInput. The ConfirmedTranscriptHashInput and InterimTranscriptHashInput. The
former is used to update the confirmed transcript hash and the latter former is used to update the confirmed transcript hash and the latter
to update the interim transcript hash. is used to update the interim transcript hash.
struct { struct {
WireFormat wire_format; WireFormat wire_format;
FramedContent content; /* with content_type == commit */ FramedContent content; /* with content_type == commit */
opaque signature<V>; opaque signature<V>;
} ConfirmedTranscriptHashInput; } ConfirmedTranscriptHashInput;
struct { struct {
MAC confirmation_tag; MAC confirmation_tag;
} InterimTranscriptHashInput; } InterimTranscriptHashInput;
skipping to change at page 76, line 48 skipping to change at line 3055
| | | |
| V | V
| +-----------------+ | +-----------------+
+------------>| interim_[N+1] | +------------>| interim_[N+1] |
+--------+--------+ +--------+--------+
| |
V V
... ...
Figure 23: Evolution of the transcript hashes through two epoch Figure 23: Evolution of the Transcript Hashes through Two Epoch
changes Changes
8.3. External Initialization 8.3. External Initialization
In addition to initializing a new epoch via KDF invocations as In addition to initializing a new epoch via KDF invocations as
described above, an MLS group can also initialize a new epoch via an described above, an MLS group can also initialize a new epoch via an
asymmetric interaction using the external key pair for the previous asymmetric interaction using the external key pair for the previous
epoch. This is done when a new member is joining via an external epoch. This is done when a new member is joining via an external
commit. commit.
In this process, the joiner sends a new init_secret value to the In this process, the joiner sends a new init_secret value to the
skipping to change at page 77, line 29 skipping to change at line 3082
kem_output, context = SetupBaseS(external_pub, "") kem_output, context = SetupBaseS(external_pub, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh) init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
Members of the group receive the kem_output in an ExternalInit Members of the group receive the kem_output in an ExternalInit
proposal and perform the corresponding calculation to retrieve the proposal and perform the corresponding calculation to retrieve the
init_secret value. init_secret value.
context = SetupBaseR(kem_output, external_priv, "") context = SetupBaseR(kem_output, external_priv, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh) init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
In both cases, the info input to HPKE is set to the GroupInfo for the
previous epoch, encoded using the TLS serialization.
8.4. Pre-Shared Keys 8.4. Pre-Shared Keys
Groups that already have an out-of-band mechanism to generate shared Groups that already have an out-of-band mechanism to generate shared
group secrets can inject those into the MLS key schedule to seed the group secrets can inject them into the MLS key schedule to
MLS group secrets computations by this external entropy. incorporate this external entropy in the computation of MLS group
secrets.
Injecting an external PSK can improve security in the case where Injecting an external PSK can improve security in the case where
having a full run of Updates across members is too expensive, or if having a full run of Updates across members is too expensive, or if
the external group key establishment mechanism provides stronger the external group key establishment mechanism provides stronger
security against classical or quantum adversaries. security against classical or quantum adversaries.
Note that, as a PSK may have a different lifetime than an Update, it Note that, as a PSK may have a different lifetime than an Update, it
does not necessarily provide the same Forward Secrecy (FS) or Post- does not necessarily provide the same forward secrecy or post-
Compromise Security (PCS) guarantees as a Commit message. Unlike the compromise security guarantees as a Commit message. Unlike the key
key pairs populated in the tree by an Update or Commit, which are pairs populated in the tree by an Update or Commit, which are always
always freshly generated, PSKs may be pre-distributed and stored. freshly generated, PSKs may be pre-distributed and stored. This
This creates the risk that a PSK may be compromised in the process of creates the risk that a PSK may be compromised in the process of
distribution and storage. The security that the group gets from distribution and storage. The security that the group gets from
injecting a PSK thus depends on both the entropy of the PSK and the injecting a PSK thus depends on both the entropy of the PSK and the
risk of compromise. These factors are outside of the scope of this risk of compromise. These factors are outside of the scope of this
document, but should be considered by application designers relying document, but they should be considered by application designers
on PSKs. relying on PSKs.
Each PSK in MLS has a type that designates how it was provisioned. Each PSK in MLS has a type that designates how it was provisioned.
External PSKs are provided by the application, while resumption PSKs External PSKs are provided by the application, while resumption PSKs
are derived from the MLS key schedule and used in cases where it is are derived from the MLS key schedule and used in cases where it is
necessary to authenticate a member's participation in a prior epoch. necessary to authenticate a member's participation in a prior epoch.
The injection of one or more PSKs into the key schedule is signaled The injection of one or more PSKs into the key schedule is signaled
in two ways: Existing members are informed via PreSharedKey proposals in two ways: Existing members are informed via PreSharedKey proposals
covered by a Commit, and new members added in the Commit are informed covered by a Commit, and new members added in the Commit are informed
by the GroupSecrets object in the Welcome message corresponding to by the GroupSecrets object in the Welcome message corresponding to
skipping to change at page 79, line 36 skipping to change at line 3150
case resumption: case resumption:
ResumptionPSKUsage usage; ResumptionPSKUsage usage;
opaque psk_group_id<V>; opaque psk_group_id<V>;
uint64 psk_epoch; uint64 psk_epoch;
}; };
opaque psk_nonce<V>; opaque psk_nonce<V>;
} PreSharedKeyID; } PreSharedKeyID;
Each time a client injects a PSK into a group, the psk_nonce of its Each time a client injects a PSK into a group, the psk_nonce of its
PreSharedKeyID MUST be set to a fresh random value of length KDF.Nh, PreSharedKeyID MUST be set to a fresh random value of length KDF.Nh,
where KDF is the KDF for the ciphersuite of the group into which the where KDF is the KDF for the cipher suite of the group into which the
PSK is being injected. This ensures that even when a PSK is used PSK is being injected. This ensures that even when a PSK is used
multiple times, the value used as an input into the key schedule is multiple times, the value used as an input into the key schedule is
different each time. different each time.
Upon receiving a Commit with a PreSharedKey proposal or a Upon receiving a Commit with a PreSharedKey proposal or a
GroupSecrets object with the psks field set, the receiving Client GroupSecrets object with the psks field set, the receiving client
includes them in the key schedule in the order listed in the Commit, includes them in the key schedule in the order listed in the Commit,
or in the psks field respectively. For resumption PSKs, the PSK is or in the psks field, respectively. For resumption PSKs, the PSK is
defined as the resumption_psk of the group and epoch specified in the defined as the resumption_psk of the group and epoch specified in the
PreSharedKeyID object. Specifically, psk_secret is computed as PreSharedKeyID object. Specifically, psk_secret is computed as
follows: follows:
struct { struct {
PreSharedKeyID id; PreSharedKeyID id;
uint16 index; uint16 index;
uint16 count; uint16 count;
} PSKLabel; } PSKLabel;
skipping to change at page 80, line 41 skipping to change at line 3199
0 | 0 |
| | | |
V V V V
psk_[1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[2] psk_[1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[2]
| |
0 ... 0 ...
| | | |
V V V V
psk_[n-1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[n] psk_[n-1] --> Extract --> ExpandWithLabel --> Extract = psk_secret_[n]
Figure 24: Computatation of a PSK secret from a set of PSKs Figure 24: Computation of a PSK Secret from a Set of PSKs
In particular, if there are no PreSharedKey proposals in a given In particular, if there are no PreSharedKey proposals in a given
Commit, then the resulting psk_secret is psk_secret_[0], the all-zero Commit, then the resulting psk_secret is psk_secret_[0], the all-zero
vector. vector.
8.5. Exporters 8.5. Exporters
The main MLS key schedule provides an exporter_secret which can be The main MLS key schedule provides an exporter_secret that can be
used by an application to derive new secrets for use outside of MLS. used by an application to derive new secrets for use outside of MLS.
MLS-Exporter(Label, Context, Length) = MLS-Exporter(Label, Context, Length) =
ExpandWithLabel(DeriveSecret(exporter_secret, Label), ExpandWithLabel(DeriveSecret(exporter_secret, Label),
"exported", Hash(Context), Length) "exported", Hash(Context), Length)
Applications SHOULD provide a unique label to MLS-Exporter that Applications SHOULD provide a unique label to MLS-Exporter that
identifies the secret's intended purpose. This is to help prevent identifies the secret's intended purpose. This is to help prevent
the same secret from being generated and used in two different the same secret from being generated and used in two different
places. To help avoid the same label being used in different places. To help avoid the same label being used in different
skipping to change at page 81, line 28 skipping to change at line 3233
the group. the group.
It is RECOMMENDED for the application generating exported values to It is RECOMMENDED for the application generating exported values to
refresh those values after a Commit is processed. refresh those values after a Commit is processed.
8.6. Resumption PSK 8.6. Resumption PSK
The main MLS key schedule provides a resumption_psk that is used as a The main MLS key schedule provides a resumption_psk that is used as a
PSK to inject entropy from one epoch into another. This PSK to inject entropy from one epoch into another. This
functionality is used in the reinitialization and branching processes functionality is used in the reinitialization and branching processes
described in Section 11.2 and Section 11.3, but may be used by described in Sections 11.2 and 11.3, but it may be used by
applications for other purposes. applications for other purposes.
Some uses of resumption PSKs might call for the use of PSKs from Some uses of resumption PSKs might call for the use of PSKs from
historical epochs. The application SHOULD specify an upper limit on historical epochs. The application SHOULD specify an upper limit on
the number of past epochs for which the resumption_psk may be stored. the number of past epochs for which the resumption_psk may be stored.
8.7. Epoch Authenticators 8.7. Epoch Authenticators
The main MLS key schedule provides a per-epoch epoch_authenticator. The main MLS key schedule provides a per-epoch epoch_authenticator.
If one member of the group is being impersonated by an active If one member of the group is being impersonated by an active
attacker, the epoch_authenticator computed by their client will attacker, the epoch_authenticator computed by their client will
differ from those computed by the other group members. differ from those computed by the other group members.
This property can be used to construct defenses against impersonation This property can be used to construct defenses against impersonation
attacks that are effective even if members' signature keys are attacks that are effective even if members' signature keys are
compromised. As a trivial example, if the users of the clients in an compromised. As a trivial example, if the users of the clients in an
MLS group were to meet in person and reliably confirm that their MLS group were to meet in person and reliably confirm that their
epoch authenticator values were equal (using some suitable user epoch authenticator values were equal (using some suitable user
interface), then each user would be assured that the others were not interface), then each user would be assured that the others were not
being impersonated in the current epoch. As soon as the epoch being impersonated in the current epoch. As soon as the epoch
changed, though, they would need to re-do this confirmation. The changed, though, they would need to redo this confirmation. The
state of the group would have changed, possibly introducing an state of the group would have changed, possibly introducing an
attacker. attacker.
More generally, in order for the members of an MLS group to obtain More generally, in order for the members of an MLS group to obtain
concrete authentication protections using the epoch_authenticator, concrete authentication protections using the epoch_authenticator,
they will need to use it in some secondary protocol (such as the they will need to use it in some secondary protocol (such as the
face-to-face protocol above). The details of that protocol will then face-to-face protocol above). The details of that protocol will then
determine the specific authentication protections provided to the MLS determine the specific authentication protections provided to the MLS
group. group.
9. Secret Tree 9. Secret Tree
For the generation of encryption keys and nonces, the key schedule For the generation of encryption keys and nonces, the key schedule
begins with the encryption_secret at the root and derives a tree of begins with the encryption_secret at the root and derives a tree of
secrets with the same structure as the group's ratchet tree. Each secrets with the same structure as the group's ratchet tree. Each
leaf in the Secret Tree is associated with the same group member as leaf in the secret tree is associated with the same group member as
the corresponding leaf in the ratchet tree. the corresponding leaf in the ratchet tree.
If N is a parent node in the Secret Tree then the secrets of the If N is a parent node in the secret tree, then the secrets of the
children of N are defined as follows (where left(N) and right(N) children of N are defined as follows (where left(N) and right(N)
denote the children of N): denote the children of N):
tree_node_[N]_secret tree_node_[N]_secret
| |
| |
+--> ExpandWithLabel(., "tree", "left", KDF.Nh) +--> ExpandWithLabel(., "tree", "left", KDF.Nh)
| = tree_node_[left(N)]_secret | = tree_node_[left(N)]_secret
| |
+--> ExpandWithLabel(., "tree", "right", KDF.Nh) +--> ExpandWithLabel(., "tree", "right", KDF.Nh)
= tree_node_[right(N)]_secret = tree_node_[right(N)]_secret
Figure 25: Derivation of secrets from parent to children within a Figure 25: Derivation of Secrets from Parent to Children within a
secret tree Secret Tree
The secret in the leaf of the Secret Tree is used to initiate two The secret in the leaf of the secret tree is used to initiate two
symmetric hash ratchets, from which a sequence of single-use keys and symmetric hash ratchets, from which a sequence of single-use keys and
nonces are derived, as described in Section 9.1. The root of each nonces are derived, as described in Section 9.1. The root of each
ratchet is computed as: ratchet is computed as:
tree_node_[N]_secret tree_node_[N]_secret
| |
| |
+--> ExpandWithLabel(., "handshake", "", KDF.Nh) +--> ExpandWithLabel(., "handshake", "", KDF.Nh)
| = handshake_ratchet_secret_[N]_[0] | = handshake_ratchet_secret_[N]_[0]
| |
+--> ExpandWithLabel(., "application", "", KDF.Nh) +--> ExpandWithLabel(., "application", "", KDF.Nh)
= application_ratchet_secret_[N]_[0] = application_ratchet_secret_[N]_[0]
Figure 26: Initialization of the hash ratchets from the leaves of Figure 26: Initialization of the Hash Ratchets from the Leaves of
a secret tree a Secret Tree
9.1. Encryption Keys 9.1. Encryption Keys
As described in Section 6, MLS encrypts three different types of As described in Section 6, MLS encrypts three different types of
information: information:
* Metadata (sender information) * Metadata (sender information)
* Handshake messages (Proposal and Commit) * Handshake messages (Proposal and Commit)
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HR0 AR0--+--K0 HR0 AR0--+--K0
| | | |
| +--N0 | +--N0
| |
AR1--+--K1 AR1--+--K1
| | | |
| +--N1 | +--N1
| |
AR2 AR2
Figure 27: Secret tree for a four-member group Figure 27: Secret Tree for a Four-Member Group
A sender ratchet starts from a per-sender base secret derived from a A sender ratchet starts from a per-sender base secret derived from a
Secret Tree, as described in Section 9. The base secret initiates a Secret Tree, as described in Section 9. The base secret initiates a
symmetric hash ratchet which generates a sequence of keys and nonces. symmetric hash ratchet, which generates a sequence of keys and
The sender uses the j-th key/nonce pair in the sequence to encrypt nonces. The sender uses the j-th key/nonce pair in the sequence to
(using the AEAD) the j-th message they send during that epoch. Each encrypt (using the AEAD) the j-th message they send during that
key/nonce pair MUST NOT be used to encrypt more than one message. epoch. Each key/nonce pair MUST NOT be used to encrypt more than one
message.
Keys, nonces, and the secrets in ratchets are derived using Keys, nonces, and the secrets in ratchets are derived using
DeriveTreeSecret. The context in a given call consists of the DeriveTreeSecret. The context in a given call consists of the
current position in the ratchet. current position in the ratchet.
DeriveTreeSecret(Secret, Label, Generation, Length) = DeriveTreeSecret(Secret, Label, Generation, Length) =
ExpandWithLabel(Secret, Label, Generation, Length) ExpandWithLabel(Secret, Label, Generation, Length)
Where Generation is encoded as a big endian uint32. Where Generation is encoded as a big endian uint32.
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+--> DeriveTreeSecret(., "nonce", j, AEAD.Nn) +--> DeriveTreeSecret(., "nonce", j, AEAD.Nn)
| = ratchet_nonce_[N]_[j] | = ratchet_nonce_[N]_[j]
| |
+--> DeriveTreeSecret(., "key", j, AEAD.Nk) +--> DeriveTreeSecret(., "key", j, AEAD.Nk)
| = ratchet_key_[N]_[j] | = ratchet_key_[N]_[j]
| |
V V
DeriveTreeSecret(., "secret", j, KDF.Nh) DeriveTreeSecret(., "secret", j, KDF.Nh)
= ratchet_secret_[N]_[j+1] = ratchet_secret_[N]_[j+1]
Here, AEAD.Nn and AEAD.Nk denote the lengths in bytes of the nonce Here AEAD.Nn and AEAD.Nk denote the lengths in bytes of the nonce and
and key for the AEAD scheme defined by the ciphersuite. key for the AEAD scheme defined by the cipher suite.
9.2. Deletion Schedule 9.2. Deletion Schedule
It is important to delete all security-sensitive values as soon as It is important to delete all security-sensitive values as soon as
they are _consumed_. A sensitive value S is said to be _consumed_ if they are _consumed_. A sensitive value S is said to be _consumed_ if:
* S was used to encrypt or (successfully) decrypt a message, or if * S was used to encrypt or (successfully) decrypt a message, or
* a key, nonce, or secret derived from S has been consumed. (This * a key, nonce, or secret derived from S has been consumed. (This
goes for values derived via DeriveSecret as well as goes for values derived via DeriveSecret as well as
ExpandWithLabel.) ExpandWithLabel.)
Here, S may be the init_secret, commit_secret, epoch_secret, Here S may be the init_secret, commit_secret, epoch_secret, or
encryption_secret as well as any secret in a Secret Tree or one of encryption_secret as well as any secret in a secret tree or one of
the ratchets. the ratchets.
As soon as a group member consumes a value they MUST immediately As soon as a group member consumes a value, they MUST immediately
delete (all representations of) that value. This is crucial to delete (all representations of) that value. This is crucial to
ensuring forward secrecy for past messages. Members MAY keep ensuring forward secrecy for past messages. Members MAY keep
unconsumed values around for some reasonable amount of time to handle unconsumed values around for some reasonable amount of time to handle
out-of-order message delivery. out-of-order message delivery.
For example, suppose a group member encrypts or (successfully) For example, suppose a group member encrypts or (successfully)
decrypts an application message using the j-th key and nonce in the decrypts an application message using the j-th key and nonce in the
ratchet of leaf node L in some epoch n. Then, for that member, at ratchet of leaf node L in some epoch n. Then, for that member, at
least the following values have been consumed and MUST be deleted: least the following values have been consumed and MUST be deleted:
* the commit_secret, joiner_secret, epoch_secret, encryption_secret * the commit_secret, joiner_secret, epoch_secret, and
of that epoch n as well as the init_secret of the previous epoch encryption_secret of that epoch n as well as the init_secret of
n-1, the previous epoch n-1,
* all node secrets in the Secret Tree on the path from the root to * all node secrets in the secret tree on the path from the root to
the leaf with node L, the leaf with node L,
* the first j secrets in the application data ratchet of node L and * the first j secrets in the application data ratchet of node L, and
* application_ratchet_nonce_[L]_[j] and * application_ratchet_nonce_[L]_[j] and
application_ratchet_key_[L]_[j]. application_ratchet_key_[L]_[j].
Concretely, consider the Secret Tree shown in Figure 27. Client A, Concretely, consider the secret tree shown in Figure 27. Client A,
B, or C would generate the illustrated values on receiving a message B, or C would generate the illustrated values on receiving a message
from D with generation equal to 1, having not received a message with from D with generation equal to 1, having not received a message with
generation 0 (e.g., due to out-of-order delivery). In such a case, generation 0 (e.g., due to out-of-order delivery). In such a case,
the following values would be consumed: the following values would be consumed:
* The key K1 and nonce N1 used to decrypt the message * The key K1 and nonce N1 used to decrypt the message
* The application ratchet secrets AR1 and AR0 * The application ratchet secrets AR1 and AR0
* The tree secrets D, F, G (recall that G is the encryption_secret * The tree secrets D, F, and G (recall that G is the
for the epoch) encryption_secret for the epoch)
* The epoch_secret, commit_secret, psk_secret, and joiner_secret for * The epoch_secret, commit_secret, psk_secret, and joiner_secret for
the current epoch the current epoch
Other values may be retained (not consumed): Other values may be retained (not consumed):
* K0 and N0 for decryption of an out-of-order message with * K0 and N0 for decryption of an out-of-order message with
generation 0 generation 0
* AR2 for derivation of further message decryption keys and nonces * AR2 for derivation of further message decryption keys and nonces
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the current epoch the current epoch
Other values may be retained (not consumed): Other values may be retained (not consumed):
* K0 and N0 for decryption of an out-of-order message with * K0 and N0 for decryption of an out-of-order message with
generation 0 generation 0
* AR2 for derivation of further message decryption keys and nonces * AR2 for derivation of further message decryption keys and nonces
* HR0 for protection of handshake messages from D * HR0 for protection of handshake messages from D
* E and C for deriving secrets used by senders A, B, and C * E and C for deriving secrets used by senders A, B, and C
10. Key Packages 10. Key Packages
In order to facilitate the asynchronous addition of clients to a In order to facilitate the asynchronous addition of clients to a
group, key packages are pre-published that provide some public group, clients can pre-publish KeyPackage objects that provide some
information about a user. A KeyPackage object specifies: public information about a user. A KeyPackage object specifies:
1. A protocol version and ciphersuite that the client supports, 1. a protocol version and cipher suite that the client supports,
2. a public key that others can use to encrypt a Welcome message to 2. a public key that others can use to encrypt a Welcome message to
this client (an "init key"), and this client (an "init key"), and
3. the content of the leaf node that should be added to the tree to 3. the content of the leaf node that should be added to the tree to
represent this client. represent this client.
KeyPackages are intended to be used only once and SHOULD NOT be KeyPackages are intended to be used only once and SHOULD NOT be
reused except in the case of last resort (see Section 16.8). Clients reused except in the case of a "last resort" KeyPackage (see
MAY generate and publish multiple KeyPackages to support multiple Section 16.8). Clients MAY generate and publish multiple KeyPackages
ciphersuites. to support multiple cipher suites.
The value for init_key MUST be a public key for the asymmetric The value for init_key MUST be a public key for the asymmetric
encryption scheme defined by cipher_suite, and it MUST be unique encryption scheme defined by cipher_suite, and it MUST be unique
among the set of KeyPackages created by this client. Likewise, the among the set of KeyPackages created by this client. Likewise, the
leaf_node field MUST be valid for the ciphersuite, including both the leaf_node field MUST be valid for the cipher suite, including both
encryption_key and signature_key fields. The whole structure is the encryption_key and signature_key fields. The whole structure is
signed using the client's signature key. A KeyPackage object with an signed using the client's signature key. A KeyPackage object with an
invalid signature field MUST be considered malformed. invalid signature field MUST be considered malformed.
The signature is computed by the function SignWithLabel with a label The signature is computed by the function SignWithLabel with a label
KeyPackageTBS and a Content input comprising all of the fields except "KeyPackageTBS" and a Content input comprising all of the fields
for the signature field. except for the signature field.
struct { struct {
ProtocolVersion version; ProtocolVersion version;
CipherSuite cipher_suite; CipherSuite cipher_suite;
HPKEPublicKey init_key; HPKEPublicKey init_key;
LeafNode leaf_node; LeafNode leaf_node;
Extension extensions<V>; Extension extensions<V>;
/* SignWithLabel(., "KeyPackageTBS", KeyPackageTBS) */ /* SignWithLabel(., "KeyPackageTBS", KeyPackageTBS) */
opaque signature<V>; opaque signature<V>;
} KeyPackage; } KeyPackage;
skipping to change at page 87, line 31 skipping to change at line 3504
} KeyPackageTBS; } KeyPackageTBS;
If a client receives a KeyPackage carried within an MLSMessage If a client receives a KeyPackage carried within an MLSMessage
object, then it MUST verify that the version field of the KeyPackage object, then it MUST verify that the version field of the KeyPackage
has the same value as the version field of the MLSMessage. The has the same value as the version field of the MLSMessage. The
version field in the KeyPackage provides an explicit signal of the version field in the KeyPackage provides an explicit signal of the
intended version to the other members of group when they receive the intended version to the other members of group when they receive the
KeyPackage in an Add proposal. KeyPackage in an Add proposal.
The field leaf_node.capabilities indicates what protocol versions, The field leaf_node.capabilities indicates what protocol versions,
ciphersuites, credential types, and non-default proposal/extension cipher suites, credential types, and non-default proposal/extension
types are supported by the client. (Proposal and extension types types are supported by the client. (As discussed in Section 7.2,
defined in this document are considered "default" and not listed.) some proposal and extension types defined in this document are
This information allows MLS session establishment to be safe from considered "default" and thus are not listed.) This information
downgrade attacks on the parameters described (as discussed in allows MLS session establishment to be safe from downgrade attacks on
Section 11), while still only advertising one version / ciphersuite the parameters described (as discussed in Section 11), while still
per KeyPackage. only advertising one version and one cipher suite per KeyPackage.
The field leaf_node.leaf_node_source of the LeafNode in a KeyPackage The field leaf_node.leaf_node_source of the LeafNode in a KeyPackage
MUST be set to key_package. MUST be set to key_package.
Extensions included in the extensions or leaf_node.extensions fields Extensions included in the extensions or leaf_node.extensions fields
MUST be included in the leaf_node.capabilities field. As discussed MUST be included in the leaf_node.capabilities field. As discussed
in Section 13, unknown extensions in KeyPackage.extensions MUST be in Section 13, unknown extensions in KeyPackage.extensions MUST be
ignored, and the creator of a KeyPackage object SHOULD include some ignored, and the creator of a KeyPackage object SHOULD include some
random GREASE extensions to help ensure that other clients correctly random GREASE extensions to help ensure that other clients correctly
ignore unknown extensions. ignore unknown extensions.
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The validity of a KeyPackage needs to be verified at a few stages: The validity of a KeyPackage needs to be verified at a few stages:
* When a KeyPackage is downloaded by a group member, before it is * When a KeyPackage is downloaded by a group member, before it is
used to add the client to the group used to add the client to the group
* When a KeyPackage is received by a group member in an Add message * When a KeyPackage is received by a group member in an Add message
The client verifies the validity of a KeyPackage using the following The client verifies the validity of a KeyPackage using the following
steps: steps:
* Verify that the ciphersuite and protocol version of the KeyPackage * Verify that the cipher suite and protocol version of the
match those in the GroupContext. KeyPackage match those in the GroupContext.
* Verify that the leaf_node of the KeyPackage is valid for a * Verify that the leaf_node of the KeyPackage is valid for a
KeyPackage according to Section 7.3. KeyPackage according to Section 7.3.
* Verify that the signature on the KeyPackage is valid using the * Verify that the signature on the KeyPackage is valid using the
public key in leaf_node.credential. public key in leaf_node.credential.
* Verify that the value of leaf_node.encryption_key is different * Verify that the value of leaf_node.encryption_key is different
from the value of the init_key field. from the value of the init_key field.
11. Group Creation 11. Group Creation
A group is always created with a single member, the "creator". Other A group is always created with a single member, the "creator". Other
members are then added to the group using the usual Add/Commit members are then added to the group using the usual Add/Commit
mechanism. mechanism.
The creator of a group is responsible for setting the group ID, The creator of a group is responsible for setting the group ID,
ciphersuite, and initial extensions for the group. If the creator cipher suite, and initial extensions for the group. If the creator
intends to add other members at the time of creation, then it SHOULD intends to add other members at the time of creation, then it SHOULD
Fetch KeyPackages for the members to be added, and select a fetch KeyPackages for the members to be added, and select a cipher
ciphersuite and extensions according to the capabilities of the suite and extensions according to the capabilities of the members.
members. To protect against downgrade attacks, the creator MUST use To protect against downgrade attacks, the creator MUST use the
the capabilities information in these KeyPackages to verify that the capabilities information in these KeyPackages to verify that the
chosen version and ciphersuite is the best option supported by all chosen version and cipher suite is the best option supported by all
members. members.
Group IDs SHOULD be constructed in such a way that there's an Group IDs SHOULD be constructed in such a way that there is an
overwhelmingly low probability of honest group creators generating overwhelmingly low probability of honest group creators generating
the same group ID, even without assistance from the Delivery Service. the same group ID, even without assistance from the Delivery Service.
For example, by making the group ID a freshly generated random value This can be done, for example, by making the group ID a freshly
of size KDF.Nh. The Delivery Service MAY attempt to ensure that generated random value of size KDF.Nh. The Delivery Service MAY
group IDs are globally unique by rejecting the creation of new groups attempt to ensure that group IDs are globally unique by rejecting the
with a previously used ID. creation of new groups with a previously used ID.
The creator of a group MUST take the following steps to initialize To initialize a group, the creator of the group MUST take the
the group: following steps:
* Initialize a one-member group with the following initial values: * Initialize a one-member group with the following initial values:
- Ratchet tree: A tree with a single node, a leaf containing an - Ratchet tree: A tree with a single node, a leaf node containing
HPKE public key and credential for the creator an HPKE public key and credential for the creator
- Group ID: A value set by the creator - Group ID: A value set by the creator
- Epoch: 0 - Epoch: 0
- Tree hash: The root hash of the above ratchet tree - Tree hash: The root hash of the above ratchet tree
- Confirmed transcript hash: The zero-length octet string - Confirmed transcript hash: The zero-length octet string
- Epoch secret: A fresh random value of size KDF.Nh - Epoch secret: A fresh random value of size KDF.Nh
skipping to change at page 89, line 31 skipping to change at line 3601
- Derive the confirmation_key for the epoch as described in - Derive the confirmation_key for the epoch as described in
Section 8. Section 8.
- Compute a confirmation_tag over the empty - Compute a confirmation_tag over the empty
confirmed_transcript_hash using the confirmation_key as confirmed_transcript_hash using the confirmation_key as
described in Section 6.1. described in Section 6.1.
- Compute the updated interim_transcript_hash from the - Compute the updated interim_transcript_hash from the
confirmed_transcript_hash and the confirmation_tag as described confirmed_transcript_hash and the confirmation_tag as described
in Section 8.2 in Section 8.2.
At this point, the creator's state represents a one-member group with At this point, the creator's state represents a one-member group with
a fully initialized key schedule, transcript hashes, etc. Proposals a fully initialized key schedule, transcript hashes, etc. Proposals
and Commits can be generated for this group state just like any other and Commits can be generated for this group state just like any other
state of the group, such as Add proposals and Commits to add other state of the group, such as Add proposals and Commits to add other
members to the group. A GroupInfo object for this group state can members to the group. A GroupInfo object for this group state can
also be published to facilitate external joins. also be published to facilitate external joins.
Members other than the creator join either by being sent a Welcome Members other than the creator join either by being sent a Welcome
message (as described in Section 12.4.3.1) or by sending an external message (as described in Section 12.4.3.1) or by sending an external
skipping to change at page 90, line 9 skipping to change at line 3625
creator directly create a tree and choose a random value for first creator directly create a tree and choose a random value for first
epoch's epoch secret. We follow the steps above because it removes epoch's epoch secret. We follow the steps above because it removes
unnecessary choices, by which, for example, bad randomness could be unnecessary choices, by which, for example, bad randomness could be
introduced. The only choices the creator makes here are its own introduced. The only choices the creator makes here are its own
KeyPackage and the leaf secret from which the Commit is built. KeyPackage and the leaf secret from which the Commit is built.
11.1. Required Capabilities 11.1. Required Capabilities
The configuration of a group imposes certain requirements on clients The configuration of a group imposes certain requirements on clients
in the group. At a minimum, all members of the group need to support in the group. At a minimum, all members of the group need to support
the ciphersuite and protocol version in use. Additional requirements the cipher suite and protocol version in use. Additional
can be imposed by including a required_capabilities extension in the requirements can be imposed by including a required_capabilities
GroupContext. extension in the GroupContext.
struct { struct {
ExtensionType extension_types<V>; ExtensionType extension_types<V>;
ProposalType proposal_types<V>; ProposalType proposal_types<V>;
CredentialType credential_types<V>; CredentialType credential_types<V>;
} RequiredCapabilities; } RequiredCapabilities;
This extension lists the extensions, proposals, and credential types This extension lists the extensions, proposals, and credential types
that must be supported by all members of the group. The "default" that must be supported by all members of the group. The "default"
proposal and extension types defined in this document are assumed to proposal and extension types defined in this document are assumed to
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default capabilities not supported by all current members. default capabilities not supported by all current members.
11.2. Reinitialization 11.2. Reinitialization
A group may be reinitialized by creating a new group with the same A group may be reinitialized by creating a new group with the same
membership and different parameters, and linking it to the old group membership and different parameters, and linking it to the old group
via a resumption PSK. The members of a group reinitialize it using via a resumption PSK. The members of a group reinitialize it using
the following steps: the following steps:
1. A member of the old group sends a ReInit proposal (see 1. A member of the old group sends a ReInit proposal (see
Section 12.1.5) Section 12.1.5).
2. A member of the old group sends a Commit covering the ReInit 2. A member of the old group sends a Commit covering the ReInit
proposal proposal.
3. A member of the old group creates an initial Commit that sets up
a new group that matches the ReInit and sends a Welcome message:
3. A member of the old group creates an initial Commit setting up a
new group that matches the ReInit and sends a Welcome message
* The version, cipher_suite, group_id, and extensions fields of * The version, cipher_suite, group_id, and extensions fields of
the GroupContext object in the Welcome message MUST be the the GroupContext object in the Welcome message MUST be the
same as the corresponding fields in the ReInit proposal. The same as the corresponding fields in the ReInit proposal. The
epoch in the Welcome message MUST be 1. epoch in the Welcome message MUST be 1.
* The Welcome MUST specify a PreSharedKeyID of type resumption * The Welcome message MUST specify a PreSharedKeyID of type
with usage reinit, where the group_id field matches the old resumption with usage reinit, where the group_id field matches
group and the epoch field indicates the epoch after the Commit the old group and the epoch field indicates the epoch after
covering the ReInit. the Commit covering the ReInit.
Note that these three steps may be done by the same group member or Note that these three steps may be done by the same group member or
different members. For example, if a group member sends a Commit different members. For example, if a group member sends a Commit
with an inline ReInit proposal (steps 1 and 2) but then goes offline, with an inline ReInit proposal (steps 1 and 2) but then goes offline,
another group member may recreate the group instead. This another group member may recreate the group instead. This
flexibility avoids situations where a group gets stuck between steps flexibility avoids situations where a group gets stuck between steps
2 and 3. 2 and 3.
Resumption PSKs with usage reinit MUST NOT be used in other contexts. Resumption PSKs with usage reinit MUST NOT be used in other contexts.
A PreSharedKey proposal with type resumption and usage reinit MUST be A PreSharedKey proposal with type resumption and usage reinit MUST be
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A new group can be formed from a subset of an existing group's A new group can be formed from a subset of an existing group's
members, using the same parameters as the old group. members, using the same parameters as the old group.
A member can create a subgroup by performing the following steps: A member can create a subgroup by performing the following steps:
1. Fetch a new KeyPackage for each group member that should be 1. Fetch a new KeyPackage for each group member that should be
included in the subgroup. included in the subgroup.
2. Create an initial Commit message that sets up the new group and 2. Create an initial Commit message that sets up the new group and
contains a PreSharedKey proposal of type resumption with usage contains a PreSharedKey proposal of type resumption with usage
branch. To avoid key re-use, the psk_nonce included in the branch. To avoid key reuse, the psk_nonce included in the
PreSharedKeyID object MUST be a randomly sampled nonce of length PreSharedKeyID object MUST be a randomly sampled nonce of length
KDF.Nh. KDF.Nh.
3. Send the corresponding Welcome message to the subgroup members. 3. Send the corresponding Welcome message to the subgroup members.
A client receiving a Welcome including a PreSharedKey of type A client receiving a Welcome message including a PreSharedKey of type
resumption with usage branch MUST verify that the new group reflects resumption with usage branch MUST verify that the new group reflects
a subgroup branched from the referenced group by checking: a subgroup branched from the referenced group by checking that:
* The version and ciphersuite values in the Welcome are the same as * The version and cipher_suite values in the Welcome message are the
those used by the old group. same as those used by the old group.
* The epoch in the Welcome message MUST be 1. * The epoch in the Welcome message MUST be 1.
* Each LeafNode in a new subgroup MUST match some LeafNode in the * Each LeafNode in a new subgroup MUST match some LeafNode in the
original group. In this context, a pair of LeafNodes is said to original group. In this context, a pair of LeafNodes is said to
"match" if the identifiers presented by their respective "match" if the identifiers presented by their respective
credentials are considered equivalent by the application. credentials are considered equivalent by the application.
Resumption PSKs with usage branch MUST NOT be used in other contexts. Resumption PSKs with usage branch MUST NOT be used in other contexts.
A PreSharedKey proposal with type resumption and usage branch MUST be A PreSharedKey proposal with type resumption and usage branch MUST be
considered invalid. considered invalid.
12. Group Evolution 12. Group Evolution
Over the lifetime of a group, its membership can change, and existing Over the lifetime of a group, its membership can change, and existing
members might want to change their keys in order to achieve post- members might want to change their keys in order to achieve post-
compromise security. In MLS, each such change is accomplished by a compromise security. In MLS, each such change is accomplished by a
two-step process: two-step process:
1. A proposal to make the change is broadcast to the group in a 1. A proposal to make the change is broadcast to the group in a
Proposal message Proposal message.
2. A member of the group or a new member broadcasts a Commit message 2. A member of the group or a new member broadcasts a Commit message
that causes one or more proposed changes to enter into effect that causes one or more proposed changes to enter into effect.
In cases where the Proposal and Commit are sent by the same member, In cases where the Proposal and Commit are sent by the same member,
these two steps can be combined by sending the proposals in the these two steps can be combined by sending the proposals in the
commit. commit.
The group thus evolves from one cryptographic state to another each The group thus evolves from one cryptographic state to another each
time a Commit message is sent and processed. These states are time a Commit message is sent and processed. These states are
referred to as "epochs" and are uniquely identified among states of referred to as "epochs" and are uniquely identified among states of
the group by eight-octet epoch values. When a new group is the group by eight-octet epoch values. When a new group is
initialized, its initial state epoch is 0x0000000000000000. Each initialized, its initial state epoch is 0x0000000000000000. Each
time a state transition occurs, the epoch number is incremented by time a state transition occurs, the epoch number is incremented by
one. one.
12.1. Proposals 12.1. Proposals
Proposals are included in a FramedContent by way of a Proposal Proposals are included in a FramedContent by way of a Proposal
structure that indicates their type: structure that indicates their type:
// See IANA registry for registered values // See the "MLS Proposal Types" IANA registry for values
uint16 ProposalType; uint16 ProposalType;
struct { struct {
ProposalType proposal_type; ProposalType proposal_type;
select (Proposal.proposal_type) { select (Proposal.proposal_type) {
case add: Add; case add: Add;
case update: Update; case update: Update;
case remove: Remove; case remove: Remove;
case psk: PreSharedKey; case psk: PreSharedKey;
case reinit: ReInit; case reinit: ReInit;
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An Add is applied after being included in a Commit message. The An Add is applied after being included in a Commit message. The
position of the Add in the list of proposals determines the leaf node position of the Add in the list of proposals determines the leaf node
where the new member will be added. For the first Add in the Commit, where the new member will be added. For the first Add in the Commit,
the corresponding new member will be placed in the leftmost empty the corresponding new member will be placed in the leftmost empty
leaf in the tree, for the second Add, the next empty leaf to the leaf in the tree, for the second Add, the next empty leaf to the
right, etc. If no empty leaf exists, the tree is extended to the right, etc. If no empty leaf exists, the tree is extended to the
right. right.
* Identify the leaf L for the new member: if there are empty leaves * Identify the leaf L for the new member: if there are empty leaves
in the tree, L is the leftmost empty leaf. Otherwise, the tree is in the tree, L is the leftmost empty leaf. Otherwise, the tree is
extended to the right as described in Section 7.7 and L is extended to the right as described in Section 7.7, and L is
assigned the leftmost new blank leaf. assigned the leftmost new blank leaf.
* For each non-blank intermediate node along the path from the leaf * For each non-blank intermediate node along the path from the leaf
L to the root, add L's leaf index to the unmerged_leaves list for L to the root, add L's leaf index to the unmerged_leaves list for
the node. the node.
* Set the leaf node L to a new node containing the LeafNode object * Set the leaf node L to a new node containing the LeafNode object
carried in the leaf_node field of the KeyPackage in the Add. carried in the leaf_node field of the KeyPackage in the Add.
12.1.2. Update 12.1.2. Update
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LeafNode leaf_node; LeafNode leaf_node;
} Update; } Update;
An Update proposal is invalid if the LeafNode is invalid for an An Update proposal is invalid if the LeafNode is invalid for an
Update proposal according to Section 7.3. Update proposal according to Section 7.3.
A member of the group applies an Update message by taking the A member of the group applies an Update message by taking the
following steps: following steps:
* Replace the sender's LeafNode with the one contained in the Update * Replace the sender's LeafNode with the one contained in the Update
proposal proposal.
* Blank the intermediate nodes along the path from the sender's leaf * Blank the intermediate nodes along the path from the sender's leaf
to the root to the root.
12.1.3. Remove 12.1.3. Remove
A Remove proposal requests that the member with the leaf index A Remove proposal requests that the member with the leaf index
removed be removed from the group. removed be removed from the group.
struct { struct {
uint32 removed; uint32 removed;
} Remove; } Remove;
A Remove proposal is invalid if the removed field does not identify a A Remove proposal is invalid if the removed field does not identify a
non-blank leaf node. non-blank leaf node.
A member of the group applies a Remove message by taking the A member of the group applies a Remove message by taking the
following steps: following steps:
* Identify the leaf node matching removed. Let L be this leaf node. * Identify the leaf node matching removed. Let L be this leaf node.
* Replace the leaf node L with a blank node * Replace the leaf node L with a blank node.
* Blank the intermediate nodes along the path from L to the root
* Blank the intermediate nodes along the path from L to the root.
* Truncate the tree by removing the right subtree until there is at * Truncate the tree by removing the right subtree until there is at
least one non-blank leaf node in the right subtree. If the least one non-blank leaf node in the right subtree. If the
rightmost non-blank leaf has index L, then this will result in the rightmost non-blank leaf has index L, then this will result in the
tree having 2^d leaves, where d is the smallest value such that tree having 2^d leaves, where d is the smallest value such that
2^d > L. 2^d > L.
12.1.4. PreSharedKey 12.1.4. PreSharedKey
A PreSharedKey proposal can be used to request that a pre-shared key A PreSharedKey proposal can be used to request that a pre-shared key
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The psk_nonce MUST be randomly sampled. When processing a Commit The psk_nonce MUST be randomly sampled. When processing a Commit
message that includes one or more PreSharedKey proposals, group message that includes one or more PreSharedKey proposals, group
members derive psk_secret as described in Section 8.4, where the members derive psk_secret as described in Section 8.4, where the
order of the PSKs corresponds to the order of the PreSharedKey order of the PSKs corresponds to the order of the PreSharedKey
proposals in the Commit. proposals in the Commit.
12.1.5. ReInit 12.1.5. ReInit
A ReInit proposal represents a request to reinitialize the group with A ReInit proposal represents a request to reinitialize the group with
different parameters, for example, to increase the version number or different parameters, for example, to increase the version number or
to change the ciphersuite. The reinitialization is done by creating to change the cipher suite. The reinitialization is done by creating
a completely new group and shutting down the old one. a completely new group and shutting down the old one.
struct { struct {
opaque group_id<V>; opaque group_id<V>;
ProtocolVersion version; ProtocolVersion version;
CipherSuite cipher_suite; CipherSuite cipher_suite;
Extension extensions<V>; Extension extensions<V>;
} ReInit; } ReInit;
A ReInit proposal is invalid if the version field is less than the A ReInit proposal is invalid if the version field is less than the
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A GroupContextExtensions proposal is used to update the list of A GroupContextExtensions proposal is used to update the list of
extensions in the GroupContext for the group. extensions in the GroupContext for the group.
struct { struct {
Extension extensions<V>; Extension extensions<V>;
} GroupContextExtensions; } GroupContextExtensions;
A GroupContextExtensions proposal is invalid if it includes a A GroupContextExtensions proposal is invalid if it includes a
required_capabilities extension and some members of the group do not required_capabilities extension and some members of the group do not
support some of the required capabilities (including those added in support some of the required capabilities (including those added in
the same commit, and excluding those removed). the same Commit, and excluding those removed).
A member of the group applies a GroupContextExtensions proposal with A member of the group applies a GroupContextExtensions proposal with
the following steps: the following steps:
* Remove all of the existing extensions from the GroupContext object * Remove all of the existing extensions from the GroupContext object
for the group and replacing them with the list of extensions in for the group and replace them with the list of extensions in the
the proposal. (This is a wholesale replacement, not a merge. An proposal. (This is a wholesale replacement, not a merge. An
extension is only carried over if the sender of the proposal extension is only carried over if the sender of the proposal
includes it in the new list.) includes it in the new list.)
Note that once the GroupContext is updated, its inclusion in the Note that once the GroupContext is updated, its inclusion in the
confirmation_tag by way of the key schedule will confirm that all confirmation_tag by way of the key schedule will confirm that all
members of the group agree on the extensions in use. members of the group agree on the extensions in use.
12.1.8. External Proposals 12.1.8. External Proposals
Proposals can be constructed and sent to the group by a party that is Proposals can be constructed and sent to the group by a party that is
outside the group in two cases. One case, indicated by an external outside the group in two cases. One case, indicated by the external
SenderType is useful in cases where, for example, an automated SenderType, allows an entity outside the group to submit proposals to
service might propose to remove a member of a group who has been the group. For example, an automated service might propose removing
inactive for a long time, or propose adding a newly-hired staff a member of a group who has been inactive for a long time, or propose
member to a group representing a real-world team. An external sender adding a newly hired staff member to a group representing a real-
might send a ReInit proposal, to enforce a changed policy regarding world team. An external sender might send a ReInit proposal to
MLS version or ciphersuite. enforce a changed policy regarding MLS versions or cipher suites.
The external SenderType requires that signers are pre-provisioned to The external SenderType requires that signers are pre-provisioned to
the clients within a group and can only be used if the the clients within a group and can only be used if the
external_senders extension is present in the group's GroupContext. external_senders extension is present in the group's GroupContext.
The other case, indicated by a new_member_proposal SenderType is The other case, indicated by the new_member_proposal SenderType, is
useful when existing members of the group can independently authorize useful when existing members of the group can independently verify
the addition of an MLS client proposing it be added to the group. that an Add proposal sent by the new joiner itself (not an existing
External proposals which are not authorized are considered invalid. member) is authorized. External proposals that are not authorized
are considered invalid.
An external proposal MUST be sent as a PublicMessage object, since An external proposal MUST be sent as a PublicMessage object, since
the sender will not have the keys necessary to construct a the sender will not have the keys necessary to construct a
PrivateMessage object. PrivateMessage object.
Some types of proposal cannot be sent by an external sender. Among Proposals of some types cannot be sent by an external sender. Among
the proposal types defined in this document, only the following types the proposal types defined in this document, only the following types
may be sent by an external sender: may be sent by an external sender:
* add * add
* remove * remove
* psk * psk
* reinit * reinit
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* add * add
* remove * remove
* psk * psk
* reinit * reinit
* group_context_extensions * group_context_extensions
Messages from external senders containing proposal types other than Messages from external senders containing proposal types other than
the above MUST be rejected as malformed. New proposal types defined the above MUST be rejected as malformed. New proposal types defined
in the future MUST define whether they may be sent by external in the future MUST define whether they may be sent by external
senders. A column is defined in the relevant IANA registry senders. The "Ext" column in the "MLS Proposal Types" registry
(Section 17.4) to reflect this property. (Section 17.4) reflects this property.
12.1.8.1. External Senders Extension 12.1.8.1. External Senders Extension
The external_senders extension is a group context extension that The external_senders extension is a group context extension that
contains the credentials and signature keys of senders that are contains the credentials and signature keys of senders that are
permitted to send external proposals to the group. permitted to send external proposals to the group.
struct { struct {
SignaturePublicKey signature_key; SignaturePublicKey signature_key;
Credential credential; Credential credential;
} ExternalSender; } ExternalSender;
ExternalSender external_senders<V>; ExternalSender external_senders<V>;
12.2. Proposal List Validation 12.2. Proposal List Validation
A group member creating a commit and a group member processing a A group member creating a Commit and a group member processing a
Commit MUST verify that the list of committed proposals is valid Commit MUST verify that the list of committed proposals is valid
using one of the following procedures, depending on whether the using one of the following procedures, depending on whether the
commit is external or not. If the list of proposals is invalid, then Commit is external or not. If the list of proposals is invalid, then
the Commit message MUST be rejected as invalid. the Commit message MUST be rejected as invalid.
For a regular, i.e. not external, commit the list is invalid if any For a regular, i.e., not external, Commit, the list is invalid if any
of the following occurs: of the following occurs:
* It contains an individual proposal that is invalid as specified in * It contains an individual proposal that is invalid as specified in
Section 12.1. Section 12.1.
* It contains an Update proposal generated by the committer. * It contains an Update proposal generated by the committer.
* It contains a Remove proposal that removes the committer. * It contains a Remove proposal that removes the committer.
* It contains multiple Update and/or Remove proposals that apply to * It contains multiple Update and/or Remove proposals that apply to
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* It contains multiple GroupContextExtensions proposals. * It contains multiple GroupContextExtensions proposals.
* It contains a ReInit proposal together with any other proposal. * It contains a ReInit proposal together with any other proposal.
If the committer has received other proposals during the epoch, If the committer has received other proposals during the epoch,
they SHOULD prefer them over the ReInit proposal, allowing the they SHOULD prefer them over the ReInit proposal, allowing the
ReInit to be resent and applied in a subsequent epoch. ReInit to be resent and applied in a subsequent epoch.
* It contains an ExternalInit proposal. * It contains an ExternalInit proposal.
* It contains a proposal with a non-default proposal type that is * It contains a Proposal with a non-default proposal type that is
not supported by some members of the group that will process the not supported by some members of the group that will process the
Commit (i.e., members being added or removed by the Commit do not Commit (i.e., members being added or removed by the Commit do not
need to support the proposal type). need to support the proposal type).
* After processing the commit the ratchet tree is invalid, in * After processing the Commit the ratchet tree is invalid, in
particular, if it contains any leaf node that is invalid according particular, if it contains any leaf node that is invalid according
to Section 7.3. to Section 7.3.
An application may extend the above procedure by additional rules, An application may extend the above procedure by additional rules,
for example, requiring application-level permissions to add members, for example, requiring application-level permissions to add members,
or rules concerning non-default proposal types. or rules concerning non-default proposal types.
For an external commit, the list is valid if it contains only the For an external Commit, the list is valid if it contains only the
following proposals (not necessarily in this order): following proposals (not necessarily in this order):
* Exactly one ExternalInit * Exactly one ExternalInit
* At most one Remove proposal, with which the joiner removes an old * At most one Remove proposal, with which the joiner removes an old
version of themselves. If a Remove proposal is present, then the version of themselves. If a Remove proposal is present, then the
LeafNode in the path field of the external commit MUST meet the LeafNode in the path field of the external Commit MUST meet the
same criteria as would the LeafNode in an Update for the removed same criteria as would the LeafNode in an Update for the removed
leaf (see Section 12.1.2). In particular, the credential in the leaf (see Section 12.1.2). In particular, the credential in the
LeafNode MUST present a set of identifiers that is acceptable to LeafNode MUST present a set of identifiers that is acceptable to
the application for the removed participant. the application for the removed participant.
* Zero or more PreSharedKey proposals. * Zero or more PreSharedKey proposals
* No other proposals. * No other proposals
Proposal types defined in the future may make updates to the above Proposal types defined in the future may make updates to the above
validation logic to incorporate considerations related to proposals validation logic to incorporate considerations related to proposals
of the new type. of the new type.
12.3. Applying a Proposal List 12.3. Applying a Proposal List
The sections above defining each proposal type describe how each The sections above defining each proposal type describe how each
individual proposal is applied. When creating or processing a individual proposal is applied. When creating or processing a
Commit, a client applies a list of proposals to the ratchet tree and Commit, a client applies a list of proposals to the ratchet tree and
GroupContext. The client MUST apply the proposals in the list in the GroupContext. The client MUST apply the proposals in the list in the
following order: following order:
* If there is a GroupContextExtensions proposal, replace the * If there is a GroupContextExtensions proposal, replace the
extensions field of the GroupContext for the group with the extensions field of the GroupContext for the group with the
contents of the proposal. The new extensions MUST be used for contents of the proposal. The new extensions MUST be used when
evaluating other proposals in this list. For example, if a evaluating other proposals in this list. For example, if a
GroupContextExtensions proposal adds a required_capabilities GroupContextExtensions proposal adds a required_capabilities
extension, then any Add proposals need to indicate support for extension, then any Add proposals need to indicate support for
those capabilities. those capabilities.
* Apply any Update proposals to the ratchet tree, in any order. * Apply any Update proposals to the ratchet tree, in any order.
* Apply any Remove proposals to the ratchet tree, in any order. * Apply any Remove proposals to the ratchet tree, in any order.
* Apply any Add proposals to the ratchet tree, in the order they * Apply any Add proposals to the ratchet tree, in the order they
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proposed during the epoch are actually removed before any application proposed during the epoch are actually removed before any application
data is transmitted. data is transmitted.
A sender and a receiver of a Commit MUST verify that the committed A sender and a receiver of a Commit MUST verify that the committed
list of proposals is valid as specified in Section 12.2. A list is list of proposals is valid as specified in Section 12.2. A list is
invalid if, for example, it includes an Update and a Remove for the invalid if, for example, it includes an Update and a Remove for the
same member, or an Add when the sender does not have the application- same member, or an Add when the sender does not have the application-
level permission to add new users. level permission to add new users.
The sender of a Commit SHOULD include all proposals that it has The sender of a Commit SHOULD include all proposals that it has
received during the current epoch, that are valid according to the received during the current epoch that are valid according to the
rules for their proposal types and according to application policy, rules for their proposal types and according to application policy,
as long as this results in a valid proposal list. as long as this results in a valid proposal list.
Due to the asynchronous nature of proposals, receivers of a Commit Due to the asynchronous nature of proposals, receivers of a Commit
SHOULD NOT enforce that all valid proposals sent within the current SHOULD NOT enforce that all valid proposals sent within the current
epoch are referenced by the next Commit. In the event that a valid epoch are referenced by the next Commit. In the event that a valid
proposal is omitted from the next Commit, and that proposal is still proposal is omitted from the next Commit, and that proposal is still
valid in the current epoch, the sender of the proposal MAY resend it valid in the current epoch, the sender of the proposal MAY resend it
after updating it to reflect the current epoch. after updating it to reflect the current epoch.
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* add * add
* psk * psk
* reinit * reinit
New proposal types MUST state whether they require a path. If any New proposal types MUST state whether they require a path. If any
instance of a proposal type requires a path, then the proposal type instance of a proposal type requires a path, then the proposal type
requires a path. This attribute of a proposal type is reflected in requires a path. This attribute of a proposal type is reflected in
the "Path Required" field of the proposal type registry defined in the "Path Required" field of the "MLS Proposal Types" registry
Section 17.4. defined in Section 17.4.
Update and Remove proposals are the clearest examples of proposals Update and Remove proposals are the clearest examples of proposals
that require a path. An UpdatePath is required to evict the removed that require a path. An UpdatePath is required to evict the removed
member or the old appearance of the updated member. member or the old appearance of the updated member.
In pseudocode, the logic for validating the path field of a Commit is In pseudocode, the logic for validating the path field of a Commit is
as follows: as follows:
pathRequiredTypes = [ pathRequiredTypes = [
update, update,
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assert(commit.path != null) assert(commit.path != null)
To summarize, a Commit can have three different configurations, with To summarize, a Commit can have three different configurations, with
different uses: different uses:
1. An "empty" Commit that references no proposals, which updates the 1. An "empty" Commit that references no proposals, which updates the
committer's contribution to the group and provides PCS with committer's contribution to the group and provides PCS with
regard to the committer. regard to the committer.
2. A "partial" Commit that references proposals that do not require 2. A "partial" Commit that references proposals that do not require
a path, and where the path is empty. Such a commit doesn't a path, and where the path is empty. Such a Commit doesn't
provide PCS with regard to the committer. provide PCS with regard to the committer.
3. A "full" Commit that references proposals of any type, which 3. A "full" Commit that references proposals of any type, which
provides FS with regard to any removed members and PCS for the provides FS with regard to any removed members and PCS for the
committer and any updated members. committer and any updated members.
12.4.1. Creating a Commit 12.4.1. Creating a Commit
When creating or processing a Commit, a client updates the ratchet When creating or processing a Commit, a client updates the ratchet
tree and GroupContext for the group. These values advance from an tree and GroupContext for the group. These values advance from an
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reflects changes due to the proposals and UpdatePath, but with the reflects changes due to the proposals and UpdatePath, but with the
old confirmed transcript hash. old confirmed transcript hash.
A member of the group creates a Commit message and the corresponding A member of the group creates a Commit message and the corresponding
Welcome message at the same time, by taking the following steps: Welcome message at the same time, by taking the following steps:
* Verify that the list of proposals to be committed is valid as * Verify that the list of proposals to be committed is valid as
specified in Section 12.2. specified in Section 12.2.
* Construct an initial Commit object with the proposals field * Construct an initial Commit object with the proposals field
populated from Proposals received during the current epoch, and an populated from Proposals received during the current epoch, and
empty path field. with the path field empty.
* Create the new ratchet tree and GroupContext by applying the list * Create the new ratchet tree and GroupContext by applying the list
of proposals to the old ratchet tree and GroupContext, as defined of proposals to the old ratchet tree and GroupContext, as defined
in Section 12.3 in Section 12.3.
* Decide whether to populate the path field: If the path field is * Decide whether to populate the path field: If the path field is
required based on the proposals that are in the commit (see required based on the proposals that are in the Commit (see
above), then it MUST be populated. Otherwise, the sender MAY omit above), then it MUST be populated. Otherwise, the sender MAY omit
the path field at its discretion. the path field at its discretion.
* If populating the path field: * If populating the path field:
- If this is an external commit, assign the sender the leftmost - If this is an external Commit, assign the sender the leftmost
blank leaf node in the new ratchet tree. If there are no blank blank leaf node in the new ratchet tree. If there are no blank
leaf nodes in the new ratchet tree, expand the tree to the leaf nodes in the new ratchet tree, expand the tree to the
right as defined in Section 7.7 and assign the leftmost new right as defined in Section 7.7 and assign the leftmost new
blank leaf to the sender. blank leaf to the sender.
- Update the sender's direct path in the ratchet tree as - Update the sender's direct path in the ratchet tree as
described in Section 7.5. Define commit_secret as the value described in Section 7.5. Define commit_secret as the value
path_secret[n+1] derived from the last path secret value path_secret[n+1] derived from the last path secret value
(path_secret[n]) derived for the UpdatePath. (path_secret[n]) derived for the UpdatePath.
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of PSKs in the derivation corresponds to the order of PreSharedKey of PSKs in the derivation corresponds to the order of PreSharedKey
proposals in the proposals vector. proposals in the proposals vector.
* Construct a FramedContent object containing the Commit object. * Construct a FramedContent object containing the Commit object.
Sign the FramedContent using the old GroupContext as context. Sign the FramedContent using the old GroupContext as context.
- Use the FramedContent to update the confirmed transcript hash - Use the FramedContent to update the confirmed transcript hash
and update the new GroupContext. and update the new GroupContext.
- Use the init_secret from the previous epoch, the commit_secret - Use the init_secret from the previous epoch, the commit_secret
and the psk_secret as defined in the previous steps, and the and psk_secret defined in the previous steps, and the new
new GroupContext to compute the new joiner_secret, GroupContext to compute the new joiner_secret, welcome_secret,
welcome_secret, epoch_secret, and derived secrets for the new epoch_secret, and derived secrets for the new epoch.
epoch.
- Use the confirmation_key for the new epoch to compute the - Use the confirmation_key for the new epoch to compute the
confirmation_tag value. confirmation_tag value.
- Calculate the interim transcript hash using the new confirmed - Calculate the interim transcript hash using the new confirmed
transcript hash and the confirmation_tag from the transcript hash and the confirmation_tag from the
FramedContentAuthData. FramedContentAuthData.
* Protect the AuthenticatedContent object using keys from the old * Protect the AuthenticatedContent object using keys from the old
epoch: epoch:
- If encoding as PublicMessage, compute the membership_tag value - If encoding as PublicMessage, compute the membership_tag value
using the membership_key. using the membership_key.
- If encoding as a PrivateMessage, encrypt the message using the - If encoding as a PrivateMessage, encrypt the message using the
sender_data_secret and the next (key, nonce) pair from the sender_data_secret and the next (key, nonce) pair from the
sender's handshake ratchet. sender's handshake ratchet.
* Construct a GroupInfo reflecting the new state: * Construct a GroupInfo reflecting the new state:
- Group ID, epoch, tree, confirmed transcript hash, interim - Set the group_id, epoch, tree, confirmed_transcript_hash,
transcript hash, and group context extensions from the new interim_transcript_hash, and group_context_extensions fields to
state reflect the new state.
- The confirmation_tag from the FramedContentAuthData object - Set the confirmation_tag field to the value of the
corresponding field in the FramedContentAuthData object.
- Other extensions as defined by the application - Add any other extensions as defined by the application.
- Optionally derive an external keypair as described in Section 8
(required for External Commits, see Section 12.4.3.2)
- Sign the GroupInfo using the member's private signing key - Optionally derive an external key pair as described in
Section 8. (required for external Commits, see
Section 12.4.3.2).
- Sign the GroupInfo using the member's private signing key.
- Encrypt the GroupInfo using the key and nonce derived from the - Encrypt the GroupInfo using the key and nonce derived from the
joiner_secret for the new epoch (see Section 12.4.3.1) joiner_secret. for the new epoch (see Section 12.4.3.1).
* For each new member in the group: * For each new member in the group:
- Identify the lowest common ancestor in the tree of the new - Identify the lowest common ancestor in the tree of the new
member's leaf node and the member sending the Commit member's leaf node and the member sending the Commit.
- If the path field was populated above: Compute the path secret - If the path field was populated above: Compute the path secret
corresponding to the common ancestor node corresponding to the common ancestor node.
- Compute an EncryptedGroupSecrets object that encapsulates the - Compute an EncryptedGroupSecrets object that encapsulates the
init_secret for the current epoch and the path secret (if init_secret for the current epoch and the path secret (if
present). present).
* Construct one or more Welcome messages from the encrypted * Construct one or more Welcome messages from the encrypted
GroupInfo object, the encrypted key packages, and any PSKs for GroupInfo object, the encrypted key packages, and any PSKs for
which a proposal was included in the Commit. The order of the which a proposal was included in the Commit. The order of the
psks MUST be the same as the order of PreSharedKey proposals in psks MUST be the same as the order of PreSharedKey proposals in
the proposals vector. As discussed on Section 12.4.3.1, the the proposals vector. As discussed in Section 12.4.3.1, the
committer is free to choose how many Welcome messages to committer is free to choose how many Welcome messages to
construct. However, the set of Welcome messages produced in this construct. However, the set of Welcome messages produced in this
step MUST cover every new member added in the Commit. step MUST cover every new member added in the Commit.
* If a ReInit proposal was part of the Commit, the committer MUST * If a ReInit proposal was part of the Commit, the committer MUST
create a new group with the parameters specified in the ReInit create a new group with the parameters specified in the ReInit
proposal, and with the same members as the original group. The proposal, and with the same members as the original group. The
Welcome message MUST include a PreSharedKeyID with the following Welcome message MUST include a PreSharedKeyID with the following
parameters: parameters:
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- group_id: The group ID for the current group - group_id: The group ID for the current group
- epoch: The epoch that the group will be in after this Commit - epoch: The epoch that the group will be in after this Commit
12.4.2. Processing a Commit 12.4.2. Processing a Commit
A member of the group applies a Commit message by taking the A member of the group applies a Commit message by taking the
following steps: following steps:
* Verify that the epoch field of the enclosing FramedContent is * Verify that the epoch field of the enclosing FramedContent is
equal to the epoch field of the current GroupContext object equal to the epoch field of the current GroupContext object.
* Unprotect the Commit using the keys from the current epoch: * Unprotect the Commit using the keys from the current epoch:
- If the message is encoded as PublicMessage, verify the - If the message is encoded as PublicMessage, verify the
membership MAC using the membership_key membership MAC using the membership_key.
- If the message is encoded as PrivateMessage, decrypt the - If the message is encoded as PrivateMessage, decrypt the
message using the sender_data_secret and the (key, nonce) pair message using the sender_data_secret and the (key, nonce) pair
from the step on the sender's hash ratchet indicated by the from the step on the sender's hash ratchet indicated by the
generation field. generation field.
* Verify that the signature on the FramedContent message as * Verify the signature on the FramedContent message as described in
described in Section 6.1. Section 6.1.
* Verify that the proposals vector is valid as specified in * Verify that the proposals vector is valid according to the rules
Section 12.2. in Section 12.2.
* Verify that all PreSharedKey proposals in the proposals vector are * Verify that all PreSharedKey proposals in the proposals vector are
available. available.
* Create the new ratchet tree and GroupContext by applying the list * Create the new ratchet tree and GroupContext by applying the list
of proposals to the old ratchet tree and GroupContext, as defined of proposals to the old ratchet tree and GroupContext, as defined
in Section 12.3 in Section 12.3.
* Verify that the path value is populated if the proposals vector * Verify that the path value is populated if the proposals vector
contains any Update or Remove proposals, or if it's empty. contains any Update or Remove proposals, or if it's empty.
Otherwise, the path value MAY be omitted. Otherwise, the path value MAY be omitted.
* If the path value is populated, validate it and apply it to the * If the path value is populated, validate it and apply it to the
tree: tree:
- If this is an external commit, assign the sender the leftmost - If this is an external Commit, assign the sender the leftmost
blank leaf node in the new ratchet tree. If there are no blank blank leaf node in the new ratchet tree. If there are no blank
leaf nodes in the new ratchet tree, add a blank leaf to the leaf nodes in the new ratchet tree, add a blank leaf to the
right side of the new ratchet tree and assign it to the sender. right side of the new ratchet tree and assign it to the sender.
- Validate the LeafNode as specified in Section 7.3. The - Validate the LeafNode as specified in Section 7.3. The
leaf_node_source field MUST be set to commit. leaf_node_source field MUST be set to commit.
- Verify that the encryption_key value in the LeafNode is - Verify that the encryption_key value in the LeafNode is
different from the committer's current leaf node. different from the committer's current leaf node.
- Verify that none of the public keys in the UpdatePath appear in - Verify that none of the public keys in the UpdatePath appear in
any node of the new ratchet tree. any node of the new ratchet tree.
- Merge the UpdatePath into the new ratchet tree as described in - Merge the UpdatePath into the new ratchet tree, as described in
Section 7.5. Section 7.5.
- Construct a provisional GroupContext object containing the - Construct a provisional GroupContext object containing the
following values: following values:
o group_id: Same as the old GroupContext o group_id: Same as the old GroupContext
o epoch: The epoch number for the new epoch o epoch: The epoch number for the new epoch
o tree_hash: The tree hash of the new ratchet tree o tree_hash: The tree hash of the new ratchet tree
skipping to change at page 108, line 30 skipping to change at line 4484
updated by a GroupContextExtensions proposal) updated by a GroupContextExtensions proposal)
- Decrypt the path secrets for UpdatePath as described in - Decrypt the path secrets for UpdatePath as described in
Section 7.5, using the provisional GroupContext as the context Section 7.5, using the provisional GroupContext as the context
for HPKE decryption. for HPKE decryption.
- Define commit_secret as the value path_secret[n+1] derived from - Define commit_secret as the value path_secret[n+1] derived from
the last path secret value (path_secret[n]) derived for the the last path secret value (path_secret[n]) derived for the
UpdatePath. UpdatePath.
* If the path value is not populated: Define commit_secret as the * If the path value is not populated, define commit_secret as the
all-zero vector of length KDF.Nh (the same length as a path_secret all-zero vector of length KDF.Nh (the same length as a path_secret
value would be). value would be).
* Update the confirmed and interim transcript hashes using the new * Update the confirmed and interim transcript hashes using the new
Commit, and generate the new GroupContext. Commit, and generate the new GroupContext.
* Derive the psk_secret as specified in Section 8.4, where the order * Derive the psk_secret as specified in Section 8.4, where the order
of PSKs in the derivation corresponds to the order of PreSharedKey of PSKs in the derivation corresponds to the order of PreSharedKey
proposals in the proposals vector. proposals in the proposals vector.
* Use the init_secret from the previous epoch, the commit_secret and * Use the init_secret from the previous epoch, the commit_secret and
the psk_secret as defined in the previous steps, and the new psk_secret defined in the previous steps, and the new GroupContext
GroupContext to compute the new joiner_secret, welcome_secret, to compute the new joiner_secret, welcome_secret, epoch_secret,
epoch_secret, and derived secrets for the new epoch. and derived secrets for the new epoch.
* Use the confirmation_key for the new epoch to compute the * Use the confirmation_key for the new epoch to compute the
confirmation tag for this message, as described below, and verify confirmation tag for this message, as described below, and verify
that it is the same as the confirmation_tag field in the that it is the same as the confirmation_tag field in the
FramedContentAuthData object. FramedContentAuthData object.
* If the above checks are successful, consider the new GroupContext * If the above checks are successful, consider the new GroupContext
object as the current state of the group. object as the current state of the group.
* If the Commit included a ReInit proposal, the client MUST NOT use * If the Commit included a ReInit proposal, the client MUST NOT use
the group to send messages anymore. Instead, it MUST wait for a the group to send messages anymore. Instead, it MUST wait for a
Welcome message from the committer meeting the requirements of Welcome message from the committer meeting the requirements of
Section 11.2. Section 11.2.
Note that clients need to be prepared to receive a valid Commit Note that clients need to be prepared to receive a valid Commit
message which removes them from the group. In this case, the client message that removes them from the group. In this case, the client
cannot send any more messages in the group and SHOULD promptly delete cannot send any more messages in the group and SHOULD promptly delete
its group state and secret tree. (A client might keep the secret its group state and secret tree. (A client might keep the secret
tree for a short time to decrypt late messages in the previous tree for a short time to decrypt late messages in the previous
epoch.) epoch.)
12.4.3. Adding Members to the Group 12.4.3. Adding Members to the Group
New members can join the group in two ways. Either by being added by New members can join the group in two ways: by being added by a group
a group member, or by adding themselves through an external Commit. member or by adding themselves through an external Commit. In both
In both cases, the new members need information to bootstrap their cases, the new members need information to bootstrap their local
local group state. group state.
struct { struct {
GroupContext group_context; GroupContext group_context;
Extension extensions<V>; Extension extensions<V>;
MAC confirmation_tag; MAC confirmation_tag;
uint32 signer; uint32 signer;
/* SignWithLabel(., "GroupInfoTBS", GroupInfoTBS) */ /* SignWithLabel(., "GroupInfoTBS", GroupInfoTBS) */
opaque signature<V>; opaque signature<V>;
} GroupInfo; } GroupInfo;
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GroupInfo may recompute the confirmation tag as MAC(confirmation_key, GroupInfo may recompute the confirmation tag as MAC(confirmation_key,
confirmed_transcript_hash).) confirmed_transcript_hash).)
As discussed in Section 13, unknown extensions in As discussed in Section 13, unknown extensions in
GroupInfo.extensions MUST be ignored, and the creator of a GroupInfo GroupInfo.extensions MUST be ignored, and the creator of a GroupInfo
object SHOULD include some random GREASE extensions to help ensure object SHOULD include some random GREASE extensions to help ensure
that other clients correctly ignore unknown extensions. Extensions that other clients correctly ignore unknown extensions. Extensions
in GroupInfo.group_context.extensions, however, MUST be supported by in GroupInfo.group_context.extensions, however, MUST be supported by
the new joiner. the new joiner.
New members MUST verify that group_id is unique among the groups New members MUST verify that group_id is unique among the groups they
they're currently participating in. are currently participating in.
New members also MUST verify the signature using the public key taken New members also MUST verify the signature using the public key taken
from the leaf node of the ratchet tree with leaf index signer. The from the leaf node of the ratchet tree with leaf index signer. The
signature covers the following structure, comprising all the fields signature covers the following structure, comprising all the fields
in the GroupInfo above signature: in the GroupInfo above signature:
struct { struct {
GroupContext group_context; GroupContext group_context;
Extension extensions<V>; Extension extensions<V>;
MAC confirmation_tag; MAC confirmation_tag;
skipping to change at page 110, line 34 skipping to change at line 4584
could create one Welcome that is encrypted for all new members, a could create one Welcome that is encrypted for all new members, a
different Welcome for each new member, or Welcome messages for different Welcome for each new member, or Welcome messages for
batches of new members (according to some batching scheme that works batches of new members (according to some batching scheme that works
well for the application). The processes for creating and processing well for the application). The processes for creating and processing
the Welcome are the same in all cases, aside from the set of new the Welcome are the same in all cases, aside from the set of new
members for whom a given Welcome is encrypted. members for whom a given Welcome is encrypted.
The Welcome message provides the new members with the current state The Welcome message provides the new members with the current state
of the group after the application of the Commit message. The new of the group after the application of the Commit message. The new
members will not be able to decrypt or verify the Commit message, but members will not be able to decrypt or verify the Commit message, but
will have the secrets they need to participate in the epoch initiated they will have the secrets they need to participate in the epoch
by the Commit message. initiated by the Commit message.
In order to allow the same Welcome message to be sent to multiple new In order to allow the same Welcome message to be sent to multiple new
members, information describing the group is encrypted with a members, information describing the group is encrypted with a
symmetric key and nonce derived from the joiner_secret for the new symmetric key and nonce derived from the joiner_secret for the new
epoch. The joiner_secret is then encrypted to each new member using epoch. The joiner_secret is then encrypted to each new member using
HPKE. In the same encrypted package, the committer transmits the HPKE. In the same encrypted package, the committer transmits the
path secret for the lowest (closest to the leaf) node which is path secret for the lowest (closest to the leaf) node that is
contained in the direct paths of both the committer and the new contained in the direct paths of both the committer and the new
member. This allows the new member to compute private keys for nodes member. This allows the new member to compute private keys for nodes
in its direct path that are being reset by the corresponding Commit. in its direct path that are being reset by the corresponding Commit.
If the sender of the Welcome message wants the receiving member to If the sender of the Welcome message wants the receiving member to
include a PSK in the derivation of the epoch_secret, they can include a PSK in the derivation of the epoch_secret, they can
populate the psks field indicating which PSK to use. populate the psks field indicating which PSK to use.
struct { struct {
opaque path_secret<V>; opaque path_secret<V>;
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otherwise (e.g., provided by a caching service on the Delivery otherwise (e.g., provided by a caching service on the Delivery
Service), then the client MUST download the tree before processing Service), then the client MUST download the tree before processing
the Welcome. the Welcome.
On receiving a Welcome message, a client processes it using the On receiving a Welcome message, a client processes it using the
following steps: following steps:
* Identify an entry in the secrets array where the new_member value * Identify an entry in the secrets array where the new_member value
corresponds to one of this client's KeyPackages, using the hash corresponds to one of this client's KeyPackages, using the hash
indicated by the cipher_suite field. If no such field exists, or indicated by the cipher_suite field. If no such field exists, or
if the ciphersuite indicated in the KeyPackage does not match the if the cipher suite indicated in the KeyPackage does not match the
one in the Welcome message, return an error. one in the Welcome message, return an error.
* Decrypt the encrypted_group_secrets value with the algorithms * Decrypt the encrypted_group_secrets value with the algorithms
indicated by the ciphersuite and the private key init_key_priv indicated by the cipher suite and the private key init_key_priv
corresponding to init_key in the referenced KeyPackage. corresponding to init_key in the referenced KeyPackage.
encrypted_group_secrets = EncryptWithLabel(init_key, "Welcome", encrypted_group_secrets =
encrypted_group_info, group_secrets) EncryptWithLabel(init_key, "Welcome",
encrypted_group_info, group_secrets)
group_secrets = DecryptWithLabel(init_key_priv, "Welcome", group_secrets =
encrypted_group_info, kem_output, ciphertext) DecryptWithLabel(init_key_priv, "Welcome",
encrypted_group_info, kem_output, ciphertext)
* If a PreSharedKeyID is part of the GroupSecrets and the client is * If a PreSharedKeyID is part of the GroupSecrets and the client is
not in possession of the corresponding PSK, return an error. not in possession of the corresponding PSK, return an error.
Additionally, if a PreSharedKeyID has type resumption with usage Additionally, if a PreSharedKeyID has type resumption with usage
reinit or branch, verify that it is the only such PSK. reinit or branch, verify that it is the only such PSK.
* From the joiner_secret in the decrypted GroupSecrets object and * From the joiner_secret in the decrypted GroupSecrets object and
the PSKs specified in the GroupSecrets, derive the welcome_secret the PSKs specified in the GroupSecrets, derive the welcome_secret
and using that the welcome_key and welcome_nonce. Use the key and and then the welcome_key and welcome_nonce. Use the key and nonce
nonce to decrypt the encrypted_group_info field. to decrypt the encrypted_group_info field.
welcome_nonce = ExpandWithLabel(welcome_secret, "nonce", "", AEAD.Nn) welcome_nonce = ExpandWithLabel(welcome_secret, "nonce", "", AEAD.Nn)
welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk) welcome_key = ExpandWithLabel(welcome_secret, "key", "", AEAD.Nk)
* Verify the signature on the GroupInfo object. The signature input * Verify the signature on the GroupInfo object. The signature input
comprises all of the fields in the GroupInfo object except the comprises all of the fields in the GroupInfo object except the
signature field. The public key is taken from the LeafNode of the signature field. The public key is taken from the LeafNode of the
ratchet tree with leaf index signer. If the node is blank or if ratchet tree with leaf index signer. If the node is blank or if
signature verification fails, return an error. signature verification fails, return an error.
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- For each non-empty leaf node, validate the LeafNode as - For each non-empty leaf node, validate the LeafNode as
described in Section 7.3. described in Section 7.3.
- For each non-empty parent node and each entry in the node's - For each non-empty parent node and each entry in the node's
unmerged_leaves field: unmerged_leaves field:
o Verify that the entry represents a non-blank leaf node that o Verify that the entry represents a non-blank leaf node that
is a descendant of the parent node. is a descendant of the parent node.
o Verify that every non-blank intermediate node beween the o Verify that every non-blank intermediate node between the
leaf node and the parent node also has an entry for the leaf leaf node and the parent node also has an entry for the leaf
node in its unmerged_leaves. node in its unmerged_leaves.
o Verify that the encryption key in the parent node does not o Verify that the encryption key in the parent node does not
appear in any other node of the tree. appear in any other node of the tree.
* Identify a leaf whose LeafNode is identical to the one in the * Identify a leaf whose LeafNode is identical to the one in the
KeyPackage. If no such field exists, return an error. Let KeyPackage. If no such field exists, return an error. Let
my_leaf represent this leaf in the tree. my_leaf represent this leaf in the tree.
* Construct a new group state using the information in the GroupInfo * Construct a new group state using the information in the GroupInfo
object. object.
- The GroupContext is the group_context field from the GroupInfo - Initialize the GroupContext for the group from the
object. group_context field from the GroupInfo object.
- The new member's position in the tree is at the leaf my_leaf,
as defined above.
- Update the leaf my_leaf with the private key corresponding to - Update the leaf my_leaf with the private key corresponding to
the public key in the node. the public key in the node, where my_leaf is the new member's
leaf node in the ratchet tree, as defined above.
- If the path_secret value is set in the GroupSecrets object: - If the path_secret value is set in the GroupSecrets object:
Identify the lowest common ancestor of the leaf node my_leaf Identify the lowest common ancestor of the leaf node my_leaf
and of the node of the member with leaf index GroupInfo.signer. and of the node of the member with leaf index GroupInfo.signer.
Set the private key for this node to the private key derived Set the private key for this node to the private key derived
from the path_secret. from the path_secret.
- For each parent of the common ancestor, up to the root of the - For each parent of the common ancestor, up to the root of the
tree, derive a new path secret and set the private key for the tree, derive a new path secret, and set the private key for the
node to the private key derived from the path secret. The node to the private key derived from the path secret. The
private key MUST be the private key that corresponds to the private key MUST be the private key that corresponds to the
public key in the node. public key in the node.
* Use the joiner_secret from the GroupSecrets object to generate the * Use the joiner_secret from the GroupSecrets object to generate the
epoch secret and other derived secrets for the current epoch. epoch secret and other derived secrets for the current epoch.
* Set the confirmed transcript hash in the new state to the value of * Set the confirmed transcript hash in the new state to the value of
the confirmed_transcript_hash in the GroupInfo. the confirmed_transcript_hash in the GroupInfo.
skipping to change at page 114, line 18 skipping to change at line 4759
- For usage branch, verify that the version and cipher_suite of - For usage branch, verify that the version and cipher_suite of
the new group match those of the old group, and that the the new group match those of the old group, and that the
members of the new group compose a subset of the members of the members of the new group compose a subset of the members of the
old group, according to the application. old group, according to the application.
12.4.3.2. Joining via External Commits 12.4.3.2. Joining via External Commits
External Commits are a mechanism for new members (external parties External Commits are a mechanism for new members (external parties
that want to become members of the group) to add themselves to a that want to become members of the group) to add themselves to a
group, without requiring that an existing member has to come online group, without requiring that an existing member has to come online
to issue a Commit that references an Add Proposal. to issue a Commit that references an Add proposal.
Whether existing members of the group will accept or reject an Whether existing members of the group will accept or reject an
External Commit follows the same rules that are applied to other external Commit follows the same rules that are applied to other
handshake messages. handshake messages.
New members can create and issue an External Commit if they have New members can create and issue an external Commit if they have
access to the following information for the group's current epoch: access to the following information for the group's current epoch:
* group ID * group ID
* epoch ID * epoch ID
* ciphersuite * cipher suite
* public tree hash * public tree hash
* confirmed transcript hash * confirmed transcript hash
* confirmation tag of the most recent Commit * confirmation tag of the most recent Commit
* group extensions * group extensions
* external public key * external public key
In other words, to join a group via an External Commit, a new member In other words, to join a group via an external Commit, a new member
needs a GroupInfo with an external_pub extension present in its needs a GroupInfo with an external_pub extension present in its
extensions field. extensions field.
struct { struct {
HPKEPublicKey external_pub; HPKEPublicKey external_pub;
} ExternalPub; } ExternalPub;
Thus, a member of the group can enable new clients to join by making Thus, a member of the group can enable new clients to join by making
a GroupInfo object available to them. Note that because a GroupInfo a GroupInfo object available to them. Note that because a GroupInfo
object is specific to an epoch, it will need to be updated as the object is specific to an epoch, it will need to be updated as the
group advances. In particular, each GroupInfo object can be used for group advances. In particular, each GroupInfo object can be used for
one external join, since that external join will cause the epoch to one external join, since that external join will cause the epoch to
change. change.
Note that the tree_hash field is used the same way as in the Welcome Note that the tree_hash field is used the same way as in the Welcome
message. The full tree can be included via the ratchet_tree message. The full tree can be included via the ratchet_tree
extension Section 12.4.3.3. extension (see Section 12.4.3.3).
The information in a GroupInfo is not generally public information, The information in a GroupInfo is not generally public information,
but applications can choose to make it available to new members in but applications can choose to make it available to new members in
order to allow External Commits. order to allow External Commits.
In principle, External Commits work like regular Commits. However, In principle, external Commits work like regular Commits. However,
their content has to meet a specific set of requirements: their content has to meet a specific set of requirements:
* External Commits MUST contain a path field (and is therefore a * External Commits MUST contain a path field (and is therefore a
"full" Commit). The joiner is added at the leftmost free leaf "full" Commit). The joiner is added at the leftmost free leaf
node (just as if they were added with an Add proposal), and the node (just as if they were added with an Add proposal), and the
path is calculated relative to that leaf node. path is calculated relative to that leaf node.
* The Commit MUST NOT include any proposals by reference, since an * The Commit MUST NOT include any proposals by reference, since an
external joiner cannot determine the validity of proposals sent external joiner cannot determine the validity of proposals sent
within the group within the group.
* External Commits MUST be signed by the new member. In particular, * External Commits MUST be signed by the new member. In particular,
the signature on the enclosing AuthenticatedContent MUST verify the signature on the enclosing AuthenticatedContent MUST verify
using the public key for the credential in the leaf_node of the using the public key for the credential in the leaf_node of the
path field. path field.
* When processing a Commit, both existing and new members MUST use * When processing a Commit, both existing and new members MUST use
the external init secret as described in Section 8.3. the external init secret as described in Section 8.3.
* The sender type for the AuthenticatedContent encapsulating the * The sender type for the AuthenticatedContent encapsulating the
External Commit MUST be new_member_commit. external Commit MUST be new_member_commit.
External Commits come in two "flavors" -- a "join" commit that adds External Commits come in two "flavors" -- a "join" Commit that adds
the sender to the group or a "resync" commit that replaces a member's the sender to the group or a "resync" Commit that replaces a member's
prior appearance with a new one. prior appearance with a new one.
Note that the "resync" operation allows an attacker that has Note that the "resync" operation allows an attacker that has
compromised a member's signature private key to introduce themselves compromised a member's signature private key to introduce themselves
into the group and remove the prior, legitimate member in a single into the group and remove the prior, legitimate member in a single
Commit. Without resync, this can still be done, but requires two Commit. Without resync, this can still be done, but it requires two
operations, the external Commit to join and a second Commit to remove operations: the external Commit to join and a second Commit to remove
the old appearance. Applications for whom this distinction is the old appearance. Applications for whom this distinction is
salient can choose to disallow external commits that contain a salient can choose to disallow external commits that contain a
Remove, or to allow such resync commits only if they contain a Remove, or to allow such resync commits only if they contain a
"reinit" PSK proposal that demonstrates the joining member's presence "reinit" PSK proposal that demonstrates the joining member's presence
in a prior epoch of the group. With the latter approach, the in a prior epoch of the group. With the latter approach, the
attacker would need to compromise the PSK as well as the signing key, attacker would need to compromise the PSK as well as the signing key,
but the application will need to ensure that continuing, non- but the application will need to ensure that continuing, non-
resynchronizing members have the required PSK. resynchronizing members have the required PSK.
12.4.3.3. Ratchet Tree Extension 12.4.3.3. Ratchet Tree Extension
By default, a GroupInfo message only provides the joiner with a hash By default, a GroupInfo message only provides the joiner with a hash
of the group's ratchet tree. In order to process or generate of the group's ratchet tree. In order to process or generate
handshake messages, the joiner will need to get a copy of the ratchet handshake messages, the joiner will need to get a copy of the ratchet
tree from some other source. (For example, the DS might provide a tree from some other source. (For example, the DS might provide a
cached copy.) The inclusion of the tree hash in the GroupInfo cached copy.) The inclusion of the tree hash in the GroupInfo
message means that the source of the ratchet tree need not be trusted message means that the source of the ratchet tree need not be trusted
to maintain the integrity of tree. to maintain the integrity of the tree.
In cases where the application does not wish to provide such an In cases where the application does not wish to provide such an
external source, the whole public state of the ratchet tree can be external source, the whole public state of the ratchet tree can be
provided in an extension of type ratchet_tree, containing a provided in an extension of type ratchet_tree, containing a
ratchet_tree object of the following form: ratchet_tree object of the following form:
struct { struct {
NodeType node_type; NodeType node_type;
select (Node.node_type) { select (Node.node_type) {
case leaf: LeafNode leaf_node; case leaf: LeafNode leaf_node;
skipping to change at page 117, line 4 skipping to change at line 4880
optional<Node> ratchet_tree<V>; optional<Node> ratchet_tree<V>;
Each entry in the ratchet_tree vector provides the value for a node Each entry in the ratchet_tree vector provides the value for a node
in the tree, or the null optional for a blank node. in the tree, or the null optional for a blank node.
The nodes are listed in the order specified by a left-to-right in- The nodes are listed in the order specified by a left-to-right in-
order traversal of the ratchet tree. Each node is listed between its order traversal of the ratchet tree. Each node is listed between its
left subtree and its right subtree. (This is the same ordering as left subtree and its right subtree. (This is the same ordering as
specified for the array-based trees outlined in Appendix C.) specified for the array-based trees outlined in Appendix C.)
If the tree has 2^d leaves, then it has 2^(d+1) - 1 nodes. The If the tree has 2^d leaves, then it has 2^(d+1) - 1 nodes. The
ratchet_tree vector logically has this number of entries, but the ratchet_tree vector logically has this number of entries, but the
sender MUST NOT include blank nodes after the last non-blank node. sender MUST NOT include blank nodes after the last non-blank node.
The receiver MUST check that the last node in ratchet_tree is non- The receiver MUST check that the last node in ratchet_tree is non-
blank, and extend it to the right until it has a length of the form blank, and then extend the tree to the right until it has a length of
2^(d+1) - 1, adding the minimum number of blank values possible. the form 2^(d+1) - 1, adding the minimum number of blank values
(Obviously, this may be done "virtually", by synthesizing blank nodes possible. (Obviously, this may be done "virtually", by synthesizing
when required, as opposed to actually changing the structure in blank nodes when required, as opposed to actually changing the
memory.) structure in memory.)
The leaves of the tree are stored in even-numbered entries in the The leaves of the tree are stored in even-numbered entries in the
array (the leaf with index L in array position 2*L). The root node array (the leaf with index L in array position 2*L). The root node
of the tree is at position 2^d - 1 of the array. Intermediate parent of the tree is at position 2^d - 1 of the array. Intermediate parent
nodes can be identified by performing the same calculation to the nodes can be identified by performing the same calculation to the
subarrays to the left and right of the root, following something like subarrays to the left and right of the root, following something like
the following algorithm: the following algorithm:
# Assuming a class Node that has left and right members # Assuming a class Node that has left and right members
def subtree_root(nodes): def subtree_root(nodes):
# If there is only one node in the array return it # If there is only one node in the array, return it
if len(nodes) == 1: if len(nodes) == 1:
return Node(nodes[0]) return Node(nodes[0])
# Otherwise, the length of the array MUST be odd # Otherwise, the length of the array MUST be odd
if len(nodes) % 2 == 0: if len(nodes) % 2 == 0:
raise Exception("Malformed node array {}", len(nodes)) raise Exception("Malformed node array {}", len(nodes))
# Identify the root of the subtree # Identify the root of the subtree
d = 0 d = 0
while (2**(d+1)) < len(nodes): while (2**(d+1)) < len(nodes):
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W _ Z _ W _ Z _
/ \ / \ / \ / \ / \ / \ / \ / \
A B C D E F _ _ A B C D E F _ _
1 1
0 1 2 3 4 5 6 7 8 9 0 0 1 2 3 4 5 6 7 8 9 0
<-----------> R <-----------> <-----------> R <----------->
<---> R <---> <---> R <---> <---> R <---> <---> R <--->
- R - - R - - R - - R - - R - - R - - R - - R -
Figure 28: Left-to-right in-order traversal of a six-member tree Figure 28: Left-to-Right In-Order Traversal of a Six-Member Tree
The presence of a ratchet_tree extension in a GroupInfo message does The presence of a ratchet_tree extension in a GroupInfo message does
not result in any changes to the GroupContext extensions for the not result in any changes to the GroupContext extensions for the
group. The ratchet tree provided is simply stored by the client and group. The ratchet tree provided is simply stored by the client and
used for MLS operations. used for MLS operations.
If this extension is not provided in a Welcome message, then the If this extension is not provided in a Welcome message, then the
client will need to fetch the ratchet tree over some other channel client will need to fetch the ratchet tree over some other channel
before it can generate or process Commit messages. Applications before it can generate or process Commit messages. Applications
should ensure that this out-of-band channel is provided with security should ensure that this out-of-band channel is provided with security
skipping to change at page 118, line 45 skipping to change at line 4968
Proposal and Commit messages. For example, an application that Proposal and Commit messages. For example, an application that
encrypts Proposal and Commit messages might distribute ratchet trees encrypts Proposal and Commit messages might distribute ratchet trees
encrypted using a key exchanged over the MLS channel. encrypted using a key exchanged over the MLS channel.
Regardless of how the client obtains the tree, the client MUST verify Regardless of how the client obtains the tree, the client MUST verify
that the root hash of the ratchet tree matches the tree_hash of the that the root hash of the ratchet tree matches the tree_hash of the
GroupContext before using the tree for MLS operations. GroupContext before using the tree for MLS operations.
13. Extensibility 13. Extensibility
The base MLS protocol can be extended in a few ways. New The base MLS protocol can be extended in a few ways. New cipher
ciphersuites can be added to enable the use of new cryptographic suites can be added to enable the use of new cryptographic
algorithms. New types of proposals can be used to perform new algorithms. New types of proposals can be used to perform new
actions within an epoch. Extension fields can be used to add actions within an epoch. Extension fields can be used to add
additional information to the protocol. In this section, we discuss additional information to the protocol. In this section, we discuss
some constraints on these extensibility mechanisms that are necessary some constraints on these extensibility mechanisms that are necessary
to ensure broad interoperability. to ensure broad interoperability.
13.1. Additional Ciphersuites 13.1. Additional Cipher Suites
As discussed in Section 5.1, MLS allows the participants in a group As discussed in Section 5.1, MLS allows the participants in a group
to negotiate the cryptographic algorithms used within the group. to negotiate the cryptographic algorithms used within the group.
This extensibility is important for maintaining the security of the This extensibility is important for maintaining the security of the
protocol over time [RFC7696]. It also creates a risk of protocol over time [RFC7696]. It also creates a risk of
interoperability failure due to clients not supporting a common interoperability failure due to clients not supporting a common
ciphersuite. cipher suite.
The ciphersuite registry defined in Section 17.1 attempts to strike a The cipher suite registry defined in Section 17.1 attempts to strike
balance on this point. On the one hand, the base policy for the a balance on this point. On the one hand, the base policy for the
registry is Specification Required, a fairly low bar designed to registry is Specification Required, a fairly low bar designed to
avoid the need for standards work in cases where different ciphers avoid the need for standards work in cases where different ciphers
are needed for niche applications. There is a higher bar (Standards are needed for niche applications. On the other hand, there is a
Action) for ciphers to set the Recommended field in the registry. higher bar (Standards Action) for ciphers to set the Recommended
This higher bar is there in part to ensure that the interoperability field in the registry. This higher bar is there in part to ensure
implications of new ciphersuites are considered. that the interoperability implications of new cipher suites are
considered.
MLS ciphersuites are defined independent of MLS versions, so that in MLS cipher suites are defined independent of MLS versions, so that in
principle the same ciphersuite can be used across versions. principle, the same cipher suite can be used across versions.
Standards work defining new versions of MLS should consider whether Standards work defining new versions of MLS should consider whether
it is desirable for the new version to be compatible with existing it is desirable for the new version to be compatible with existing
ciphersuites, or whether the new version should rule out some cipher suites, or whether the new version should rule out some cipher
ciphersuites. For example, a new version could follow the example of suites. For example, a new version could follow the example of
HTTP/2, which restricted the set of allowed TLS ciphers (see HTTP/2, which restricted the set of allowed TLS ciphers (see
Section 9.2.2 of [RFC9113]. Section 9.2.2 of [RFC9113]).
13.2. Proposals 13.2. Proposals
Commit messages do not have an extension field because the set of Commit messages do not have an extension field because the set of
proposals is extensible. As discussed in Section 12.4, Proposals proposals is extensible. As discussed in Section 12.4, Proposals
with a non-default proposal type MUST NOT be included in a commit with a non-default proposal type MUST NOT be included in a commit
unless the proposal type is supported by all the members of the group unless the proposal type is supported by all the members of the group
that will process the Commit. that will process the Commit.
13.3. Credential Extensibility 13.3. Credential Extensibility
In order to ensure that MLS provides meaningful authentication it is In order to ensure that MLS provides meaningful authentication, it is
important that each member is able to authenticate some identity important that each member is able to authenticate some identity
information for each other member. Identity information is encoded information for each other member. Identity information is encoded
in Credentials, so this property is provided by ensuring that members in Credentials, so this property is provided by ensuring that members
use compatible credential types. use compatible credential types.
The types of credential that may be used in a group is restricted to The only types of credential that may be used in a group are those
what all members of the group support, as specified by the that all members of the group support, as specified by the
capabilities field of each LeafNode in the ratchet tree. An capabilities field of each LeafNode in the ratchet tree. An
application can introduce new credential types by choosing an application can introduce new credential types by choosing an
unallocated identifier from the registry in Section 17.5 and unallocated identifier from the registry in Section 17.5 and
indicating support for the credential type in published LeafNodes, indicating support for the credential type in published LeafNodes,
whether in Update proposals to existing groups or KeyPackages that whether in Update proposals to existing groups or KeyPackages that
are added to new groups. Once all members in a group indicate are added to new groups. Once all members in a group indicate
support for the credential type, members can start using LeafNodes support for the credential type, members can start using LeafNodes
with the new credential. Application may enforce that certain with the new credential. Application may enforce that certain
credential types always remain supported by adding a credential types always remain supported by adding a
required_capabilities extension to the group's GroupContext, which required_capabilities extension to the group's GroupContext, which
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This extension mechanism is designed to allow for the secure and This extension mechanism is designed to allow for the secure and
forward-compatible negotiation of extensions. For this to work, forward-compatible negotiation of extensions. For this to work,
implementations MUST correctly handle extensible fields: implementations MUST correctly handle extensible fields:
* A client that posts a KeyPackage MUST support all parameters * A client that posts a KeyPackage MUST support all parameters
advertised in it. Otherwise, another client might fail to advertised in it. Otherwise, another client might fail to
interoperate by selecting one of those parameters. interoperate by selecting one of those parameters.
* A client processing a KeyPackage object MUST ignore all * A client processing a KeyPackage object MUST ignore all
unrecognized values in the capabilities field of the LeafNode, and unrecognized values in the capabilities field of the LeafNode and
all unknown extensions in the extensions and leaf_node.extensions all unknown extensions in the extensions and leaf_node.extensions
fields. Otherwise, it could fail to interoperate with newer fields. Otherwise, it could fail to interoperate with newer
clients. clients.
* A client processing a GroupInfo object MUST ignore all * A client processing a GroupInfo object MUST ignore all
unrecognized extensions in the extensions field. unrecognized extensions in the extensions field.
* Any field containing a list of extensions MUST NOT have more than * Any field containing a list of extensions MUST NOT have more than
one extension of any given type. one extension of any given type.
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the group MUST be supported by all members of the group. the group MUST be supported by all members of the group.
The parameters of a group may be changed by sending a The parameters of a group may be changed by sending a
GroupContextExtensions proposal to enable additional extensions GroupContextExtensions proposal to enable additional extensions
(Section 12.1.7), or by reinitializing the group (Section 11.2). (Section 12.1.7), or by reinitializing the group (Section 11.2).
13.5. GREASE 13.5. GREASE
As described in Section 13.4, clients are required to ignore unknown As described in Section 13.4, clients are required to ignore unknown
values for certain parameters. To help ensure that other clients values for certain parameters. To help ensure that other clients
implement this behavior, a client can follow the “Generate Random implement this behavior, a client can follow the "Generate Random
Extensions And Sustain Extensibility” or GREASE approach described in Extensions And Sustain Extensibility" or GREASE approach described in
[RFC8701]. In the context of MLS, this means that a client [RFC8701]. In the context of MLS, this means that a client
generating a KeyPackage, LeafNode, or GroupInfo object includes generating a KeyPackage, LeafNode, or GroupInfo object includes
random values in certain fields which would be ignored by a random values in certain fields which would be ignored by a correctly
correctly-implemented client processing the message. A client that implemented client processing the message. A client that incorrectly
incorrectly rejects unknown code points will fail to process such a rejects unknown code points will fail to process such a message,
message, providing a signal to its implementer that the client needs providing a signal to its implementer that the client needs to be
to be fixed. fixed.
When generating the following fields, an MLS client SHOULD include a When generating the following fields, an MLS client SHOULD include a
random selection of values chosen from these GREASE values: random selection of values chosen from these GREASE values:
* LeafNode.capabilities.ciphersuites * LeafNode.capabilities.cipher_suites
* LeafNode.capabilities.extensions * LeafNode.capabilities.extensions
* LeafNode.capabilities.proposals * LeafNode.capabilities.proposals
* LeafNode.capabilities.credentials * LeafNode.capabilities.credentials
* LeafNode.extensions * LeafNode.extensions
* KeyPackage.extensions * KeyPackage.extensions
* GroupInfo.extensions * GroupInfo.extensions
For the KeyPackage and GroupInfo extensions, the extension_data for For the KeyPackage and GroupInfo extensions, the extension_data for
GREASE extensions MAY have any contents selected by the sender, since GREASE extensions MAY have any contents selected by the sender, since
they will be ignored by a correctly-implemented receiver. For they will be ignored by a correctly implemented receiver. For
example, a sender might populate these extensions with a randomly- example, a sender might populate these extensions with a randomly
sized amount of random data. sized amount of random data.
Note that any GREASE values added to LeafNode.extensions need to be
reflected in LeafNode.capabilities.extensions, since the LeafNode
validation process described in Section 7.3 requires that these two
fields be consistent.
GREASE values MUST NOT be sent in the following fields, because an GREASE values MUST NOT be sent in the following fields, because an
unsupported value in one these fields (including a GREASE value), unsupported value in one these fields (including a GREASE value) will
will cause the enclosing message to be rejected: cause the enclosing message to be rejected:
* Proposal.proposal_type * Proposal.proposal_type
* Credential.credential_type * Credential.credential_type
* GroupContext.extensions * GroupContext.extensions
* GroupContextExtensions.extensions * GroupContextExtensions.extensions
A set of values reserved for GREASE have been registered in the
various registries in Section 17. This prevents conflict between Values reserved for GREASE have been registered in the various
GREASE and real future values. The following values are reserved in registries in Section 17. This prevents conflict between GREASE and
each registry: 0x0A0A, 0x1A1A, 0x2A2A, 0x3A3A, 0x4A4A, 0x5A5A, real future values. The following values are reserved in each
0x6A6A, 0x7A7A, 0x8A8A, 0x9A9A, 0xAAAA, 0xBABA, 0xCACA, 0xDADA, and registry: 0x0A0A, 0x1A1A, 0x2A2A, 0x3A3A, 0x4A4A, 0x5A5A, 0x6A6A,
0xEAEA. (The value 0xFAFA falls within the private use range.) 0x7A7A, 0x8A8A, 0x9A9A, 0xAAAA, 0xBABA, 0xCACA, 0xDADA, and 0xEAEA.
These values MUST only appear in the fields listed above, and not, (The value 0xFAFA falls within the private use range.) These values
for example, in the proposal_type field of a Proposal. Clients MUST MUST only appear in the fields listed above, and not, for example, in
NOT implement any special processing rules for how to handle these the proposal_type field of a Proposal. Clients MUST NOT implement
values when receiving them, since this negates their utility for any special processing rules for how to handle these values when
detecting extensibility failures. receiving them, since this negates their utility for detecting
extensibility failures.
GREASE values MUST be handled using normal logic for processing GREASE values MUST be handled using normal logic for processing
unsupported values. When comparing lists of capabilities to identify unsupported values. When comparing lists of capabilities to identify
mutually-supported capabilities, clients MUST represent their own mutually supported capabilities, clients MUST represent their own
capabilities with a list containing only the capabilities actually capabilities with a list containing only the capabilities actually
supported, without any GREASE values. In other words, lists supported, without any GREASE values. In other words, lists
including GREASE values are only sent to other clients; including GREASE values are only sent to other clients;
representations of a client's own capabilities MUST NOT contain representations of a client's own capabilities MUST NOT contain
GREASE values. GREASE values.
14. Sequencing of State Changes 14. Sequencing of State Changes
Each Commit message is premised on a given starting state, indicated Each Commit message is premised on a given starting state, indicated
by the epoch field of the enclosing FramedContent. If the changes by the epoch field of the enclosing FramedContent. If the changes
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been accepted. been accepted.
Regardless of how messages are kept in sequence, there is a risk that Regardless of how messages are kept in sequence, there is a risk that
in a sufficiently busy group, a given member may never be able to in a sufficiently busy group, a given member may never be able to
send a Commit message because they always lose to other members. The send a Commit message because they always lose to other members. The
degree to which this is a practical problem will depend on the degree to which this is a practical problem will depend on the
dynamics of the application. dynamics of the application.
15. Application Messages 15. Application Messages
The primary purpose of handshake messages are to provide an The primary purpose of handshake messages is to provide an
authenticated group key exchange to clients. In order to protect authenticated group key exchange to clients. In order to protect
application messages sent among the members of a group, the application messages sent among the members of a group, the
encryption_secret provided by the key schedule is used to derive a encryption_secret provided by the key schedule is used to derive a
sequence of nonces and keys for message encryption. Every epoch sequence of nonces and keys for message encryption. Every epoch
moves the key schedule forward which triggers the creation of a new moves the key schedule forward, which triggers the creation of a new
secret tree, as described in Section 9, along with a new set of secret tree, as described in Section 9, along with a new set of
symmetric ratchets of nonces and keys for each member. symmetric ratchets of nonces and keys for each member.
Each client maintains their own local copy of the key schedule for Each client maintains their own local copy of the key schedule for
each epoch during which they are a group member. They derive new each epoch during which they are a group member. They derive new
keys, nonces, and secrets as needed while deleting old ones as soon keys, nonces, and secrets as needed while deleting old ones as soon
as they have been used. as they have been used.
The group identifier and epoch allow a recipient to know which group The group identifier and epoch allow a recipient to know which group
secrets should be used and from which epoch_secret to start computing secrets should be used and from which epoch_secret to start computing
other secrets. The sender identifier and content type is used to other secrets. The sender identifier and content type are used to
identify which symmetric ratchet to use from the secret tree. The identify which symmetric ratchet to use from the secret tree. The
generation counter determines how far into the ratchet to iterate in generation counter determines how far into the ratchet to iterate in
order to produce the required nonce and key for encryption or order to produce the required nonce and key for encryption or
decryption. decryption.
15.1. Padding 15.1. Padding
Application messages MAY be padded to provide some resistance against Application messages MAY be padded to provide some resistance against
traffic analysis techniques over encrypted traffic [CLINIC] [HCJ16]. traffic analysis techniques over encrypted traffic [CLINIC] [HCJ16].
While MLS might deliver the same payload less frequently across a lot While MLS might deliver the same payload less frequently across a lot
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chosen by the sender at the time of message encryption. Senders may chosen by the sender at the time of message encryption. Senders may
use padding to reduce the ability of attackers outside the group to use padding to reduce the ability of attackers outside the group to
infer the size of the encrypted content. Note, however, that the infer the size of the encrypted content. Note, however, that the
transports used to carry MLS messages may have maximum message sizes, transports used to carry MLS messages may have maximum message sizes,
so padding schemes SHOULD avoid increasing message size beyond any so padding schemes SHOULD avoid increasing message size beyond any
such limits that exist in a given deployment scenario. such limits that exist in a given deployment scenario.
15.2. Restrictions 15.2. Restrictions
During each epoch, senders MUST NOT encrypt more data than permitted During each epoch, senders MUST NOT encrypt more data than permitted
by the security bounds of the AEAD scheme used by the security bounds of the AEAD scheme used [CFRG-AEAD-LIMITS].
[I-D.irtf-cfrg-aead-limits].
Note that each change to the group through a handshake message will Note that each change to the group through a handshake message will
also set a new encryption_secret. Hence this change MUST be applied also set a new encryption_secret. Hence this change MUST be applied
before encrypting any new application message. This is required both before encrypting any new application message. This is required both
to ensure that any users removed from the group can no longer receive to ensure that any users removed from the group can no longer receive
messages and to (potentially) recover confidentiality and messages and to (potentially) recover confidentiality and
authenticity for future messages despite a past state compromise. authenticity for future messages despite a past state compromise.
15.3. Delayed and Reordered Application messages 15.3. Delayed and Reordered Application Messages
Since each application message contains the group identifier, the Since each application message contains the group identifier, the
epoch, and a generation counter, a client can receive messages out of epoch, and a generation counter, a client can receive messages out of
order. When messages are received out of order, the client moves the order. When messages are received out of order, the client moves the
sender ratchet forward to match the received generation counter. Any sender ratchet forward to match the received generation counter. Any
unused nonce and key pairs from the ratchet are potentially stored so unused nonce and key pairs from the ratchet are potentially stored so
that they can be used to decrypt the messages which were delayed or that they can be used to decrypt the messages that were delayed or
reordered. reordered.
Applications SHOULD define a policy on how long to keep unused nonce Applications SHOULD define a policy on how long to keep unused nonce
and key pairs for a sender, and the maximum number to keep. This is and key pairs for a sender, and the maximum number to keep. This is
in addition to ensuring that these secrets are deleted according to in addition to ensuring that these secrets are deleted according to
the deletion schedule defined in Section 9.2. Applications SHOULD the deletion schedule defined in Section 9.2. Applications SHOULD
also define a policy limiting the maximum number of steps that also define a policy limiting the maximum number of steps that
clients will move the ratchet forward in response to a new message. clients will move the ratchet forward in response to a new message.
Messages received with a generation counter that's too much higher Messages received with a generation counter that is too much higher
than the last message received would then be rejected. This avoids than the last message received would then be rejected. This avoids
causing a denial-of-service attack by requiring the recipient to causing a denial-of-service attack by requiring the recipient to
perform an excessive number of key derivations. For example, a perform an excessive number of key derivations. For example, a
malicious group member could send a message with generation = malicious group member could send a message with generation =
0xffffffff at the beginning of a new epoch, forcing recipients to 0xffffffff at the beginning of a new epoch, forcing recipients to
perform billions of key derivations unless they apply limits of the perform billions of key derivations unless they apply limits of the
type discussed above. type discussed above.
16. Security Considerations 16. Security Considerations
The security goals of MLS are described in The security goals of MLS are described in [MLS-ARCH]. We describe
[I-D.ietf-mls-architecture]. We describe here how the protocol here how the protocol achieves its goals at a high level, though a
achieves its goals at a high level, though a complete security complete security analysis is outside of the scope of this document.
analysis is outside of the scope of this document. The Security The Security Considerations section of [MLS-ARCH] provides some
Considerations section of [I-D.ietf-mls-architecture] provides some
citations to detailed security analyses. citations to detailed security analyses.
16.1. Transport Security 16.1. Transport Security
Because MLS messages are protected at the message level, the Because MLS messages are protected at the message level, the
confidentiality and integrity of the group state do not depend on confidentiality and integrity of the group state do not depend on
those messages being protected in transit. However, an attacker who those messages being protected in transit. However, an attacker who
can observe those messages in transit will be able to learn about the can observe those messages in transit will be able to learn about the
group state, including potentially the group membership (see group state, including potentially the group membership (see
Section 16.4.3 below). Such an attacker might also be able to mount Section 16.4.3 below). Such an attacker might also be able to mount
denial-of-service attacks on the group or exclude new members by denial-of-service attacks on the group or exclude new members by
selectively removing messages in transit. In order to prevent this selectively removing messages in transit. In order to prevent this
form of attack, it is RECOMMENDED that all MLS messages be carried form of attack, it is RECOMMENDED that all MLS messages be carried
over a secure transport such as TLS [RFC8446] or QUIC [RFC9000]. over a secure transport such as TLS [RFC8446] or QUIC [RFC9000].
16.2. Confidentiality of the Group Secrets 16.2. Confidentiality of Group Secrets
Group secrets are partly derived from the output of a ratchet tree. Group secrets are partly derived from the output of a ratchet tree.
Ratchet trees work by assigning each member of the group to a leaf in Ratchet trees work by assigning each member of the group to a leaf in
the tree and maintaining the following property: the private key of a the tree and maintaining the following property: the private key of a
node in the tree is known only to members of the group that are node in the tree is known only to members of the group that are
assigned a leaf in the node's subtree. This is called the _ratchet assigned a leaf in the node's subtree. This is called the _tree
tree invariant_ and it makes it possible to encrypt to all group invariant_, and it makes it possible to encrypt to all group members
members except one, with a number of ciphertexts that is logarithmic except one, with a number of ciphertexts that is logarithmic in the
in the number of group members. number of group members.
The ability to efficiently encrypt to all members except one allows The ability to efficiently encrypt to all members except one allows
members to be securely removed from a group. It also allows a member members to be securely removed from a group. It also allows a member
to rotate their keypair such that the old private key can no longer to rotate their key pair such that the old private key can no longer
be used to decrypt new messages. be used to decrypt new messages.
16.3. Confidentiality of Sender Data 16.3. Confidentiality of Sender Data
The PrivateMessage framing encrypts "sender data" that identifies The PrivateMessage framing encrypts "sender data" that identifies
which group member sent an encrypted message, as described in which group member sent an encrypted message, as described in
Section 6.3.2. As with the QUIC header protection scheme [RFC9001], Section 6.3.2. As with the QUIC header protection scheme [RFC9001],
Section 5.4, this scheme is a variant of the HN1 construction Section 5.4, this scheme is a variant of the HN1 construction
analyzed in [NAN]. A sample of the ciphertext is combined with a analyzed in [NAN]. A sample of the ciphertext is combined with a
sender_data_secret to derive a key and nonce that are used for AEAD sender_data_secret to derive a key and nonce that are used for AEAD
encryption of the sender data. encryption of the sender data.
pseudocode (key, nonce) = PRF(sender_data_secret, sample) (key, nonce) = PRF(sender_data_secret, sample)
encrypted_sender_data = AEAD.Seal(key, nonce, sender_data_aad, encrypted_sender_data =
sender_data) AEAD.Seal(key, nonce, sender_data_aad, sender_data)
The only differences between this construction and HN1 as described The only differences between this construction and HN1 as described
in [NAN] are (1) that it uses authenticated encryption instead of in [NAN] are that it (1) uses authenticated encryption instead of
unauthenticated encryption and (2) that it protects information used unauthenticated encryption and (2) protects information used to
to derive a nonce instead of the nonce itself. derive a nonce instead of the nonce itself.
Since the sender_data_secret is distinct from the content encryption Since the sender_data_secret is distinct from the content encryption
key, it follows that the sender data encryption scheme achieves AE2 key, it follows that the sender data encryption scheme achieves AE2
security as defined in [NAN], and therefore guarantees the security as defined in [NAN], and therefore guarantees the
confidentiality of the sender data. confidentiality of the sender data.
Use of the same sender_data_secret and ciphertext sample more than Use of the same sender_data_secret and ciphertext sample more than
once risks compromising sender data protection by reusing an AEAD once risks compromising sender data protection by reusing an AEAD
(key, nonce) pair. For example, in many AEAD schemes, reusing a key (key, nonce) pair. For example, in many AEAD schemes, reusing a key
and nonce reveals the exclusive OR of the two plaintexts. Assuming and nonce reveals the exclusive OR of the two plaintexts. Assuming
the ciphertext output of the AEAD algorithm is indistinguishable from the ciphertext output of the AEAD algorithm is indistinguishable from
random data (i.e., the AEAD is AE1-secure in the phrasing of [NAN]), random data (i.e., the AEAD is AE1-secure in the phrasing of [NAN]),
the odds of two ciphertext samples being identical is roughly the odds of two ciphertext samples being identical is roughly
2^(-L/2), i.e., the birthday bound. 2^(-L/2), i.e., the birthday bound.
The AEAD algorithms for ciphersuites defined in this document all The AEAD algorithms for cipher suites defined in this document all
provide this property. The size of the sample depends on the provide this property. The size of the sample depends on the cipher
ciphersuite's hash function, but in all cases, the probability of suite's hash function, but in all cases, the probability of collision
collision is no more than 2^-128. Any future ciphersuite MUST use an is no more than 2^-128. Any future cipher suite MUST use an
AE1-secure AEAD algorithm. AE1-secure AEAD algorithm.
16.4. Confidentiality of Group Metadata 16.4. Confidentiality of Group Metadata
MLS does not provide confidentiality protection to some messages and MLS does not provide confidentiality protection to some messages and
fields within messages: fields within messages:
* KeyPackage messages * KeyPackage messages
* GroupInfo messages * GroupInfo messages
skipping to change at page 129, line 8 skipping to change at line 5444
16.4.1. GroupID, Epoch, and Message Frequency 16.4.1. GroupID, Epoch, and Message Frequency
MLS provides no mechanism to protect the group ID and epoch of a MLS provides no mechanism to protect the group ID and epoch of a
message from the DS, so the group ID and the frequency of messages message from the DS, so the group ID and the frequency of messages
and epoch changes are not protected against inspection by the DS. and epoch changes are not protected against inspection by the DS.
However, any modifications to these will cause decryption failure. However, any modifications to these will cause decryption failure.
16.4.2. Group Extensions 16.4.2. Group Extensions
A group's extensions are first set by the group's creator and then A group's extensions are first set by the group's creator and then
updated by GroupContextExtensions proposals. A GroupContextExtension updated by GroupContextExtensions proposals. A
proposal sent as a PublicMessage leaks the group's extensions. GroupContextExtensions proposal sent as a PublicMessage leaks the
group's extensions.
A new member learns the group's extensions via a GroupInfo object. A new member learns the group's extensions via a GroupInfo object.
When the new member joins via a Welcome message, the Welcome When the new member joins via a Welcome message, the Welcome
message's encryption protects the GroupInfo message. When the new message's encryption protects the GroupInfo message. When the new
member joins via an external join, they must be provided with a member joins via an external join, they must be provided with a
GroupInfo object. Protection of this GroupInfo object is up to the GroupInfo object. Protection of this GroupInfo object is up to the
application -- if it is transmitted over a channel that is not application -- if it is transmitted over a channel that is not
confidential to the group and the new joiner, then it will leak the confidential to the group and the new joiner, then it will leak the
group's extensions. group's extensions.
skipping to change at page 129, line 42 skipping to change at line 5479
reconstruct the group membership. reconstruct the group membership.
Welcome messages contain a hash of each KeyPackage for which the Welcome messages contain a hash of each KeyPackage for which the
Welcome message is encrypted. If a party has access to a pool of Welcome message is encrypted. If a party has access to a pool of
KeyPackages and observes a Welcome message, then they can identify KeyPackages and observes a Welcome message, then they can identify
the KeyPackage representing the new member. If the party can also the KeyPackage representing the new member. If the party can also
associate the Welcome with a group, then the party can infer that the associate the Welcome with a group, then the party can infer that the
identified new member was added to that group. identified new member was added to that group.
Note that these information leaks reveal the group's membership only Note that these information leaks reveal the group's membership only
to the degree that that membership is revealed by the contents of a to the degree that membership is revealed by the contents of a
member's LeafNode in the ratchet tree. In some cases, this may be member's LeafNode in the ratchet tree. In some cases, this may be
quite direct, e.g., due to credentials attesting to identifiers such quite direct, e.g., due to credentials attesting to identifiers such
as email addresses. An application could construct a member's leaf as email addresses. An application could construct a member's leaf
node to be less identifying, e.g., by using a pseudonymous credential node to be less identifying, e.g., by using a pseudonymous credential
and frequently rotating encryption and signature keys. and frequently rotating encryption and signature keys.
16.5. Authentication 16.5. Authentication
The first form of authentication we provide is that group members can The first form of authentication we provide is that group members can
verify a message originated from one of the members of the group. verify a message originated from one of the members of the group.
For encrypted messages, this is guaranteed because messages are For encrypted messages, this is guaranteed because messages are
encrypted with an AEAD under a key derived from the group secrets. encrypted with an AEAD under a key derived from the group secrets.
For plaintext messages, this is guaranteed by the use of a For plaintext messages, this is guaranteed by the use of a
membership_tag which constitutes a MAC over the message, under a key membership_tag, which constitutes a MAC over the message, under a key
derived from the group secrets. derived from the group secrets.
The second form of authentication is that group members can verify a The second form of authentication is that group members can verify a
message originated from a particular member of the group. This is message originated from a particular member of the group. This is
guaranteed by a digital signature on each message from the sender's guaranteed by a digital signature on each message from the sender's
signature key. signature key.
The signature keys held by group members are critical to the security The signature keys held by group members are critical to the security
of MLS against active attacks. If a member's signature key is of MLS against active attacks. If a member's signature key is
compromised, then an attacker can create LeafNodes and KeyPackages compromised, then an attacker can create LeafNodes and KeyPackages
impersonating the member; depending on the application, this can then impersonating the member; depending on the application, this can then
allow the attacker to join the group with the compromised member's allow the attacker to join the group with the compromised member's
identity. For example, if a group has enabled external parties to identity. For example, if a group has enabled external parties to
join via external commits, then an attacker that has compromised a join via external commits, then an attacker that has compromised a
member's signature key could use an external commit to insert member's signature key could use an external Commit to insert
themselves into the group -- even using a "resync"-style external themselves into the group -- even using a "resync"-style external
commit to replace the compromised member in the group. Commit to replace the compromised member in the group.
Applications can mitigate the risks of signature key compromise using Applications can mitigate the risks of signature key compromise using
pre-shared keys. If a group requires joiners to know a PSK in pre-shared keys. If a group requires joiners to know a PSK in
addition to authenticating with a credential, then in order to mount addition to authenticating with a credential, then in order to mount
an impersonation attack, the attacker would need to compromise the an impersonation attack, the attacker would need to compromise the
relevant PSK as well as the victim's signature key. The cost of this relevant PSK as well as the victim's signature key. The cost of this
mitigation is that the application needs some external arrangement mitigation is that the application needs some external arrangement
that ensures that the legitimate members of the group have the that ensures that the legitimate members of the group have the
required PSKs. required PSKs.
skipping to change at page 131, line 15 skipping to change at line 5540
Compromise Compromise
| |
| |
| V | | V |
------------------|---------|-------------------------> ------------------|---------|------------------------->
| | Time | | Time
<-----------------| |----------------> <-----------------| |---------------->
Forward Secrecy | | Post-Compromise Forward Secrecy | | Post-Compromise
| | Security | | Security
Figure 29: Forward secrecy and post-compromise security Figure 29: Forward Secrecy and Post-Compromise Security
Post-compromise security is provided between epochs by members Post-compromise security is provided between epochs by members
regularly updating their leaf key in the ratchet tree. Updating regularly updating their leaf key in the ratchet tree. Updating
their leaf key prevents group secrets from continuing to be encrypted their leaf key prevents group secrets from continuing to be encrypted
to public keys whose private keys had previously been compromised. to public keys whose private keys had previously been compromised.
Note that sending an Update proposal does not achieve PCS until Note that sending an Update proposal does not achieve PCS until
another member includes it in a Commit. Members can achieve another member includes it in a Commit. Members can achieve
immediate PCS by sending their own Commit and populating the path immediate PCS by sending their own Commit and populating the path
field, as described in Section 12.4. To be clear, in all these field, as described in Section 12.4. To be clear, in all these
cases, the PCS guarantees come into effect when the members of the cases, the PCS guarantees come into effect when the members of the
group process the relevant Commit, not when the sender creates it. group process the relevant Commit, not when the sender creates it.
Forward secrecy between epochs is provided by deleting private keys Forward secrecy between epochs is provided by deleting private keys
from past versions of the ratchet tree, as this prevents old group from past versions of the ratchet tree, as this prevents old group
secrets from being re-derived. Forward secrecy _within_ an epoch is secrets from being re-derived. Forward secrecy _within_ an epoch is
provided by deleting message encryption keys once they've been used provided by deleting message encryption keys once they've been used
to encrypt or decrypt a message. Note that group secrets and message to encrypt or decrypt a message. Note that group secrets and message
encryption keys are shared by the group, and thus there is a risk to encryption keys are shared by the group. There is thus a risk to
forward secrecy as long as any member has not deleted these keys. forward secrecy as long as any member has not deleted these keys.
This is a particular risk if a member is offline for a long period of This is a particular risk if a member is offline for a long period of
time. Applications SHOULD have mechanisms for evicting group members time. Applications SHOULD have mechanisms for evicting group members
which are offline for too long (i.e., have not changed their key that are offline for too long (i.e., have not changed their key
within some period). within some period).
New groups are also at risk of using previously compromised keys (as New groups are also at risk of using previously compromised keys (as
with post-compromise security), if a member is added to a new group with post-compromise security) if a member is added to a new group
via an old KeyPackage whose corresponding private key has been via an old KeyPackage whose corresponding private key has been
compromised. This risk can be mitigated by having clients regularly compromised. This risk can be mitigated by having clients regularly
generate new KeyPackages and upload them to the Delivery Service. generate new KeyPackages and upload them to the Delivery Service.
This way, the key material used to add a member to a new group is This way, the key material used to add a member to a new group is
more likely to be fresh and less likely to be compromised. more likely to be fresh and less likely to be compromised.
16.7. Uniqueness of Ratchet Tree Key Pairs 16.7. Uniqueness of Ratchet Tree Key Pairs
The encryption and signature keys stored in the encryption_key and The encryption and signature keys stored in the encryption_key and
signature_key fields of ratchet tree nodes MUST be distinct from one signature_key fields of ratchet tree nodes MUST be distinct from one
another. If two members' leaf nodes have the same signature key, for another. If two members' leaf nodes have the same signature key, for
example, then the data origin authentication properties afforded by example, then the data origin authentication properties afforded by
signatures within the group are degraded. signatures within the group are degraded.
Uniqueness of keys in leaf nodes is assured by explicit checks on Uniqueness of keys in leaf nodes is assured by explicitly checking
leaf nodes being added to the tree by Add or Update proposals, or in each leaf node as it is added to the tree, whether in an Add
the path field of a Commit. Details can be found in Section 7.3, proposal, in an Update proposal, or in the path field of a Commit.
Section 12.2, and Section 12.4.2. Uniqueness of encryption keys in Details can be found in Sections 7.3, 12.2, and 12.4.2. Uniqueness
parent nodes is assured by checking that the keys in an UpdatePath of encryption keys in parent nodes is assured by checking that the
are not found elsewhere in the tree (see Section 12.4.2. keys in an UpdatePath are not found elsewhere in the tree (see
Section 12.4.2).
16.8. KeyPackage Reuse 16.8. KeyPackage Reuse
KeyPackages are intended to be used only once. That is, once a KeyPackages are intended to be used only once. That is, once a
KeyPackage has been used to introduce the corresponding client to a KeyPackage has been used to introduce the corresponding client to a
group, it SHOULD be deleted from the KeyPackage publication system. group, it SHOULD be deleted from the KeyPackage publication system.
Reuse of KeyPackages can lead to replay attacks. Reuse of KeyPackages can lead to replay attacks.
An application MAY allow for reuse of a "last resort" KeyPackage in An application MAY allow for reuse of a "last resort" KeyPackage in
order to prevent denial-of-service attacks. Since a KeyPackage is order to prevent denial-of-service attacks. Since a KeyPackage is
needed to add a client to a new group, an attacker could prevent a needed to add a client to a new group, an attacker could prevent a
client being added to new groups by exhausting all available client from being added to new groups by exhausting all available
KeyPackages. To prevent such a denial-of-service attack, the KeyPackages. To prevent such a denial-of-service attack, the
KeyPackage publication system SHOULD rate-limit KeyPackage requests, KeyPackage publication system SHOULD rate-limit KeyPackage requests,
especially if not authenticated. especially if not authenticated.
16.9. Delivery Service Compromise 16.9. Delivery Service Compromise
MLS is designed to protect the confidentiality and integrity of the MLS is designed to protect the confidentiality and integrity of the
group data even in the face of a compromised DS. However, a group data even in the face of a compromised DS. However, a
compromised DS can still mount some attacks. While it cannot forge compromised DS can still mount some attacks. While it cannot forge
messages, it can selectively delay or remove them. This can in some messages, it can selectively delay or remove them. In some cases,
cases be observed by detecting gaps in the per-sender generation this can be observed by detecting gaps in the per-sender generation
counter, though it may not always be possible to distinguish an counter, though it may not always be possible to distinguish an
attack from message loss. In addition, the DS can permanently block attack from message loss. In addition, the DS can permanently block
messages to and from a group member. This will not always be messages to and from a group member. This will not always be
detectable by other members. If an application uses the DS to detectable by other members. If an application uses the DS to
resolve conflicts between simultaneous Commits (see Section 14), it resolve conflicts between simultaneous Commits (see Section 14), it
is also possible for the DS to influence which Commit is applied, is also possible for the DS to influence which Commit is applied,
even to the point of preventing a member from ever having its Commits even to the point of preventing a member from ever having its Commits
applied. applied.
When put together, these abilities potentially allow a DS to collude When put together, these abilities potentially allow a DS to collude
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16.10. Authentication Service Compromise 16.10. Authentication Service Compromise
Authentication Service compromise is much more serious than Authentication Service compromise is much more serious than
compromise of the Delivery Service. A compromised AS can assert a compromise of the Delivery Service. A compromised AS can assert a
binding for a signature key and identity pair of its choice, thus binding for a signature key and identity pair of its choice, thus
allowing impersonation of a given user. This ability is sufficient allowing impersonation of a given user. This ability is sufficient
to allow the AS to join new groups as if it were that user. to allow the AS to join new groups as if it were that user.
Depending on the application architecture, it may also be sufficient Depending on the application architecture, it may also be sufficient
to allow the compromised AS to join the group as an existing user, to allow the compromised AS to join the group as an existing user,
for instance as if it were a new device associated with the same for instance, as if it were a new device associated with the same
user. If the application uses a transparency mechanism such as user. If the application uses a transparency mechanism such as
CONIKS [CONIKS] or Key Transparency [KT], then it may be possible for CONIKS [CONIKS] or Key Transparency [KT], then it may be possible for
end users to detect this kind of misbehavior by the AS. It is also end users to detect this kind of misbehavior by the AS. It is also
possible to construct schemes in which the various clients owned by a possible to construct schemes in which the various clients owned by a
user vouch for each other, e.g., by signing each others' keys. user vouch for each other, e.g., by signing each others' keys.
16.11. Additional Policy Enforcement 16.11. Additional Policy Enforcement
The DS and AS may also apply additional policies to MLS operations to The DS and AS may also apply additional policies to MLS operations to
obtain additional security properties. For example, MLS enables any obtain additional security properties. For example, MLS enables any
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encrypts a sequence of path secrets to different subtrees of the encrypts a sequence of path secrets to different subtrees of the
group's ratchet trees. These path secrets should be derived in a group's ratchet trees. These path secrets should be derived in a
sequence as described in Section 7.4, but the UpdatePath syntax sequence as described in Section 7.4, but the UpdatePath syntax
allows the sender to encrypt arbitrary, unrelated secrets. The allows the sender to encrypt arbitrary, unrelated secrets. The
syntax also does not guarantee that the encrypted path secret for a syntax also does not guarantee that the encrypted path secret for a
given node corresponds to the public key provided for that node. given node corresponds to the public key provided for that node.
Both of these types of corruption will cause processing of a Commit Both of these types of corruption will cause processing of a Commit
to fail for some members of the group. If the public key for a node to fail for some members of the group. If the public key for a node
does not match the path secret, then the members that decrypt that does not match the path secret, then the members that decrypt that
path secret will reject the commit based on this mismatch. If the path secret will reject the Commit based on this mismatch. If the
path secret sequence is incorrect at some point, then members that path secret sequence is incorrect at some point, then members that
can decrypt nodes before that point will compute a different public can decrypt nodes before that point will compute a different public
key for the mismatched node than the one in the UpdatePath, which key for the mismatched node than the one in the UpdatePath, which
also causes the Commit to fail. Applications SHOULD provide also causes the Commit to fail. Applications SHOULD provide
mechanisms for failed commits to be reported, so that group members mechanisms for failed commits to be reported, so that group members
who were not able to recognize the error themselves can reinitialize who were not able to recognize the error themselves can reinitialize
the group if necessary. the group if necessary.
Even with such an error reporting mechanism in place, however, it is Even with such an error reporting mechanism in place, however, it is
still possible for members to get locked out of the group by a still possible for members to get locked out of the group by a
malformed commit. Since malformed Commits can only be recognized by malformed Commit. Since malformed Commits can only be recognized by
certain members of the group, in an asynchronous application, it may certain members of the group, in an asynchronous application, it may
be the case that all members that could detect a fault in a Commit be the case that all members that could detect a fault in a Commit
are offline. In such a case, the Commit will be accepted by the are offline. In such a case, the Commit will be accepted by the
group, and the resulting state possibly used as the basis for further group, and the resulting state will possibly be used as the basis for
Commits. When the affected members come back online, they will further Commits. When the affected members come back online, they
reject the first commit, and thus be unable to catch up with the will reject the first Commit, and thus be unable to catch up with the
group. These members will either need to add themselves back with an group. These members will need to either add themselves back with an
external Commit, or reinitialize the group from scratch. external Commit or reinitialize the group from scratch.
Applications can address this risk by requiring certain members of Applications can address this risk by requiring certain members of
the group to acknowledge successful processing of a Commit before the the group to acknowledge successful processing of a Commit before the
group regards the Commit as accepted. The minimum set of group regards the Commit as accepted. The minimum set of
acknowledgements necessary to verify that a Commit is well-formed acknowledgements necessary to verify that a Commit is well-formed
comprises an acknowledgement from one member per node in the comprises an acknowledgement from one member per node in the
UpdatePath, that is, one member from each subtree rooted in the UpdatePath, that is, one member from each subtree rooted in the
copath node corresponding to the node in the UpdatePath. MLS does copath node corresponding to the node in the UpdatePath. MLS does
not provide a built-in mechanism for such acknowledgements, but they not provide a built-in mechanism for such acknowledgements, but they
can be added at the application layer. can be added at the application layer.
17. IANA Considerations 17. IANA Considerations
This document requests the creation of the following new IANA IANA has created the following registries:
registries:
* MLS Ciphersuites (Section 17.1) * MLS Cipher Suites (Section 17.1)
* MLS Wire Formats (Section 17.2) * MLS Wire Formats (Section 17.2)
* MLS Extension Types (Section 17.3) * MLS Extension Types (Section 17.3)
* MLS Proposal Types (Section 17.4) * MLS Proposal Types (Section 17.4)
* MLS Credential Types (Section 17.5) * MLS Credential Types (Section 17.5)
* MLS Signature Labels (Section 17.6) * MLS Signature Labels (Section 17.6)
* MLS Public Key Encryption Labels (Section 17.7) * MLS Public Key Encryption Labels (Section 17.7)
* MLS Exporter Labels (Section 17.8) * MLS Exporter Labels (Section 17.8)
All of these registries should be under a heading of "Messaging Layer All of these registries are under the "Messaging Layer Security"
Security", and assignments are made via the Specification Required group registry heading, and assignments are made via the
policy [RFC8126]. See Section 17.9 for additional information about Specification Required policy [RFC8126]. See Section 17.9 for
the MLS Designated Experts (DEs). additional information about the MLS Designated Experts (DEs).
RFC EDITOR: Please replace XXXX throughout with the RFC number
assigned to this document
17.1. MLS Ciphersuites 17.1. MLS Cipher Suites
A ciphersuite is a combination of a protocol version and the set of A cipher suite is a combination of a protocol version and the set of
cryptographic algorithms that should be used. cryptographic algorithms that should be used.
Ciphersuite names follow the naming convention: Cipher suite names follow the naming convention:
CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE; CipherSuite MLS_LVL_KEM_AEAD_HASH_SIG = VALUE;
Where VALUE is represented as a sixteen-bit integer: Where VALUE is represented as a 16-bit integer:
uint16 CipherSuite; uint16 CipherSuite;
+===========+==================================+ +===========+==================================+
| Component | Contents | | Component | Contents |
+===========+==================================+ +===========+==================================+
| LVL | The security level (in bits) | | LVL | The security level (in bits) |
+-----------+----------------------------------+ +-----------+----------------------------------+
| KEM | The KEM algorithm used for HPKE | | KEM | The KEM algorithm used for HPKE |
| | in ratchet tree operations | | | in ratchet tree operations |
+-----------+----------------------------------+ +-----------+----------------------------------+
| AEAD | The AEAD algorithm used for HPKE | | AEAD | The AEAD algorithm used for HPKE |
| | and message protection | | | and message protection |
+-----------+----------------------------------+ +-----------+----------------------------------+
| HASH | The hash algorithm used for HPKE | | HASH | The hash algorithm used for HPKE |
| | and the MLS transcript hash | | | and the MLS transcript hash |
+-----------+----------------------------------+ +-----------+----------------------------------+
| SIG | The Signature algorithm used for | | SIG | The signature algorithm used for |
| | message authentication | | | message authentication |
+-----------+----------------------------------+ +-----------+----------------------------------+
Table 5 Table 5
The columns in the registry are as follows: The columns in the registry are as follows:
* Value: The numeric value of the ciphersuite * Value: The numeric value of the cipher suite
* Name: The name of the ciphersuite
[[ RFC EDITOR: This section should be the same as the corresponding * Name: The name of the cipher suite
text in draft-ietf-tls-rfc8447bis. Please align the two documents if
they have diverged in the approval process. ]]
* Recommended: Whether support for this ciphersuite is recommended * Recommended: Whether support for this cipher suite is recommended
by the IETF. Valid values are "Y", "N", and "D", as described by the IETF. Valid values are "Y", "N", and "D", as described
below. The default value of the "Recommended" column is "N". below. The default value of the "Recommended" column is "N".
Setting the Recommended item to "Y" or "D", or changing an item Setting the Recommended item to "Y" or "D", or changing an item
whose current value is "Y" or "D", requires Standards Action whose current value is "Y" or "D", requires Standards Action
[RFC8126]. [RFC8126].
- Y: Indicates that the IETF has consensus that the item is - Y: Indicates that the IETF has consensus that the item is
RECOMMENDED. This only means that the associated mechanism is RECOMMENDED. This only means that the associated mechanism is
fit for the purpose for which it was defined. Careful reading fit for the purpose for which it was defined. Careful reading
of the documentation for the mechanism is necessary to of the documentation for the mechanism is necessary to
understand the applicability of that mechanism. The IETF could understand the applicability of that mechanism. The IETF could
recommend mechanisms that have limited applicability, but will recommend mechanisms that have limited applicability, but it
provide applicability statements that describe any limitations will provide applicability statements that describe any
of the mechanism or necessary constraints on its use. limitations of the mechanism or necessary constraints on its
use.
- N: Indicates that the item has not been evaluated by the IETF - N: Indicates that the item has not been evaluated by the IETF
and that the IETF has made no statement about the suitability and that the IETF has made no statement about the suitability
of the associated mechanism. This does not necessarily mean of the associated mechanism. This does not necessarily mean
that the mechanism is flawed, only that no consensus exists. that the mechanism is flawed, only that no consensus exists.
The IETF might have consensus to leave an item marked as "N" on The IETF might have consensus to leave an item marked as "N" on
the basis of it having limited applicability or usage the basis of it having limited applicability or usage
constraints. constraints.
- D: Indicates that the item is discouraged and SHOULD NOT or - D: Indicates that the item is discouraged and SHOULD NOT or
MUST NOT be used. This marking could be used to identify MUST NOT be used. This marking could be used to identify
mechanisms that might result in problems if they are used, such mechanisms that might result in problems if they are used, such
as a weak cryptographic algorithm or a mechanism that might as a weak cryptographic algorithm or a mechanism that might
cause interoperability problems in deployment. cause interoperability problems in deployment.
* Reference: The document where this ciphersuite is defined * Reference: The document where this cipher suite is defined
Initial contents: Initial contents:
+========+===================================================+=+====+ +========+===================================================+=+====+
| Value |Name |R|Ref | | Value |Name |R|Ref |
+========+===================================================+=+====+ +========+===================================================+=+====+
| 0x0000 |RESERVED |-|RFC | | 0x0000 |RESERVED |-|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0001 |MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 |Y|RFC | | 0x0001 |MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0002 |MLS_128_DHKEMP256_AES128GCM_SHA256_P256 |Y|RFC | | 0x0002 |MLS_128_DHKEMP256_AES128GCM_SHA256_P256 |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0003 |MLS_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519|Y|RFC | | 0x0003 |MLS_128_DHKEMX25519_CHACHA20POLY1305_SHA256_Ed25519|Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0004 |MLS_256_DHKEMX448_AES256GCM_SHA512_Ed448 |Y|RFC | | 0x0004 |MLS_256_DHKEMX448_AES256GCM_SHA512_Ed448 |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0005 |MLS_256_DHKEMP521_AES256GCM_SHA512_P521 |Y|RFC | | 0x0005 |MLS_256_DHKEMP521_AES256GCM_SHA512_P521 |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0006 |MLS_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448 |Y|RFC | | 0x0006 |MLS_256_DHKEMX448_CHACHA20POLY1305_SHA512_Ed448 |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0007 |MLS_256_DHKEMP384_AES256GCM_SHA384_P384. |Y|RFC | | 0x0007 |MLS_256_DHKEMP384_AES256GCM_SHA384_P384 |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x0A0A |GREASE |Y|RFC | | 0x0A0A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x1A1A |GREASE |Y|RFC | | 0x1A1A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x2A2A |GREASE |Y|RFC | | 0x2A2A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x3A3A |GREASE |Y|RFC | | 0x3A3A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x4A4A |GREASE |Y|RFC | | 0x4A4A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x5A5A |GREASE |Y|RFC | | 0x5A5A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x6A6A |GREASE |Y|RFC | | 0x6A6A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x7A7A |GREASE |Y|RFC | | 0x7A7A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x8A8A |GREASE |Y|RFC | | 0x8A8A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0x9A9A |GREASE |Y|RFC | | 0x9A9A |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0xAAAA |GREASE |Y|RFC | | 0xAAAA |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0xBABA |GREASE |Y|RFC | | 0xBABA |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0xCACA |GREASE |Y|RFC | | 0xCACA |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0xDADA |GREASE |Y|RFC | | 0xDADA |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0xEAEA |GREASE |Y|RFC | | 0xEAEA |GREASE |Y|RFC |
| | | |XXXX| | | | |9420|
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
| 0xF000 |Reserved for Private Use |-|RFC | | 0xF000 |Reserved for Private Use |-|RFC |
| - | | |XXXX| | - | | |9420|
| 0xFFFF | | | | | 0xFFFF | | | |
+--------+---------------------------------------------------+-+----+ +--------+---------------------------------------------------+-+----+
Table 6 Table 6: MLS Extension Types Registry
All of these ciphersuites use HMAC [RFC2104] as their MAC function, All of the non-GREASE cipher suites use HMAC [RFC2104] as their MAC
with different hashes per ciphersuite. The mapping of ciphersuites function, with different hashes per cipher suite. The mapping of
to HPKE primitives, HMAC hash functions, and TLS signature schemes is cipher suites to HPKE primitives [RFC9180], HMAC hash functions, and
as follows [RFC9180] [RFC8446]: TLS signature schemes [RFC8446] is as follows:
+======+======+========+========+========+========================+ +======+======+========+========+========+========================+
|Value |KEM | KDF | AEAD | Hash | Signature | |Value |KEM | KDF | AEAD | Hash | Signature |
+======+======+========+========+========+========================+ +======+======+========+========+========+========================+
|0x0001|0x0020| 0x0001 | 0x0001 | SHA256 | ed25519 | |0x0001|0x0020| 0x0001 | 0x0001 | SHA256 | ed25519 |
+------+------+--------+--------+--------+------------------------+ +------+------+--------+--------+--------+------------------------+
|0x0002|0x0010| 0x0001 | 0x0001 | SHA256 | ecdsa_secp256r1_sha256 | |0x0002|0x0010| 0x0001 | 0x0001 | SHA256 | ecdsa_secp256r1_sha256 |
+------+------+--------+--------+--------+------------------------+ +------+------+--------+--------+--------+------------------------+
|0x0003|0x0020| 0x0001 | 0x0003 | SHA256 | ed25519 | |0x0003|0x0020| 0x0001 | 0x0003 | SHA256 | ed25519 |
+------+------+--------+--------+--------+------------------------+ +------+------+--------+--------+--------+------------------------+
skipping to change at page 139, line 26 skipping to change at line 5904
|0x0005|0x0012| 0x0003 | 0x0002 | SHA512 | ecdsa_secp521r1_sha512 | |0x0005|0x0012| 0x0003 | 0x0002 | SHA512 | ecdsa_secp521r1_sha512 |
+------+------+--------+--------+--------+------------------------+ +------+------+--------+--------+--------+------------------------+
|0x0006|0x0021| 0x0003 | 0x0003 | SHA512 | ed448 | |0x0006|0x0021| 0x0003 | 0x0003 | SHA512 | ed448 |
+------+------+--------+--------+--------+------------------------+ +------+------+--------+--------+--------+------------------------+
|0x0007|0x0011| 0x0002 | 0x0002 | SHA384 | ecdsa_secp384r1_sha384 | |0x0007|0x0011| 0x0002 | 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
+------+------+--------+--------+--------+------------------------+ +------+------+--------+--------+--------+------------------------+
Table 7 Table 7
The hash used for the MLS transcript hash is the one referenced in The hash used for the MLS transcript hash is the one referenced in
the ciphersuite name. In the ciphersuites defined above, "SHA256", the cipher suite name. In the cipher suites defined above, "SHA256",
"SHA384", and "SHA512" refer to the SHA-256, SHA-384, and SHA-512 "SHA384", and "SHA512" refer, respectively, to the SHA-256, SHA-384,
functions defined in [SHS]. and SHA-512 functions defined in [SHS].
In addition to the general requirements of Section 13.1, future In addition to the general requirements of Section 13.1, future
ciphersuites MUST meet the requirements of Section 16.3. cipher suites MUST meet the requirements of Section 16.3.
It is advisable to keep the number of ciphersuites low to increase It is advisable to keep the number of cipher suites low to increase
the chances clients can interoperate in a federated environment, the likelihood that clients can interoperate in a federated
therefore the ciphersuites only include modern, yet well-established environment. The cipher suites therefore include only modern, yet
algorithms. Depending on their requirements, clients can choose well-established algorithms. Depending on their requirements,
between two security levels (roughly 128-bit and 256-bit). Within clients can choose between two security levels (roughly 128-bit and
the security levels clients can choose between faster X25519/X448 256-bit). Within the security levels, clients can choose between
curves and FIPS 140-2 compliant curves for Diffie-Hellman key faster X25519/X448 curves and curves compliant with FIPS 140-2 for
negotiations. Clients may also choose ChaCha20Poly1305 or AES-GCM, Diffie-Hellman key negotiations. Clients may also choose
e.g., for performance reasons. Since ChaCha20Poly1305 is not listed ChaCha20Poly1305 or AES-GCM, e.g., for performance reasons. Since
by FIPS 140-2 it is not paired with FIPS 140-2 compliant curves. The ChaCha20Poly1305 is not listed by FIPS 140-2, it is not paired with
security level of symmetric encryption algorithms and hash functions curves compliant with FIPS 140-2. The security level of symmetric
is paired with the security level of the curves. encryption algorithms and hash functions is paired with the security
level of the curves.
The mandatory-to-implement ciphersuite for MLS 1.0 is The mandatory-to-implement cipher suite for MLS 1.0 is
MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519 which uses Curve25519 MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519, which uses Curve25519
for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and for key exchange, AES-128-GCM for HPKE, HKDF over SHA2-256, and
Ed25519 for signatures. MLS clients MUST implement this ciphersuite. Ed25519 for signatures. MLS clients MUST implement this cipher
suite.
New ciphersuite values are assigned by IANA as described in
Section 17.
17.2. MLS Wire Formats 17.2. MLS Wire Formats
This registry lists identifiers for the types of messages that can be The "MLS Wire Formats" registry lists identifiers for the types of
sent in MLS. The wire format field is two bytes wide, so the valid messages that can be sent in MLS. The wire format field is two bytes
wire format values are in the range 0x0000 to 0xFFFF. wide, so the valid wire format values are in the range 0x0000 to
0xFFFF.
Template: Template:
* Value: The numeric value of the wire format * Value: The numeric value of the wire format
* Name: The name of the wire format * Name: The name of the wire format
* Recommended: Same as in Section 17.1 * Recommended: Same as in Section 17.1
* Reference: The document where this wire format is defined * Reference: The document where this wire format is defined
Initial contents: Initial contents:
+=================+==========================+===+==========+ +=================+==========================+===+==========+
| Value | Name | R | Ref | | Value | Name | R | Ref |
+=================+==========================+===+==========+ +=================+==========================+===+==========+
| 0x0000 | RESERVED | - | RFC XXXX | | 0x0000 | RESERVED | - | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0001 | mls_public_message | Y | RFC XXXX | | 0x0001 | mls_public_message | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0002 | mls_private_message | Y | RFC XXXX | | 0x0002 | mls_private_message | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0003 | mls_welcome | Y | RFC XXXX | | 0x0003 | mls_welcome | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0004 | mls_group_info | Y | RFC XXXX | | 0x0004 | mls_group_info | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0005 | mls_key_package | Y | RFC XXXX | | 0x0005 | mls_key_package | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC XXXX | | 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
Table 8 Table 8: MLS Wire Formats Registry
17.3. MLS Extension Types 17.3. MLS Extension Types
This registry lists identifiers for extensions to the MLS protocol. The "MLS Extension Types" registry lists identifiers for extensions
The extension type field is two bytes wide, so valid extension type to the MLS protocol. The extension type field is two bytes wide, so
values are in the range 0x0000 to 0xFFFF. valid extension type values are in the range 0x0000 to 0xFFFF.
Template: Template:
* Value: The numeric value of the extension type * Value: The numeric value of the extension type
* Name: The name of the extension type * Name: The name of the extension type
* Message(s): The messages in which the extension may appear, drawn * Message(s): The messages in which the extension may appear, drawn
from the following list: from the following list:
- KP: KeyPackage objects - KP: KeyPackage objects
- LN: LeafNode objects - LN: LeafNode objects
- GC: GroupContext objects - GC: GroupContext objects
skipping to change at page 141, line 26 skipping to change at line 6002
* Recommended: Same as in Section 17.1 * Recommended: Same as in Section 17.1
* Reference: The document where this extension is defined * Reference: The document where this extension is defined
Initial contents: Initial contents:
+==========+=======================+============+===+==========+ +==========+=======================+============+===+==========+
| Value | Name | Message(s) | R | Ref | | Value | Name | Message(s) | R | Ref |
+==========+=======================+============+===+==========+ +==========+=======================+============+===+==========+
| 0x0000 | RESERVED | N/A | - | RFC XXXX | | 0x0000 | RESERVED | N/A | - | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x0001 | application_id | LN | Y | RFC XXXX | | 0x0001 | application_id | LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x0002 | ratchet_tree | GI | Y | RFC XXXX | | 0x0002 | ratchet_tree | GI | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x0003 | required_capabilities | GC | Y | RFC XXXX | | 0x0003 | required_capabilities | GC | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x0004 | external_pub | GI | Y | RFC XXXX | | 0x0004 | external_pub | GI | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x0005 | external_senders | GC | Y | RFC XXXX | | 0x0005 | external_senders | GC | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x0A0A | GREASE | KP, GI | Y | RFC XXXX | | 0x0A0A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x1A1A | GREASE | KP, GI | Y | RFC XXXX | | 0x1A1A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x2A2A | GREASE | KP, GI | Y | RFC XXXX | | 0x2A2A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x3A3A | GREASE | KP, GI | Y | RFC XXXX | | 0x3A3A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x4A4A | GREASE | KP, GI | Y | RFC XXXX | | 0x4A4A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x5A5A | GREASE | KP, GI | Y | RFC XXXX | | 0x5A5A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x6A6A | GREASE | KP, GI | Y | RFC XXXX | | 0x6A6A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x7A7A | GREASE | KP, GI | Y | RFC XXXX | | 0x7A7A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x8A8A | GREASE | KP, GI | Y | RFC XXXX | | 0x8A8A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0x9A9A | GREASE | KP, GI | Y | RFC XXXX | | 0x9A9A | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0xAAAA | GREASE | KP, GI | Y | RFC XXXX | | 0xAAAA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0xBABA | GREASE | KP, GI | Y | RFC XXXX | | 0xBABA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0xCACA | GREASE | KP, GI | Y | RFC XXXX | | 0xCACA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0xDADA | GREASE | KP, GI | Y | RFC XXXX | | 0xDADA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0xEAEA | GREASE | KP, GI | Y | RFC XXXX | | 0xEAEA | GREASE | KP, GI, LN | Y | RFC 9420 |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
| 0xF000 - | Reserved for Private | N/A | - | RFC XXXX | | 0xF000 - | Reserved for Private | N/A | - | RFC 9420 |
| 0xFFFF | Use | | | | | 0xFFFF | Use | | | |
+----------+-----------------------+------------+---+----------+ +----------+-----------------------+------------+---+----------+
Table 9 Table 9: MLS Extension Types Registry
17.4. MLS Proposal Types 17.4. MLS Proposal Types
This registry lists identifiers for types of proposals that can be The "MLS Proposal Types" registry lists identifiers for types of
made for changes to an MLS group. The extension type field is two proposals that can be made for changes to an MLS group. The
bytes wide, so valid extension type values are in the range 0x0000 to extension type field is two bytes wide, so valid extension type
0xFFFF. values are in the range 0x0000 to 0xFFFF.
Template: Template:
* Value: The numeric value of the proposal type * Value: The numeric value of the proposal type
* Name: The name of the proposal type * Name: The name of the proposal type
* Recommended: Same as in Section 17.1 * Recommended: Same as in Section 17.1
* External: Whether a proposal of this type may be sent by an * External: Whether a proposal of this type may be sent by an
external sender (see Section 12.1.8). external sender (see Section 12.1.8)
* Path Required: Whether a Commit covering a proposal of this type * Path Required: Whether a Commit covering a proposal of this type
is required to have its path field populated (see Section 12.4). is required to have its path field populated (see Section 12.4)
* Reference: The document where this extension is defined * Reference: The document where this extension is defined
Initial contents: Initial contents:
+==========+==========================+===+=====+======+==========+ +==========+==========================+===+=====+======+==========+
| Value | Name | R | Ext | Path | Ref | | Value | Name | R | Ext | Path | Ref |
+==========+==========================+===+=====+======+==========+ +==========+==========================+===+=====+======+==========+
| 0x0000 | RESERVED | - | - | - | RFC XXXX | | 0x0000 | RESERVED | - | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0001 | add | Y | Y | N | RFC XXXX | | 0x0001 | add | Y | Y | N | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0002 | update | Y | N | Y | RFC XXXX | | 0x0002 | update | Y | N | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0003 | remove | Y | Y | Y | RFC XXXX | | 0x0003 | remove | Y | Y | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0004 | psk | Y | Y | N | RFC XXXX | | 0x0004 | psk | Y | Y | N | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0005 | reinit | Y | Y | N | RFC XXXX | | 0x0005 | reinit | Y | Y | N | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0006 | external_init | Y | N | Y | RFC XXXX | | 0x0006 | external_init | Y | N | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0007 | group_context_extensions | Y | Y | Y | RFC XXXX | | 0x0007 | group_context_extensions | Y | Y | Y | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x0A0A | GREASE | Y | - | - | RFC XXXX | | 0x0A0A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x1A1A | GREASE | Y | - | - | RFC XXXX | | 0x1A1A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x2A2A | GREASE | Y | - | - | RFC XXXX | | 0x2A2A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x3A3A | GREASE | Y | - | - | RFC XXXX | | 0x3A3A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x4A4A | GREASE | Y | - | - | RFC XXXX | | 0x4A4A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x5A5A | GREASE | Y | - | - | RFC XXXX | | 0x5A5A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x6A6A | GREASE | Y | - | - | RFC XXXX | | 0x6A6A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x7A7A | GREASE | Y | - | - | RFC XXXX | | 0x7A7A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x8A8A | GREASE | Y | - | - | RFC XXXX | | 0x8A8A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0x9A9A | GREASE | Y | - | - | RFC XXXX | | 0x9A9A | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0xAAAA | GREASE | Y | - | - | RFC XXXX | | 0xAAAA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0xBABA | GREASE | Y | - | - | RFC XXXX | | 0xBABA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0xCACA | GREASE | Y | - | - | RFC XXXX | | 0xCACA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0xDADA | GREASE | Y | - | - | RFC XXXX | | 0xDADA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0xEAEA | GREASE | Y | - | - | RFC XXXX | | 0xEAEA | GREASE | Y | - | - | RFC 9420 |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
| 0xF000 - | Reserved for Private Use | - | - | - | RFC XXXX | | 0xF000 - | Reserved for Private Use | - | - | - | RFC 9420 |
| 0xFFFF | | | | | | | 0xFFFF | | | | | |
+----------+--------------------------+---+-----+------+----------+ +----------+--------------------------+---+-----+------+----------+
Table 10 Table 10: MLS Proposal Types Registry
17.5. MLS Credential Types 17.5. MLS Credential Types
This registry lists identifiers for types of credentials that can be The "MLS Credential Types" registry lists identifiers for types of
used for authentication in the MLS protocol. The credential type credentials that can be used for authentication in the MLS protocol.
field is two bytes wide, so valid credential type values are in the The credential type field is two bytes wide, so valid credential type
range 0x0000 to 0xFFFF. values are in the range 0x0000 to 0xFFFF.
Template: Template:
* Value: The numeric value of the credential type * Value: The numeric value of the credential type
* Name: The name of the credential type * Name: The name of the credential type
* Recommended: Same as in Section 17.1 * Recommended: Same as in Section 17.1
* Reference: The document where this credential is defined * Reference: The document where this credential is defined
Initial contents: Initial contents:
+=================+==========================+===+==========+ +=================+==========================+===+==========+
| Value | Name | R | Ref | | Value | Name | R | Ref |
+=================+==========================+===+==========+ +=================+==========================+===+==========+
| 0x0000 | RESERVED | - | RFC XXXX | | 0x0000 | RESERVED | - | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0001 | basic | Y | RFC XXXX | | 0x0001 | basic | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0002 | x509 | Y | RFC XXXX | | 0x0002 | x509 | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x0A0A | GREASE | Y | RFC XXXX | | 0x0A0A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x1A1A | GREASE | Y | RFC XXXX | | 0x1A1A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x2A2A | GREASE | Y | RFC XXXX | | 0x2A2A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x3A3A | GREASE | Y | RFC XXXX | | 0x3A3A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x4A4A | GREASE | Y | RFC XXXX | | 0x4A4A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x5A5A | GREASE | Y | RFC XXXX | | 0x5A5A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x6A6A | GREASE | Y | RFC XXXX | | 0x6A6A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x7A7A | GREASE | Y | RFC XXXX | | 0x7A7A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x8A8A | GREASE | Y | RFC XXXX | | 0x8A8A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0x9A9A | GREASE | Y | RFC XXXX | | 0x9A9A | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0xAAAA | GREASE | Y | RFC XXXX | | 0xAAAA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0xBABA | GREASE | Y | RFC XXXX | | 0xBABA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0xCACA | GREASE | Y | RFC XXXX | | 0xCACA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0xDADA | GREASE | Y | RFC XXXX | | 0xDADA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0xEAEA | GREASE | Y | RFC XXXX | | 0xEAEA | GREASE | Y | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
| 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC XXXX | | 0xF000 - 0xFFFF | Reserved for Private Use | - | RFC 9420 |
+-----------------+--------------------------+---+----------+ +-----------------+--------------------------+---+----------+
Table 11 Table 11: MLS Credential Types Registry
17.6. MLS Signature Labels 17.6. MLS Signature Labels
The SignWithLabel function defined in Section 5.1.2 avoids the risk The SignWithLabel function defined in Section 5.1.2 avoids the risk
of confusion between signatures in different contexts. Each context of confusion between signatures in different contexts. Each context
is assigned a distinct label that is incorporated into the signature. is assigned a distinct label that is incorporated into the signature.
This registry records the labels defined in this document, and allows The "MLS Signature Labels" registry records the labels defined in
additional labels to be registered in case extensions add other types this document and allows additional labels to be registered in case
of signature using the same signature keys used elsewhere in MLS. extensions add other types of signatures using the same signature
keys used elsewhere in MLS.
Template: Template:
* Label: The string to be used as the Label parameter to * Label: The string to be used as the Label parameter to
SignWithLabel SignWithLabel
* Recommended: Same as in Section 17.1 * Recommended: Same as in Section 17.1
* Reference: The document where this credential is defined * Reference: The document where this label is defined
Initial contents: Initial contents:
+====================+===+==========+ +====================+===+==========+
| Label | R | Ref | | Label | R | Ref |
+====================+===+==========+ +====================+===+==========+
| "FramedContentTBS" | Y | RFC XXXX | | "FramedContentTBS" | Y | RFC 9420 |
+--------------------+---+----------+ +--------------------+---+----------+
| "LeafNodeTBS" | Y | RFC XXXX | | "LeafNodeTBS" | Y | RFC 9420 |
+--------------------+---+----------+ +--------------------+---+----------+
| "KeyPackageTBS" | Y | RFC XXXX | | "KeyPackageTBS" | Y | RFC 9420 |
+--------------------+---+----------+ +--------------------+---+----------+
| "GroupInfoTBS" | Y | RFC XXXX | | "GroupInfoTBS" | Y | RFC 9420 |
+--------------------+---+----------+ +--------------------+---+----------+
Table 12 Table 12: MLS Signature Labels
Registry
17.7. MLS Public Key Encryption Labels 17.7. MLS Public Key Encryption Labels
The EncryptWithLabel function defined in Section 5.1.3 avoids the The EncryptWithLabel function defined in Section 5.1.3 avoids the
risk of confusion between ciphertexts produced for different purposes risk of confusion between ciphertexts produced for different purposes
in different contexts. Each context is assigned a distinct label in different contexts. Each context is assigned a distinct label
that is incorporated into the signature. This registry records the that is incorporated into the signature. The "MLS Public Key
labels defined in this document, and allows additional labels to be Encryption Labels" registry records the labels defined in this
registered in case extensions add other types of public-key document and allows additional labels to be registered in case
encryption using the same HPKE keys used elsewhere in MLS. extensions add other types of public key encryption using the same
HPKE keys used elsewhere in MLS.
Template: Template:
* Label: The string to be used as the Label parameter to * Label: The string to be used as the Label parameter to
EncryptWithLabel EncryptWithLabel
* Recommended: Same as in Section 17.1 * Recommended: Same as in Section 17.1
* Reference: The document where this credential is defined * Reference: The document where this label is defined
Initial contents: Initial contents:
+==================+===+==========+ +==================+===+==========+
| Label | R | Ref | | Label | R | Ref |
+==================+===+==========+ +==================+===+==========+
| "UpdatePathNode" | Y | RFC XXXX | | "UpdatePathNode" | Y | RFC 9420 |
+------------------+---+----------+ +------------------+---+----------+
| "Welcome" | Y | RFC XXXX | | "Welcome" | Y | RFC 9420 |
+------------------+---+----------+ +------------------+---+----------+
Table 13 Table 13: MLS Public Key
Encryption Labels Registry
17.8. MLS Exporter Labels 17.8. MLS Exporter Labels
The exporter function defined in Section 8.5 allows applications to The exporter function defined in Section 8.5 allows applications to
derive key material from the MLS key schedule. Like the TLS exporter derive key material from the MLS key schedule. Like the TLS exporter
[RFC8446], the MLS exporter uses a label to distinguish between [RFC8446], the MLS exporter uses a label to distinguish between
different applications' use of the exporter. This registry allows different applications' use of the exporter. The "MLS Exporter
applications to register their usage to avoid collisions. Labels" registry allows applications to register their usage to avoid
collisions.
Template: Template:
* Label: The string to be used as the Label parameter to MLS- * Label: The string to be used as the Label parameter to MLS-
Exporter Exporter
* Recommended: Same as in Section 17.1 * Recommended: Same as in Section 17.1
* Reference: The document where this credential is defined * Reference: The document where this label is defined
The registry has no initial contents, since it is intended to be used The registry has no initial contents, since it is intended to be used
by applications, not the core protocol. The table below is intended by applications, not the core protocol. The table below is intended
only to show the column layout of the registry. only to show the column layout of the registry.
+=======+=============+===========+ +=======+=============+===========+
| Label | Recommended | Reference | | Label | Recommended | Reference |
+=======+=============+===========+ +=======+=============+===========+
| (N/A) | (N/A) | (N/A) | | (N/A) | (N/A) | (N/A) |
+-------+-------------+-----------+ +-------+-------------+-----------+
Table 14 Table 14: MLS Exporter Labels
Registry
17.9. MLS Designated Expert Pool 17.9. MLS Designated Expert Pool
Specification Required [RFC8126] registry requests are registered Specification Required [RFC8126] registry requests are registered
after a three-week review period on the MLS DEs' mailing list: mls- after a three-week review period on the MLS Designated Expert (DE)
reg-review@ietf.org (mailto:mls-reg-review@ietf.org), on the advice mailing list <mailto:mls-reg-review@ietf.org> on the advice of one or
of one or more of the MLS DEs. However, to allow for the allocation more of the MLS DEs. However, to allow for the allocation of values
of values prior to publication, the MLS DEs may approve registration prior to publication, the MLS DEs may approve registration once they
once they are satisfied that such a specification will be published. are satisfied that such a specification will be published.
Registration requests sent to the MLS DEs mailing list for review Registration requests sent to the MLS DEs' mailing list for review
SHOULD use an appropriate subject (e.g., "Request to register value SHOULD use an appropriate subject (e.g., "Request to register value
in MLS Bar registry"). in MLS Bar registry").
Within the review period, the MLS DEs will either approve or deny the Within the review period, the MLS DEs will either approve or deny the
registration request, communicating this decision to the MLS DEs registration request, communicating this decision to the MLS DEs'
mailing list and IANA. Denials SHOULD include an explanation and, if mailing list and IANA. Denials SHOULD include an explanation and, if
applicable, suggestions as to how to make the request successful. applicable, suggestions as to how to make the request successful.
Registration requests that are undetermined for a period longer than Registration requests that are undetermined for a period longer than
21 days can be brought to the IESG's attention for resolution using 21 days can be brought to the IESG's attention for resolution using
the iesg@ietf.org (mailto:iesg@ietf.org) mailing list. the <mailto:iesg@ietf.org> mailing list.
Criteria that SHOULD be applied by the MLS DEs includes determining Criteria that SHOULD be applied by the MLS DEs includes determining
whether the proposed registration duplicates existing functionality, whether the proposed registration duplicates existing functionality,
whether it is likely to be of general applicability or useful only whether it is likely to be of general applicability or useful only
for a single application, and whether the registration description is for a single application, and whether the registration description is
clear. For example, the MLS DEs will apply the ciphersuite-related clear. For example, for cipher suite registrations, the MLS DEs will
advisory found in Section 17.1. apply the advisory found in Section 17.1.
IANA MUST only accept registry updates from the MLS DEs and SHOULD IANA MUST only accept registry updates from the MLS DEs and SHOULD
direct all requests for registration to the MLS DEs' mailing list. direct all requests for registration to the MLS DEs' mailing list.
It is suggested that multiple MLS DEs be appointed who are able to It is suggested that multiple MLS DEs who are able to represent the
represent the perspectives of different applications using this perspectives of different applications using this specification be
specification, in order to enable broadly informed review of appointed, in order to enable a broadly informed review of
registration decisions. In cases where a registration decision could registration decisions. In cases where a registration decision could
be perceived as creating a conflict of interest for a particular MLS be perceived as creating a conflict of interest for a particular MLS
DE, that MLS DE SHOULD defer to the judgment of the other MLS DEs. DE, that MLS DE SHOULD defer to the judgment of the other MLS DEs.
17.10. The "message/mls" MIME Type 17.10. The "message/mls" Media Type
This document registers the "message/mls" MIME media type in order to This document registers the "message/mls" media type in the "message"
allow other protocols (e.g., HTTP [RFC9113]) to convey MLS messages. registry in order to allow other protocols (e.g., HTTP [RFC9113]) to
convey MLS messages.
Type name: message Type name: message
Subtype name: mls Subtype name: mls
Required parameters: none Required parameters: none
Optional parameters: version Optional parameters: version
version: The MLS protocol version expressed as version: The MLS protocol version expressed as
a string <major>.<minor>. If omitted the version is "1.0", a string <major>.<minor>. If omitted, the version is "1.0",
which corresponds to MLS ProtocolVersion mls10. If for some which corresponds to MLS ProtocolVersion mls10. If for some
reason the version number in the MIME type parameter differs reason the version number in the media type parameter differs
from the ProtocolVersion embedded in the protocol, the protocol from the ProtocolVersion embedded in the protocol, the protocol
takes precedence. takes precedence.
Encoding considerations: MLS messages are represented using the TLS Encoding considerations: MLS messages are represented using the TLS
presentation language [RFC8446]. Therefore MLS messages need to presentation language [RFC8446]. Therefore, MLS messages need to
be treated as binary data. be treated as binary data.
Security considerations: MLS is an encrypted messaging layer Security considerations: MLS is an encrypted messaging layer
designed to be transmitted over arbitrary lower layer protocols. designed to be transmitted over arbitrary lower-layer protocols.
The security considerations in this document (RFC XXXX) also The security considerations in this document (RFC 9420) also
apply. apply.
Interoperability considerations: N/A Interoperability considerations: N/A
Published specification: RFC XXXX Published specification: RFC 9420
Applications that use this media type: MLS-based messaging Applications that use this media type: MLS-based messaging
applications applications
Fragment identifier considerations: N/A Fragment identifier considerations: N/A
Additional information: Additional information:
* Deprecated alias names for this type: N/A Deprecated alias names for this type: N/A
Magic number(s): N/A
* Magic number(s): N/A File extension(s): N/A
Macintosh file type code(s): N/A
* File extension(s): N/A
* Macintosh file type code(s): N/A
Person & email address to contact for further information: IETF MLS Person & email address to contact for further information: IETF MLS
Working Group mls@ietf.org (mailto:mls@ietf.org) Working Group <mailto:mls@ietf.org>
Intended usage: COMMON Intended usage: COMMON
Restrictions on usage: N/A Restrictions on usage: N/A
Author: IETF MLS Working Group Author: IETF MLS Working Group
Change controller: IESG Change controller: IETF
Provisional registration? (standards tree only): No
18. References 18. References
18.1. Normative References 18.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997, DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/rfc/rfc2104>. <https://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/rfc/rfc8126>. <https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid [RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180, Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/rfc/rfc9180>. February 2022, <https://www.rfc-editor.org/info/rfc9180>.
18.2. Informative References 18.2. Informative References
[ART] Cohn-Gordon, K., Cremers, C., Garratt, L., Millican, J., [ART] Cohn-Gordon, K., Cremers, C., Garratt, L., Millican, J.,
and K. Milner, "On Ends-to-Ends Encryption: Asynchronous and K. Milner, "On Ends-to-Ends Encryption: Asynchronous
Group Messaging with Strong Security Guarantees", 18 Group Messaging with Strong Security Guarantees", Version
January 2018, <https://eprint.iacr.org/2017/666.pdf>. 2.3, DOI 10.1145/3243734.3243747, March 2020,
<https://eprint.iacr.org/2017/666.pdf>.
[CFRG-AEAD-LIMITS]
Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-07, 31 May 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-07>.
[CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know [CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
Why You Went to the Clinic: Risks and Realization of HTTPS Why You Went to the Clinic: Risks and Realization of HTTPS
Traffic Analysis", Privacy Enhancing Technologies pp. Traffic Analysis", Privacy Enhancing Technologies, pp.
143-163, DOI 10.1007/978-3-319-08506-7_8, 2014, 143-163, DOI 10.1007/978-3-319-08506-7_8, 2014,
<https://doi.org/10.1007/978-3-319-08506-7_8>. <https://doi.org/10.1007/978-3-319-08506-7_8>.
[CONIKS] Melara, M. S., Blankstein, A., Bonneau, J., Felten, E. W., [CONIKS] Melara, M. S., Blankstein, A., Bonneau, J., Felten, E. W.,
and M. J. Freedman, "CONIKS: Bringing Key Transparency to and M. J. Freedman, "CONIKS: Bringing Key Transparency to
End Users", 2015, End Users", Proceedings of the 24th USENIX Security
Symposium, ISBN 978-1-939133-11-3, August 2015,
<https://www.usenix.org/system/files/conference/ <https://www.usenix.org/system/files/conference/
usenixsecurity15/sec15-paper-melara.pdf>. usenixsecurity15/sec15-paper-melara.pdf>.
[DoubleRatchet] [DoubleRatchet]
Cohn-Gordon, K., Cremers, C., Dowling, B., Garratt, L., Cohn-Gordon, K., Cremers, C., Dowling, B., Garratt, L.,
and D. Stebila, "A Formal Security Analysis of the Signal and D. Stebila, "A Formal Security Analysis of the Signal
Messaging Protocol", 2017 IEEE European Symposium on Messaging Protocol", 2017 IEEE European Symposium on
Security and Privacy (EuroS&P), Security and Privacy (EuroS&P),
DOI 10.1109/eurosp.2017.27, April 2017, DOI 10.1109/eurosp.2017.27, April 2017,
<https://doi.org/10.1109/eurosp.2017.27>. <https://doi.org/10.1109/eurosp.2017.27>.
[HCJ16] Husák, M., Čermák, M., Jirsík, T., and P. Čeleda, "HTTPS [HCJ16] Husák, M., Čermák, M., Jirsík, T., and P. Čeleda, "HTTPS
traffic analysis and client identification using passive traffic analysis and client identification using passive
SSL/TLS fingerprinting", EURASIP Journal on Information SSL/TLS fingerprinting", EURASIP Journal on Information
Security vol. 2016, no. 1, DOI 10.1186/s13635-016-0030-7, Security, Vol. 2016, Issue 1,
February 2016, DOI 10.1186/s13635-016-0030-7, February 2016,
<https://doi.org/10.1186/s13635-016-0030-7>. <https://doi.org/10.1186/s13635-016-0030-7>.
[I-D.ietf-mls-architecture] [KT] "Key Transparency Design Doc", commit fb0f87f, June 2020,
Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., and <https://github.com/google/keytransparency/blob/master/
docs/design.md>.
[MLS-ARCH] Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., and
A. Duric, "The Messaging Layer Security (MLS) A. Duric, "The Messaging Layer Security (MLS)
Architecture", Work in Progress, Internet-Draft, draft- Architecture", Work in Progress, Internet-Draft, draft-
ietf-mls-architecture-10, 16 December 2022, ietf-mls-architecture-10, 16 December 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls- <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
architecture-10>. architecture-10>.
[I-D.irtf-cfrg-aead-limits]
Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-06, 30 January 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-06>.
[KT] "Key Transparency Design Doc", 26 June 2020,
<https://github.com/google/keytransparency/blob/master/
docs/design.md>.
[NAN] Bellare, M., Ng, R., and B. Tackmann, "Nonces Are Noticed: [NAN] Bellare, M., Ng, R., and B. Tackmann, "Nonces Are Noticed:
AEAD Revisited", Advances in Cryptology - CRYPTO 2019 pp. AEAD Revisited", Advances in Cryptology - CRYPTO 2019, pp.
235-265, DOI 10.1007/978-3-030-26948-7_9, 2019, 235-265, DOI 10.1007/978-3-030-26948-7_9, August 2019,
<https://doi.org/10.1007/978-3-030-26948-7_9>. <https://doi.org/10.1007/978-3-030-26948-7_9>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/rfc/rfc5116>. <https://www.rfc-editor.org/info/rfc5116>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/rfc/rfc5905>. <https://www.rfc-editor.org/info/rfc5905>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509 within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer (PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/rfc/rfc6125>. 2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm [RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms", Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015, BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/rfc/rfc7696>. <https://www.rfc-editor.org/info/rfc7696>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032, Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017, DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/rfc/rfc8032>. <https://www.rfc-editor.org/info/rfc8032>.
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And [RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility", Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020, RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/rfc/rfc8701>. <https://www.rfc-editor.org/info/rfc8701>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based [RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000, Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021, DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>. <https://www.rfc-editor.org/info/rfc9000>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure [RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>. <https://www.rfc-editor.org/info/rfc9001>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113, [RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022, DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/rfc/rfc9113>. <https://www.rfc-editor.org/info/rfc9113>.
[SHS] Dang, Q., "Secure Hash Standard", National Institute of [SHS] National Institute of Standards and Technology (NIST),
Standards and Technology report, "Secure Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/nist.fips.180-4, July 2015, DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://doi.org/10.6028/nist.fips.180-4>. <https://doi.org/10.6028/NIST.FIPS.180-4>.
[Signal] Perrin(ed), T. and M. Marlinspike, "The Double Ratchet [Signal] Perrin(ed), T. and M. Marlinspike, "The Double Ratchet
Algorithm", 20 November 2016, Algorithm", Revision 1, November 2016,
<https://www.signal.org/docs/specifications/ <https://www.signal.org/docs/specifications/
doubleratchet/>. doubleratchet/>.
Appendix A. Protocol Origins of Example Trees Appendix A. Protocol Origins of Example Trees
Protocol operations in MLS give rise to specific forms of ratchet Protocol operations in MLS give rise to specific forms of ratchet
tree, typically affecting a whole direct path at once. In this tree, typically affecting a whole direct path at once. In this
section, we describe the protocol operations that could have given section, we describe the protocol operations that could have given
rise to the various example trees in this document. rise to the various example trees in this document.
To construct the tree in Figure 11: To construct the tree in Figure 11:
* A creates a group with B, ..., G * A creates a group with B, ..., G
* F sends an empty Commit, setting X, Y, W * F sends an empty Commit, setting X, Y, and W
* G removes C and D, blanking V, U, and setting Y, W * G removes C and D, blanking V, U, and setting Y and W
* B sends an empty Commit, setting T and W * B sends an empty Commit, setting T and W
To construct the tree in Figure 10: To construct the tree in Figure 10:
* A creates a group with B, ..., H, as well as some members outside * A creates a group with B, ..., H, as well as some members outside
this subtree this subtree
* F sends an empty Commit, setting Y and its ancestors * F sends an empty Commit, setting Y and its ancestors
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of D of D
* Someone outside this subtree removes G, blanking the direct path * Someone outside this subtree removes G, blanking the direct path
of G of G
* A adds a new member at B with a partial Commit, adding B as * A adds a new member at B with a partial Commit, adding B as
unmerged at X unmerged at X
To construct the tree in Figure 13: To construct the tree in Figure 13:
* A creates a group with B, C, D * A creates a group with B, C, and D
* B sends a full Commit, setting X and Y * B sends a full Commit, setting X and Y
* D removes C, setting Z and Y * D removes C, setting Z and Y
* B adds a new member at C with a full Commit * B adds a new member at C with a full Commit
- The Add proposal adds C as unmerged at Z and Y - The Add proposal adds C as unmerged at Z and Y
- The path in the Commit resets X and Y, clearing Y's unmerged - The path in the Commit resets X and Y, clearing Y's unmerged
leaves leaves
To construct the tree in Figure 21: To construct the tree in Figure 21:
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To better understand how parent hashes are maintained, let's look in To better understand how parent hashes are maintained, let's look in
detail at how they evolve in a small group. Consider the following detail at how they evolve in a small group. Consider the following
sequence of operations: sequence of operations:
1. A initializes a new group 1. A initializes a new group
2. A adds B to the group with a full Commit 2. A adds B to the group with a full Commit
3. B adds C and D to the group with a full Commit 3. B adds C and D to the group with a full Commit
4. C sends an empty Commit. 4. C sends an empty Commit
Y Y' Y Y'
| | | |
.-+-. .-+-. .-+-. .-+-.
==> ==> / \ ==> / \ ==> ==> / \ ==> / \
X X' _=Z X' Z' X X' _=Z X' Z'
/ \ / \ / \ / \ / \ / \ / \ / \ / \ / \
A A B A B C D A B C D A A B A B C D A B C D
Figure 30: Building a four-member tree to illustrate parent hashes Figure 30: Building a Four-Member Tree to Illustrate Parent Hashes
Then the parent hashes associated to the nodes will be updated as Then the parent hashes associated to the nodes will be updated as
follows (where we use the shorthand ph for parent hash, th for tree follows (where we use the shorthand ph for parent hash, th for tree
hash, and osth for original sibling tree hash): hash, and osth for original sibling tree hash):
1. A adds B: set X 1. A adds B: set X
* A.parent_hash = ph(X) = H(X, ph="", osth=th(B)) * A.parent_hash = ph(X) = H(X, ph="", osth=th(B))
2. B adds C, D: set B', X', Y 2. B adds C, D: set B', X', and Y
* X'.parent_hash = ph(Y) = H(Y, ph="", osth=th(Z)), where th(Z) * X'.parent_hash = ph(Y) = H(Y, ph="", osth=th(Z)), where th(Z)
covers (C, _, D) covers (C, _, D)
* B'.parent_hash = ph(X') = H(X', ph=X'.parent_hash, osth=th(A)) * B'.parent_hash = ph(X') = H(X', ph=X'.parent_hash, osth=th(A))
3. C sends empty Commit: set C', Z', Y' 3. C sends empty Commit: set C', Z', Y'
* Z'.parent_hash = ph(Y') = H(Y', ph="", osth=th(X')), where * Z'.parent_hash = ph(Y') = H(Y', ph="", osth=th(X')), where
th(X') covers (A, X', B') th(X') covers (A, X', B')
* C'.parent_hash = ph(Z') = H(Z', ph=Z'.parent_hash, osth=th(D)) * C'.parent_hash = ph(Z') = H(Z', ph=Z'.parent_hash, osth=th(D))
When a new member joins, they will receive a tree that has the When a new member joins, they will receive a tree that has the
following parent hash values, and compute the indicated parent-hash following parent hash values and compute the indicated parent hash
validity relationships: validity relationships:
+======+======================================+=====================+ +======+======================================+=====================+
| Node | Parent hash value | Valid? | | Node | Parent Hash Value | Valid? |
+======+======================================+=====================+ +======+======================================+=====================+
| A | H(X, ph="", osth=th(B)) | No, B changed | | A | H(X, ph="", osth=th(B)) | No, B changed |
+------+--------------------------------------+---------------------+ +------+--------------------------------------+---------------------+
| B' | H(X', ph=X'.parent_hash, osth=th(A)) | Yes | | B' | H(X', ph=X'.parent_hash, osth=th(A)) | Yes |
+------+--------------------------------------+---------------------+ +------+--------------------------------------+---------------------+
| C' | H(Z', ph=Z'.parent_hash, osth=th(D)) | Yes | | C' | H(Z', ph=Z'.parent_hash, osth=th(D)) | Yes |
+------+--------------------------------------+---------------------+ +------+--------------------------------------+---------------------+
| D | (none, never sent an UpdatePath) | N/A | | D | (none, never sent an UpdatePath) | N/A |
+------+--------------------------------------+---------------------+ +------+--------------------------------------+---------------------+
| X' | H(Y, ph="", osth=th(Z)) | No, Y and Z | | X' | H(Y, ph="", osth=th(Z)) | No, Y and Z |
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Figure 31: Parent-hash links connect all non-empty parent nodes Figure 31: Parent-hash links connect all non-empty parent nodes
to leaves to leaves
Since these chains collectively cover all non-blank parent nodes in Since these chains collectively cover all non-blank parent nodes in
the tree, the tree is parent-hash valid. the tree, the tree is parent-hash valid.
Note that this tree, though valid, contains invalid parent-hash Note that this tree, though valid, contains invalid parent-hash
links. If a client were checking parent hashes top-down from Y', for links. If a client were checking parent hashes top-down from Y', for
example, they would find that X' has an invalid parent hash relative example, they would find that X' has an invalid parent hash relative
to Y', but that Z' has valid parent hash. Likewise, if the client to Y', but that Z' has a valid parent hash. Likewise, if the client
were checking bottom-up, they would find that the chain from B' ends were checking bottom-up, they would find that the chain from B' ends
in an invalid link from X' to Y'. These invalid links are the in an invalid link from X' to Y'. These invalid links are the
natural result of multiple clients having committed. natural result of multiple clients having committed.
Note also the way the tree hash and the parent hash interact. The Note also the way the tree hash and the parent hash interact. The
parent hash of node C' includes the tree hash of node D. The parent parent hash of node C' includes the tree hash of node D. The parent
hash of node Z' includes the tree hash of X', which covers nodes A hash of node Z' includes the tree hash of X', which covers nodes A
and B' (including the parent hash of B'). Although the tree hash and and B' (including the parent hash of B'). Although the tree hash and
the parent hash depend on each other, the dependency relationships the parent hash depend on each other, the dependency relationships
are structured so that there's never a circular dependency. are structured so that there is never a circular dependency.
In the particular case where a new member first receives the tree for In the particular case where a new member first receives the tree for
a group (e.g., in a ratchet tree GroupInfo extension a group (e.g., in a ratchet tree GroupInfo extension
Section 12.4.3.3), the parent hashes will be expressed in the tree Section 12.4.3.3), the parent hashes will be expressed in the tree
representation, but the tree hash need not be. Instead, the new representation, but the tree hash need not be. Instead, the new
member will recompute the tree hashes for all the nodes in the tree, member will recompute the tree hashes for all the nodes in the tree,
verifying that this matches the tree hash in the GroupInfo object. verifying that this matches the tree hash in the GroupInfo object.
Then, if the tree is valid, then the subtree hashes computed in this If the tree is valid, then the subtree hashes computed in this way
way will align with the inputs needed for parent hash validation will align with the inputs needed for parent hash validation (except
(except where recomputation is needed to account for unmerged where recomputation is needed to account for unmerged leaves).
leaves).
Appendix C. Array-Based Trees Appendix C. Array-Based Trees
One benefit of using complete balanced trees is that they admit a One benefit of using complete balanced trees is that they admit a
simple flat array representation. In this representation, leaf nodes simple flat array representation. In this representation, leaf nodes
are even-numbered nodes, with the n-th leaf at 2*n. Intermediate are even-numbered nodes, with the n-th leaf at 2*n. Intermediate
nodes are held in odd-numbered nodes. For example, the tree with 8 nodes are held in odd-numbered nodes. For example, the tree with 8
leaves has the following structure: leaves has the following structure:
X X
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/ \ / \ / \ / \
X X X X X X X X
/ \ / \ / \ / \ / \ / \ / \ / \
/ \ / \ / \ / \ / \ / \ / \ / \
X X X X X X X X X X X X X X X X
Node: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Node: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Leaf: 0 1 2 3 4 5 6 7 Leaf: 0 1 2 3 4 5 6 7
Figure 32: An 8-member tree represented as an array Figure 32: An Eight-Member Tree Represented as an Array
This allows us to compute relationships between tree nodes simply by This allows us to compute relationships between tree nodes simply by
manipulating indices, rather than having to maintain complicated manipulating indices, rather than having to maintain complicated
structures in memory. The basic rule is that the high-order bits of structures in memory. The basic rule is that the high-order bits of
parent and child nodes indices have the following relation (where x parent and child nodes indices have the following relation (where x
is an arbitrary bit string): is an arbitrary bit string):
parent=01x => left=00x, right=10x parent=01x => left=00x, right=10x
Since node relationships are implicit, the algorithms for adding and Since node relationships are implicit, the algorithms for adding and
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return min(dxy, key=level) return min(dxy, key=level)
# The common ancestor of two nodes is the lowest node that is in the # The common ancestor of two nodes is the lowest node that is in the
# direct paths of both leaves. # direct paths of both leaves.
def common_ancestor_direct(x, y, _): def common_ancestor_direct(x, y, _):
# Handle cases where one is an ancestor of the other # Handle cases where one is an ancestor of the other
lx, ly = level(x)+1, level(y)+1 lx, ly = level(x)+1, level(y)+1
if (lx <= ly) and (x>>ly == y>>ly): if (lx <= ly) and (x>>ly == y>>ly):
return y return y
elif (ly <= lx) and (x>>lx == y>>lx): elif (ly <= lx) and (x>>lx == y>>lx):
return x return x
# Handle other cases # Handle other cases
xn, yn = x, y xn, yn = x, y
k = 0 k = 0
while xn != yn: while xn != yn:
xn, yn = xn >> 1, yn >> 1 xn, yn = xn >> 1, yn >> 1
k += 1 k += 1
return (xn << k) + (1 << (k-1)) - 1 return (xn << k) + (1 << (k-1)) - 1
Appendix D. Link-Based Trees Appendix D. Link-Based Trees
An implementation may choose to store ratchet trees in a "link-based" An implementation may choose to store ratchet trees in a "link-based"
representation, where each node stores references to its parents and/ representation, where each node stores references to its parents and/
or children. (As opposed to the array-based representation suggested or children (as opposed to the array-based representation suggested
above, where these relationships are computed from relationships above, where these relationships are computed from relationships
between nodes' indices in the array.) Such an implementation needs between nodes' indices in the array). Such an implementation needs
to update these links to maintain the balanced structure of the tree to update these links to maintain the balanced structure of the tree
as the tree is extended to add new members, or truncated when members as the tree is extended to add new members or truncated when members
are removed. are removed.
The following code snippet shows how these algorithms could be The following code snippet shows how these algorithms could be
implemented in Python. implemented in Python.
class Node: class Node:
def __init__(self, value, left=None, right=None): def __init__(self, value, left=None, right=None):
self.value = value # Value of the node self.value = value # Value of the node
self.left = left # Left child node self.left = left # Left child node
self.right = right # Right child node self.right = right # Right child node
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def blank_subtree(depth): def blank_subtree(depth):
if depth == 1: if depth == 1:
return Node(None) return Node(None)
L = Node.blank_subtree(depth-1) L = Node.blank_subtree(depth-1)
R = Node.blank_subtree(depth-1) R = Node.blank_subtree(depth-1)
return Node(None, left=L, right=R) return Node(None, left=L, right=R)
def empty(self): def empty(self):
L_empty = (self.left == None) or self.left.empty() L_empty = (self.left == None) or self.left.empty()
R_empty = (self.left == None) or self.left.empty() R_empty = (self.right == None) or self.right.empty()
return (self.value == None) and L_empty and R_empty return (self.value == None) and L_empty and R_empty
class Tree: class Tree:
def __init__(self): def __init__(self):
self.depth = 0 # Depth of the tree self.depth = 0 # Depth of the tree
self.root = None # Root node of the tree, initially empty self.root = None # Root node of the tree, initially empty
# Add a blank subtree to the right # Add a blank subtree to the right
def extend(self): def extend(self):
if self.depth == 0: if self.depth == 0:
self.depth = 1 self.depth = 1
self.root = Node(None) self.root = Node(None)
L = self.root L = self.root
R = Node.blank_subtree(self.depth) R = Node.blank_subtree(self.depth)
self.root = Node(None, left=L, right=R) self.root = Node(None, left=L, right=R)
self.depth += 1 self.depth += 1
# Truncate the right subtree # Truncate the right subtree
def truncate(self): def truncate(self):
if self.root == None or self.root.right == None: if self.root == None:
raise Exception("Cannot truncate a tree with 0 or 1 nodes") return
if not self.root.right.empty(): if not self.root.right.empty():
raise Exception("Cannot truncate non-blank subtree") raise Exception("Cannot truncate non-blank subtree")
self.depth -= 1 self.depth -= 1
self.root = self.root.left self.root = self.root.left
Contributors Contributors
Joel Alwen Joel Alwen
skipping to change at page 164, line 4 skipping to change at line 7009
Inria & Mozilla Inria & Mozilla
Email: ietf@beurdouche.com Email: ietf@beurdouche.com
Raphael Robert Raphael Robert
Phoenix R&D Phoenix R&D
Email: ietf@raphaelrobert.com Email: ietf@raphaelrobert.com
Jon Millican Jon Millican
Meta Platforms Meta Platforms
Email: jmillican@meta.com Email: jmillican@meta.com
Emad Omara Emad Omara
Google Email: emad.omara@gmail.com
Email: emadomara@google.com
Katriel Cohn-Gordon Katriel Cohn-Gordon
University of Oxford University of Oxford
Email: me@katriel.co.uk Email: me@katriel.co.uk
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