rfc9261.original   rfc9261.txt 
TLS N. Sullivan Internet Engineering Task Force (IETF) N. Sullivan
Internet-Draft Cloudflare Inc. Request for Comments: 9261 Cloudflare Inc.
Intended status: Standards Track 4 March 2022 Category: Standards Track July 2022
Expires: 5 September 2022 ISSN: 2070-1721
Exported Authenticators in TLS Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-15
Abstract Abstract
This document describes a mechanism that builds on Transport Layer This document describes a mechanism that builds on Transport Layer
Security (TLS) or Datagram Transport Layer Security (DTLS) and Security (TLS) or Datagram Transport Layer Security (DTLS) and
enables peers to provide a proof of ownership of an identity, such as enables peers to provide proof of ownership of an identity, such as
an X.509 certificate. This proof can be exported by one peer, an X.509 certificate. This proof can be exported by one peer,
transmitted out-of-band to the other peer, and verified by the transmitted out of band to the other peer, and verified by the
receiving peer. receiving peer.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
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Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on 5 September 2022. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9261.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 2. Conventions and Terminology
3. Message Sequences . . . . . . . . . . . . . . . . . . . . . . 4 3. Message Sequences
4. Authenticator Request . . . . . . . . . . . . . . . . . . . . 4 4. Authenticator Request
5. Authenticator . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Authenticator
5.1. Authenticator Keys . . . . . . . . . . . . . . . . . . . 6 5.1. Authenticator Keys
5.2. Authenticator Construction . . . . . . . . . . . . . . . 7 5.2. Authenticator Construction
5.2.1. Certificate . . . . . . . . . . . . . . . . . . . . . 8 5.2.1. Certificate
5.2.2. CertificateVerify . . . . . . . . . . . . . . . . . . 8 5.2.2. CertificateVerify
5.2.3. Finished . . . . . . . . . . . . . . . . . . . . . . 10 5.2.3. Finished
5.2.4. Authenticator Creation . . . . . . . . . . . . . . . 10 5.2.4. Authenticator Creation
6. Empty Authenticator . . . . . . . . . . . . . . . . . . . . . 10 6. Empty Authenticator
7. API considerations . . . . . . . . . . . . . . . . . . . . . 11 7. API Considerations
7.1. The "request" API . . . . . . . . . . . . . . . . . . . . 11 7.1. The "request" API
7.2. The "get context" API . . . . . . . . . . . . . . . . . . 11 7.2. The "get context" API
7.3. The "authenticate" API . . . . . . . . . . . . . . . . . 11 7.3. The "authenticate" API
7.4. The "validate" API . . . . . . . . . . . . . . . . . . . 12 7.4. The "validate" API
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. IANA Considerations
8.1. Update of the TLS ExtensionType Registry . . . . . . . . 13 8.1. Update of the TLS ExtensionType Registry
8.2. Update of the TLS Exporter Labels Registry . . . . . . . 13 8.2. Update of the TLS Exporter Labels Registry
8.3. Update of the TLS HandshakeType Registry . . . . . . . . 13 8.3. Update of the TLS HandshakeType Registry
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13 9. Security Considerations
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 10. References
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 10.1. Normative References
11.1. Normative References . . . . . . . . . . . . . . . . . . 14 10.2. Informative References
11.2. Informative References . . . . . . . . . . . . . . . . . 15 Acknowledgements
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 Author's Address
1. Introduction 1. Introduction
This document provides a way to authenticate one party of a Transport This document provides a way to authenticate one party of a Transport
Layer Security (TLS) or Datagram Transport Layer Security (DTLS) Layer Security (TLS) or Datagram Transport Layer Security (DTLS)
connection to its peer using authentication messages created after connection to its peer using authentication messages created after
the session has been established. This allows both the client and the session has been established. This allows both the client and
server to prove ownership of additional identities at any time after server to prove ownership of additional identities at any time after
the handshake has completed. This proof of authentication can be the handshake has completed. This proof of authentication can be
exported and transmitted out-of-band from one party to be validated exported and transmitted out of band from one party to be validated
by its peer. by its peer.
This mechanism provides two advantages over the authentication that This mechanism provides two advantages over the authentication that
TLS and DTLS natively provide: TLS and DTLS natively provide:
multiple identities - Endpoints that are authoritative for multiple multiple identities: Endpoints that are authoritative for multiple
identities - but do not have a single certificate that includes identities, but that do not have a single certificate that
all of the identities - can authenticate additional identities includes all of the identities, can authenticate additional
over a single connection. identities over a single connection.
spontaneous authentication - Endpoints can authenticate after a spontaneous authentication: After a connection is established,
connection is established, in response to events in a higher-layer endpoints can authenticate in response to events in a higher-layer
protocol, as well as integrating more context (such as context protocol; they can also integrate more context (such as context
from the application). from the application).
Versions of TLS prior to TLS 1.3 used renegotiation as a way to Versions of TLS prior to TLS 1.3 used renegotiation as a way to
enable post-handshake client authentication given an existing TLS enable post-handshake client authentication given an existing TLS
connection. The mechanism described in this document may be used to connection. The mechanism described in this document may be used to
replace the post-handshake authentication functionality provided by replace the post-handshake authentication functionality provided by
renegotiation. Unlike renegotiation, exported Authenticator-based renegotiation. Unlike renegotiation, Exported Authenticator-based
post-handshake authentication does not require any changes at the TLS post-handshake authentication does not require any changes at the TLS
layer. layer.
Post-handshake authentication is defined in section 4.6.3 of TLS 1.3 Post-handshake authentication is defined in TLS 1.3 Section 4.6.2 of
[RFC8446], but it has the disadvantage of requiring additional state [RFC8446], but it has the disadvantage of requiring additional state
to be stored as part of the TLS state machine. Furthermore, the to be stored as part of the TLS state machine. Furthermore, the
authentication boundaries of TLS 1.3 post-handshake authentication authentication boundaries of TLS 1.3 post-handshake authentication
align with TLS record boundaries, which are often not aligned with align with TLS record boundaries, which are often not aligned with
the authentication boundaries of the higher-layer protocol. For the authentication boundaries of the higher-layer protocol. For
example, multiplexed connection protocols like HTTP/2 [RFC7540] do example, multiplexed connection protocols like HTTP/2 [RFC9113] do
not have a notion of which TLS record a given message is a part of. not have a notion of which TLS record a given message is a part of.
Exported Authenticators are meant to be used as a building block for Exported Authenticators are meant to be used as a building block for
application protocols. Mechanisms such as those required to application protocols. Mechanisms such as those required to
advertise support and handle authentication errors are not handled by advertise support and handle authentication errors are not handled by
TLS (or DTLS). TLS (or DTLS).
The minimum version of TLS and DTLS required to implement the The minimum version of TLS and DTLS required to implement the
mechanisms decribed in this document are TLS 1.2 [RFC6347] and DTLS mechanisms described in this document are TLS 1.2 [RFC5246] and DTLS
1.2 [RFC5246]. 1.2 [RFC6347].
2. Conventions and Terminology 2. Conventions and 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
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This document uses terminology such as client, server, connection, This document uses terminology such as client, server, connection,
handshake, endpoint, peer that are defined in section 1.1 of handshake, endpoint, and peer that are defined in Section 1.1 of
[RFC8446]. The term "initial connection" refers to the (D)TLS [RFC8446]. The term "initial connection" refers to the (D)TLS
connection from which the exported authenticator messages are connection from which the Exported Authenticator messages are
derived. derived.
3. Message Sequences 3. Message Sequences
There are two types of messages defined in this document: There are two types of messages defined in this document:
Authenticator Requests and Authenticators. These can be combined in authenticator requests and authenticators. These can be combined in
the following three sequences: the following three sequences:
Client Authentication Client Authentication
* Server generates Authenticator Request * Server generates authenticator request
* Client generates Authenticator from Server's Authenticator Request * Client generates Authenticator from Server's authenticator request
* Server validates Client's Authenticator * Server validates Client's authenticator
Server Authentication Server Authentication
* Client generates Authenticator Request * Client generates authenticator request
* Server generates Authenticator from Client's Authenticator Request * Server generates authenticator from Client's authenticator request
* Client validates Server's Authenticator * Client validates Server's authenticator
Spontaneous Server Authentication Spontaneous Server Authentication
* Server generates Authenticator * Server generates authenticator
* Client validates Server's Authenticator * Client validates Server's authenticator
4. Authenticator Request 4. Authenticator Request
The authenticator request is a structured message that can be created The authenticator request is a structured message that can be created
by either party of a (D)TLS connection using data exported from that by either party of a (D)TLS connection using data exported from that
connection. It can be transmitted to the other party of the (D)TLS connection. It can be transmitted to the other party of the (D)TLS
connection at the application layer. The application layer protocol connection at the application layer. The application-layer protocol
used to send the authenticator request SHOULD use a secure transport used to send the authenticator request SHOULD use a secure transport
channel with equivalent security to TLS, such as QUIC [RFC9001], as channel with equivalent security to TLS, such as QUIC [RFC9001], as
its underlying transport to keep the request confidential. The its underlying transport to keep the request confidential. The
application MAY use the existing (D)TLS connection to transport the application MAY use the existing (D)TLS connection to transport the
authenticator. authenticator.
An authenticator request message can be constructed by either the An authenticator request message can be constructed by either the
client or the server. Server-generated authenticator requests use client or the server. Server-generated authenticator requests use
the CertificateRequest message from Section 4.3.2 of [RFC8446]. the CertificateRequest message from Section 4.3.2 of [RFC8446].
Client-generated authenticator requests use a new message, called the Client-generated authenticator requests use a new message, called the
ClientCertificateRequest, which uses the same structure as "ClientCertificateRequest", that uses the same structure as
CertificateRequest. (Note that the latter is not a request for a CertificateRequest. (Note that the latter is not a request for a
client certificate, but rather a certificate request generated by the client certificate, but rather a certificate request generated by the
client.) These message structures are used even if the connection client.) These message structures are used even if the connection
protocol is TLS 1.2 or DTLS 1.2. protocol is TLS 1.2 or DTLS 1.2.
The CertificateRequest and ClientCertificateRequest messages are used The CertificateRequest and ClientCertificateRequest messages are used
to define the parameters in a request for an authenticator. These to define the parameters in a request for an authenticator. These
are encoded as TLS handshake messages, including length and type are encoded as TLS handshake messages, including length and type
fields. They do not include any TLS record layer framing and are not fields. They do not include any TLS record-layer framing and are not
encrypted with a handshake or application-data key. encrypted with a handshake or application-data key.
The structures are defined to be: The structures are defined to be:
struct { struct {
opaque certificate_request_context<0..2^8-1>; opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>; Extension extensions<2..2^16-1>;
} ClientCertificateRequest; } ClientCertificateRequest;
struct { struct {
opaque certificate_request_context<0..2^8-1>; opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>; Extension extensions<2..2^16-1>;
} CertificateRequest; } CertificateRequest;
certificate_request_context: An opaque string which identifies the certificate_request_context: An opaque string that identifies the
authenticator request and which will be echoed in the authenticator request and that will be echoed in the authenticator
authenticator message. A certificate_request_context value MUST message. A certificate_request_context value MUST be unique for
be unique for each authenticator request within the scope of a each authenticator request within the scope of a connection
connection (preventing replay and context confusion). The (preventing replay and context confusion). The
certificate_request_context SHOULD be chosen to be unpredictable certificate_request_context SHOULD be chosen to be unpredictable
to the peer (e.g., by randomly generating it) in order to prevent to the peer (e.g., by randomly generating it) in order to prevent
an attacker who has temporary access to the peer's private key an attacker who has temporary access to the peer's private key
from pre-computing valid authenticators. For example, the from precomputing valid authenticators. For example, the
application may choose this value to correspond to a value used in application may choose this value to correspond to a value used in
an existing datastructure in the software to simplify an existing data structure in the software to simplify
implementation. implementation.
extensions: The set of extensions allowed in the CertificateRequest extensions: The set of extensions allowed in the structures of
structure and the ClientCertificateRequest structure are those CertificateRequest and ClientCertificateRequest is comprised of
defined in the TLS ExtensionType Values IANA registry [RFC8447] those defined in the "TLS ExtensionType Values" IANA registry
containing CR in the TLS 1.3 column. In addition, the set of containing CR in the "TLS 1.3" column (see [IANA-TLS] and
extensions in the ClientCertificateRequest structure MAY include [RFC8447]). In addition, the set of extensions in the
the server_name [RFC6066] extension. ClientCertificateRequest structure MAY include the server_name
extension [RFC6066].
The uniqueness requirements of the certificate_request_context apply The uniqueness requirements of the certificate_request_context apply
only to CertificateRequest and ClientCertificateRequest messages that across CertificateRequest and ClientCertificateRequest messages that
are used as part of authenticator requests, but do apply across are used as part of authenticator requests. A
CertificateRequest and ClientCertificateRequest messages. A
certificate_request_context value used in a ClientCertificateRequest certificate_request_context value used in a ClientCertificateRequest
cannot be used in an authenticator CertificateRequest on the same cannot be used in an authenticator CertificateRequest on the same
connection, and vice versa. There is no impact if the value of a connection, and vice versa. There is no impact if the value of a
certificate_request_context used in an authenticator request matches certificate_request_context used in an authenticator request matches
the value of a certificate_request_context in the handshake or in a the value of a certificate_request_context in the handshake or in a
post-handshake message. post-handshake message.
5. Authenticator 5. Authenticator
The authenticator is a structured message that can be exported from The authenticator is a structured message that can be exported from
either party of a (D)TLS connection. It can be transmitted to the either party of a (D)TLS connection. It can be transmitted to the
other party of the (D)TLS connection at the application layer. The other party of the (D)TLS connection at the application layer. The
application layer protocol used to send the authenticator SHOULD use application-layer protocol used to send the authenticator SHOULD use
a secure transport channel with equivalent security to TLS, such as a secure transport channel with equivalent security to TLS, such as
QUIC [RFC9001], as its underlying transport to keep the authenticator QUIC [RFC9001], as its underlying transport to keep the authenticator
confidential. The application MAY use the existing (D)TLS connection confidential. The application MAY use the existing (D)TLS connection
to transport the authenticator. to transport the authenticator.
An authenticator message can be constructed by either the client or An authenticator message can be constructed by either the client or
the server given an established (D)TLS connection, an identity, such the server given an established (D)TLS connection; an identity, such
as an X.509 certificate, and a corresponding private key. Clients as an X.509 certificate; and a corresponding private key. Clients
MUST NOT send an authenticator without a preceding authenticator MUST NOT send an authenticator without a preceding authenticator
request; for servers an authenticator request is optional. For request; for servers, an authenticator request is optional. For
authenticators that do not correspond to authenticator requests, the authenticators that do not correspond to authenticator requests, the
certificate_request_context is chosen by the server. certificate_request_context is chosen by the server.
5.1. Authenticator Keys 5.1. Authenticator Keys
Each authenticator is computed using a Handshake Context and Finished Each authenticator is computed using a Handshake Context and Finished
MAC Key derived from the (D)TLS connection. These values are derived MAC (Message Authentication Code) Key derived from the (D)TLS
using an exporter as described in Section 4 of [RFC5705] (for (D)TLS connection. These values are derived using an exporter as described
1.2) or Section 7.5 of [RFC8446] (for (D)TLS 1.3). For (D)TLS 1.3, in Section 4 of [RFC5705] (for (D)TLS 1.2) or Section 7.5 of
the exporter_master_secret MUST be used, not the [RFC8446] (for (D)TLS 1.3). For (D)TLS 1.3, the
exporter_master_secret MUST be used, not the
early_exporter_master_secret. These values use different labels early_exporter_master_secret. These values use different labels
depending on the role of the sender: depending on the role of the sender:
* The Handshake Context is an exporter value that is derived using * The Handshake Context is an exporter value that is derived using
the label "EXPORTER-client authenticator handshake context" or the label "EXPORTER-client authenticator handshake context" or
"EXPORTER-server authenticator handshake context" for "EXPORTER-server authenticator handshake context" for
authenticators sent by the client or server respectively. authenticators sent by the client or server, respectively.
* The Finished MAC Key is an exporter value derived using the label * The Finished MAC Key is an exporter value derived using the label
"EXPORTER-client authenticator finished key" or "EXPORTER-server "EXPORTER-client authenticator finished key" or "EXPORTER-server
authenticator finished key" for authenticators sent by the client authenticator finished key" for authenticators sent by the client
or server respectively. or server, respectively.
The context_value used for the exporter is empty (zero length) for The context_value used for the exporter is empty (zero length) for
all four values. There is no need to include additional context all four values. There is no need to include additional context
information at this stage since the application-supplied context is information at this stage because the application-supplied context is
included in the authenticator itself. The length of the exported included in the authenticator itself. The length of the exported
value is equal to the length of the output of the hash function value is equal to the length of the output of the hash function
associated with the selected cipher suite (for TLS 1.3) or the hash associated with the selected ciphersuite (for TLS 1.3) or the hash
function used for the pseudorandom function (PRF) (for (D)TLS 1.2). function used for the pseudorandom function (PRF) (for (D)TLS 1.2).
Exported authenticators cannot be used with (D)TLS 1.2 cipher suites Exported Authenticators cannot be used with (D)TLS 1.2 ciphersuites
that do not use the TLS PRF and with TLS 1.3 cipher suites that do that do not use the TLS PRF and with TLS 1.3 ciphersuites that do not
not have an associated hash function. This hash is referred to as have an associated hash function. This hash is referred to as the
the authenticator hash. "authenticator hash".
To avoid key synchronization attacks, Exported Authenticators MUST To avoid key synchronization attacks, Exported Authenticators MUST
NOT be generated or accepted on (D)TLS 1.2 connections that did not NOT be generated or accepted on (D)TLS 1.2 connections that did not
negotiate the extended master secret extension [RFC7627]. negotiate the extended master secret extension [RFC7627].
5.2. Authenticator Construction 5.2. Authenticator Construction
An authenticator is formed from the concatenation of TLS 1.3 An authenticator is formed from the concatenation of TLS 1.3
[RFC8446] Certificate, CertificateVerify, and Finished messages. Certificate, CertificateVerify, and Finished messages [RFC8446].
These messages are encoded as TLS handshake messages, including These messages are encoded as TLS handshake messages, including
length and type fields. They do not include any TLS record layer length and type fields. They do not include any TLS record-layer
framing and are not encrypted with a handshake or application-data framing and are not encrypted with a handshake or application-data
key. key.
If the peer populating the certificate_request_context field in an If the peer populating the certificate_request_context field in an
authenticator's Certificate message has already created or correctly authenticator's Certificate message has already created or correctly
validated an authenticator with the same value, then no authenticator validated an authenticator with the same value, then no authenticator
should be constructed. If there is no authenticator request, the should be constructed. If there is no authenticator request, the
extensions are chosen from those presented in the (D)TLS handshake's extensions are chosen from those presented in the (D)TLS handshake's
ClientHello. Only servers can provide an authenticator without a ClientHello. Only servers can provide an authenticator without a
corresponding request. corresponding request.
skipping to change at page 8, line 9 skipping to change at line 325
the general model for extensions in (D)TLS in which extensions can the general model for extensions in (D)TLS in which extensions can
only be included as part of a Certificate message if they were only be included as part of a Certificate message if they were
previously sent as part of a CertificateRequest message or previously sent as part of a CertificateRequest message or
ClientHello message. This ensures that the recipient will be able to ClientHello message. This ensures that the recipient will be able to
process such extensions. process such extensions.
5.2.1. Certificate 5.2.1. Certificate
The Certificate message contains the identity to be used for The Certificate message contains the identity to be used for
authentication, such as the end-entity certificate and any supporting authentication, such as the end-entity certificate and any supporting
certificates in the chain. This structure is defined in [RFC8446], certificates in the chain. This structure is defined in
Section 4.4.2. Section 4.4.2 of [RFC8446].
The Certificate message contains an opaque string called The Certificate message contains an opaque string called
certificate_request_context, which is extracted from the "certificate_request_context", which is extracted from the
authenticator request if present. If no authenticator request is authenticator request, if present. If no authenticator request is
provided, the certificate_request_context can be chosen arbitrarily provided, the certificate_request_context can be chosen arbitrarily;
but MUST be unique within the scope of the connection and be however, it MUST be unique within the scope of the connection and be
unpredictable to the peer. unpredictable to the peer.
Certificates chosen in the Certificate message MUST conform to the Certificates chosen in the Certificate message MUST conform to the
requirements of a Certificate message in the negotiated version of requirements of a Certificate message in the negotiated version of
(D)TLS. In particular, the entries of certificate_list MUST be valid (D)TLS. In particular, the entries of certificate_list MUST be valid
for the signature algorithms indicated by the peer in the for the signature algorithms indicated by the peer in the
"signature_algorithms" and "signature_algorithms_cert" extension, as "signature_algorithms" and "signature_algorithms_cert" extensions, as
described in Section 4.2.3 of [RFC8446] for (D)TLS 1.3 or from described in Section 4.2.3 of [RFC8446] for (D)TLS 1.3 or in Sections
Sections 7.4.2 and 7.4.6 of [RFC5246] for (D)TLS 1.2. 7.4.2 and 7.4.6 of [RFC5246] for (D)TLS 1.2.
In addition to "signature_algorithms" and In addition to "signature_algorithms" and
"signature_algorithms_cert", the "server_name" [RFC6066], "signature_algorithms_cert", the "server_name" [RFC6066],
"certificate_authorities" (Section 4.2.4. of [RFC8446]), and "certificate_authorities" (Section 4.2.4 of [RFC8446]), and
"oid_filters" (Section 4.2.5. of [RFC8446]) extensions are used to "oid_filters" (Section 4.2.5 of [RFC8446]) extensions are used to
guide certificate selection. guide certificate selection.
Only the X.509 certificate type defined in [RFC8446] is supported. Only the X.509 certificate type defined in [RFC8446] is supported.
Alternative certificate formats such as [RFC7250] Raw Public Keys are Alternative certificate formats such as Raw Public Keys as described
not supported in this version of the specification and their use in in [RFC7250] are not supported in this version of the specification
this context has not yet been analysed. and their use in this context has not yet been analyzed.
If an authenticator request was provided, the Certificate message If an authenticator request was provided, the Certificate message
MUST contain only extensions present in the authenticator request. MUST contain only extensions present in the authenticator request.
Otherwise, the Certificate message MUST contain only extensions Otherwise, the Certificate message MUST contain only extensions
present in the (D)TLS ClientHello. Unrecognized extensions in the present in the (D)TLS ClientHello. Unrecognized extensions in the
authenticator request MUST be ignored. authenticator request MUST be ignored.
5.2.2. CertificateVerify 5.2.2. CertificateVerify
This message is used to provide explicit proof that an endpoint This message is used to provide explicit proof that an endpoint
skipping to change at page 9, line 16 skipping to change at line 377
SignatureScheme algorithm; SignatureScheme algorithm;
opaque signature<0..2^16-1>; opaque signature<0..2^16-1>;
} CertificateVerify; } CertificateVerify;
The algorithm field specifies the signature algorithm used (see The algorithm field specifies the signature algorithm used (see
Section 4.2.3 of [RFC8446] for the definition of this field). The Section 4.2.3 of [RFC8446] for the definition of this field). The
signature is a digital signature using that algorithm. signature is a digital signature using that algorithm.
The signature scheme MUST be a valid signature scheme for TLS 1.3. The signature scheme MUST be a valid signature scheme for TLS 1.3.
This excludes all RSASSA-PKCS1-v1_5 algorithms and combinations of This excludes all RSASSA-PKCS1-v1_5 algorithms and combinations of
ECDSA and hash algorithms that are not supported in TLS 1.3. Elliptic Curve Digital Signature Algorithm (ECDSA) and hash
algorithms that are not supported in TLS 1.3.
If an authenticator request is present, the signature algorithm MUST If an authenticator request is present, the signature algorithm MUST
be chosen from one of the signature schemes present in the be chosen from one of the signature schemes present in the
"signature_algorithms" extensino of the authenticator request. "signature_algorithms" extension of the authenticator request.
Otherwise, with spontaneous server authentication, the signature Otherwise, with spontaneous server authentication, the signature
algorithm used MUST be chosen from the "signature_algorithms" sent by algorithm used MUST be chosen from the "signature_algorithms" sent by
the peer in the ClientHello of the (D)TLS handshake. If there are no the peer in the ClientHello of the (D)TLS handshake. If there are no
available signature algorithms, then no authenticator should be available signature algorithms, then no authenticator should be
constructed. constructed.
The signature is computed using the chosen signature scheme over the The signature is computed using the chosen signature scheme over the
concatenation of: concatenation of:
* A string that consists of octet 32 (0x20) repeated 64 times * a string that consists of octet 32 (0x20) repeated 64 times,
* The context string "Exported Authenticator" (which is not NUL- * the context string "Exported Authenticator" (which is not NUL-
terminated) terminated),
* A single 0 octet which serves as the separator * a single 0 octet that serves as the separator, and
* The hashed authenticator transcript * the hashed authenticator transcript.
The authenticator transcript is the hash of the concatenated The authenticator transcript is the hash of the concatenated
Handshake Context, authenticator request (if present), and Handshake Context, authenticator request (if present), and
Certificate message: Certificate message:
Hash(Handshake Context || authenticator request || Certificate) Hash(Handshake Context || authenticator request || Certificate)
Where Hash is the authenticator hash defined in section 4.1. If the Where Hash is the authenticator hash defined in Section 5.1. If the
authenticator request is not present, it is omitted from this authenticator request is not present, it is omitted from this
construction, i.e., it is zero-length. construction, i.e., it is zero-length.
If the party that generates the exported authenticator does so with a If the party that generates the authenticator does so with a
different connection than the party that is validating it, then the different connection than the party that is validating it, then the
Handshake Context will not match, resulting in a CertificateVerify Handshake Context will not match, resulting in a CertificateVerify
message that does not validate. This includes situations in which message that does not validate. This includes situations in which
the application data is sent via TLS-terminating proxy. Given a the application data is sent via TLS-terminating proxy. Given a
failed CertificateVerify validation, it may be helpful for the failed CertificateVerify validation, it may be helpful for the
application to confirm that both peers share the same connection application to confirm that both peers share the same connection
using a value derived from the connection secrets (such as the using a value derived from the connection secrets (such as the
Handshake Context) before taking a user-visible action. Handshake Context) before taking a user-visible action.
5.2.3. Finished 5.2.3. Finished
An HMAC [HMAC] over the hashed authenticator transcript, which is the An HMAC [HMAC] over the hashed authenticator transcript is the
concatenation of the Handshake Context, authenticator request (if concatenation of the Handshake Context, authenticator request (if
present), Certificate, and CertificateVerify. The HMAC is computed present), Certificate, and CertificateVerify. The HMAC is computed
using the authenticator hash, using the Finished MAC Key as a key. using the authenticator hash, using the Finished MAC Key as a key.
Finished = HMAC(Finished MAC Key, Hash(Handshake Context || Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate || CertificateVerify)) authenticator request || Certificate || CertificateVerify))
5.2.4. Authenticator Creation 5.2.4. Authenticator Creation
An endpoint constructs an authenticator by serializing the An endpoint constructs an authenticator by serializing the
skipping to change at page 10, line 38 skipping to change at line 449
An authenticator is valid if the CertificateVerify message is An authenticator is valid if the CertificateVerify message is
correctly constructed given the authenticator request (if used) and correctly constructed given the authenticator request (if used) and
the Finished message matches the expected value. When validating an the Finished message matches the expected value. When validating an
authenticator, constant-time comparisons SHOULD be used for signature authenticator, constant-time comparisons SHOULD be used for signature
and MAC validation. and MAC validation.
6. Empty Authenticator 6. Empty Authenticator
If, given an authenticator request, the endpoint does not have an If, given an authenticator request, the endpoint does not have an
appropriate identity or does not want to return one, it constructs an appropriate identity or does not want to return one, it constructs an
authenticated refusal called an empty authenticator. This is a authenticated refusal called an "empty authenticator". This is a
Finished message sent without a Certificate or CertificateVerify. Finished message sent without a Certificate or CertificateVerify.
This message is an HMAC over the hashed authenticator transcript with This message is an HMAC over the hashed authenticator transcript with
a Certificate message containing no CertificateEntries and the a Certificate message containing no CertificateEntries and the
CertificateVerify message omitted. The HMAC is computed using the CertificateVerify message omitted. The HMAC is computed using the
authenticator hash, using the Finished MAC Key as a key. This authenticator hash, using the Finished MAC Key as a key. This
message is encoded as a TLS handshake message, including length and message is encoded as a TLS handshake message, including length and
type field. It does not include TLS record layer framing and is not type field. It does not include TLS record-layer framing and is not
encrypted with a handshake or application-data key. encrypted with a handshake or application-data key.
Finished = HMAC(Finished MAC Key, Hash(Handshake Context || Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate)) authenticator request || Certificate))
7. API considerations 7. API Considerations
The creation and validation of both authenticator requests and The creation and validation of both authenticator requests and
authenticators SHOULD be implemented inside the (D)TLS library even authenticators SHOULD be implemented inside the (D)TLS library even
if it is possible to implement it at the application layer. (D)TLS if it is possible to implement it at the application layer. (D)TLS
implementations supporting the use of exported authenticators SHOULD implementations supporting the use of Exported Authenticators SHOULD
provide application programming interfaces by which clients and provide application programming interfaces by which clients and
servers may request and verify exported authenticator messages. servers may request and verify Exported Authenticator messages.
Notwithstanding the success conditions described below, all APIs MUST Notwithstanding the success conditions described below, all APIs MUST
fail if: fail if:
* the connection uses a (D)TLS version of 1.1 or earlier, or * the connection uses a (D)TLS version of 1.1 or earlier, or
* the connection is (D)TLS 1.2 and the extended master secret * the connection is (D)TLS 1.2 and the extended master secret
extension [RFC7627] was not negotiated extension [RFC7627] was not negotiated
The following sections describe APIs that are considered necessary to The following sections describe APIs that are considered necessary to
implement exported authenticators. These are informative only. implement Exported Authenticators. These are informative only.
7.1. The "request" API 7.1. The "request" API
The "request" API takes as input: The "request" API takes as input:
* certificate_request_context (from 0 to 255 octets) * certificate_request_context (from 0 to 255 octets)
* set of extensions to include (this MUST include * the set of extensions to include (this MUST include
signature_algorithms) and the contents thereof signature_algorithms) and the contents thereof
It returns an authenticator request, which is a sequence of octets It returns an authenticator request, which is a sequence of octets
that comprises a CertificateRequest or ClientCertificateRequest that comprises a CertificateRequest or ClientCertificateRequest
message. message.
7.2. The "get context" API 7.2. The "get context" API
The "get context" API takes as input: The "get context" API takes as input:
skipping to change at page 12, line 4 skipping to change at line 508
* authenticator or authenticator request * authenticator or authenticator request
It returns the certificate_request_context. It returns the certificate_request_context.
7.3. The "authenticate" API 7.3. The "authenticate" API
The "authenticate" API takes as input: The "authenticate" API takes as input:
* a reference to the initial connection * a reference to the initial connection
* an identity, such as a set of certificate chains and associated * an identity, such as a set of certificate chains and associated
extensions (OCSP [RFC6960], SCT [RFC6962], etc.) extensions (OCSP [RFC6960], SCT [RFC6962] (obsoleted by
[RFC9162]), etc.)
* a signer (either the private key associated with the identity, or * a signer (either the private key associated with the identity or
interface to perform private key operations) for each chain the interface to perform private key operations) for each chain
* an authenticator request or certificate_request_context (from 0 to * an authenticator request or certificate_request_context (from 0 to
255 octets) 255 octets)
It returns either the exported authenticator or an empty It returns either the authenticator or an empty authenticator as a
authenticator as a sequence of octets. It is recommended that the sequence of octets. It is RECOMMENDED that the logic for selecting
logic for selecting the certificates and extensions to include in the the certificates and extensions to include in the exporter be
exporter is implemented in the TLS library. Implementing this in the implemented in the TLS library. Implementing this in the TLS library
TLS library lets the implementer take advantage of existing extension lets the implementer take advantage of existing extension and
and certificate selection logic and more easily remember which certificate selection logic, and the implementer can more easily
extensions were sent in the ClientHello. remember which extensions were sent in the ClientHello.
It is also possible to implement this API outside of the TLS library It is also possible to implement this API outside of the TLS library
using TLS exporters. This may be preferable in cases where the using TLS exporters. This may be preferable in cases where the
application does not have access to a TLS library with these APIs or application does not have access to a TLS library with these APIs or
when TLS is handled independently of the application layer protocol. when TLS is handled independently of the application-layer protocol.
7.4. The "validate" API 7.4. The "validate" API
The "validate" API takes as input: The "validate" API takes as input:
* a reference to the initial connection * a reference to the initial connection
* an optional authenticator request * an optional authenticator request
* an authenticator * an authenticator
* a function for validating a certificate chain * a function for validating a certificate chain
It returns a status to indicate whether the authenticator is valid or It returns a status to indicate whether or not the authenticator is
not after applying the function for validating the certificate chain valid after applying the function for validating the certificate
to the chain contained in the authenticator. If validation is chain to the chain contained in the authenticator. If validation is
successful, it also returns the identity, such as the certificate successful, it also returns the identity, such as the certificate
chain and its extensions. chain and its extensions.
The API should return a failure if the certificate_request_context of The API should return a failure if the certificate_request_context of
the authenticator was used in a different authenticator that was the authenticator was used in a different authenticator that was
previously validated. Well-formed empty authenticators are returned previously validated. Well-formed empty authenticators are returned
as invalid. as invalid.
When validating an authenticator, constant-time comparison should be When validating an authenticator, constant-time comparison should be
used. used.
8. IANA Considerations 8. IANA Considerations
8.1. Update of the TLS ExtensionType Registry 8.1. Update of the TLS ExtensionType Registry
IANA is requested to update the entry for server_name(0) in the IANA has updated the entry for server_name(0) in the "TLS
registry for ExtensionType (defined in [RFC8446]) by replacing the ExtensionType Values" registry [IANA-TLS] (defined in [RFC8446]) by
value in the "TLS 1.3" column with the value "CH, EE, CR" and adding replacing the value in the "TLS 1.3" column with the value "CH, EE,
this document in the "Reference" column. CR" and listing this document in the "Reference" column.
IANA is also requested to add the following note to the registry: IANA has also added the following note to the registry:
The addition of the "CR" to the "TLS 1.3" column for the | The addition of the "CR" to the "TLS 1.3" column for the
server_name(0) extension only marks the extension as valid in a | server_name(0) extension only marks the extension as valid in a
ClientCertificateRequest created as part of client-generated | ClientCertificateRequest created as part of client-generated
authenticator requests. | authenticator requests.
8.2. Update of the TLS Exporter Labels Registry 8.2. Update of the TLS Exporter Labels Registry
IANA is requested to add the following entries to the registry for IANA has added the following entries to the "TLS Exporter Labels"
Exporter Labels (defined in [RFC5705]): "EXPORTER-client registry [IANA-EXPORT] (defined in [RFC5705]): "EXPORTER-client
authenticator handshake context", "EXPORTER-server authenticator authenticator handshake context", "EXPORTER-server authenticator
handshake context", "EXPORTER-client authenticator handshake handshake context", "EXPORTER-client authenticator finished key" and
context", "EXPORTER-client authenticator finished key" and "EXPORTER- "EXPORTER-server authenticator finished key" with "DTLS-OK" and
server authenticator finished key" with "DTLS-OK" and "Recommended" "Recommended" set to "Y" and this document listed as the reference.
set to "Y" and this document added to the "Reference" column.
8.3. Update of the TLS HandshakeType Registry 8.3. Update of the TLS HandshakeType Registry
IANA is requested to add the following entry to the registry for IANA has added the following entry to the "TLS HandshakeType"
HandshakeType (defined in [RFC8446]): "client_certificate_request" registry [IANA-HANDSHAKE] (defined in [RFC8446]):
with "DTLS-OK" and "Recommended" set to "Y" and this document added "client_certificate_request" (17) with "DTLS-OK" set to "Y" and this
to the "Reference" column with the following in the "Note" column: document listed as the reference. In addition, the following appears
"Used in TLS versions prior to 1.3." in the "Comment" column:
| Used in TLS versions prior to 1.3.
9. Security Considerations 9. Security Considerations
The Certificate/Verify/Finished pattern intentionally looks like the The Certificate/Verify/Finished pattern intentionally looks like the
TLS 1.3 pattern which now has been analyzed several times. For TLS 1.3 pattern that now has been analyzed several times. For
example, [SIGMAC] presents a relevant framework for analysis, and example, [SIGMAC] presents a relevant framework for analysis, and
section 10. of [RFC8446] contains a conprehensive set of references. Appendix E.1.6 of [RFC8446] contains a comprehensive set of
references.
Authenticators are independent and unidirectional. There is no Authenticators are independent and unidirectional. There is no
explicit state change inside TLS when an authenticator is either explicit state change inside TLS when an authenticator is either
created or validated. The application in possession of a validated created or validated. The application in possession of a validated
authenticator can rely on any semantics associated with data in the authenticator can rely on any semantics associated with data in the
certificate_request_context. certificate_request_context.
* This property makes it difficult to formally prove that a server * This property makes it difficult to formally prove that a server
is jointly authoritative over multiple identities, rather than is jointly authoritative over multiple identities, rather than
individually authoritative over each. individually authoritative over each.
* There is no indication in (D)TLS about which point in time an * There is no indication in (D)TLS about which point in time an
authenticator was computed. Any feedback about the time of authenticator was computed. Any feedback about the time of
creation or validation of the authenticator should be tracked as creation or validation of the authenticator should be tracked as
part of the application layer semantics if required. part of the application-layer semantics if required.
The signatures generated with this API cover the context string The signatures generated with this API cover the context string
"Exported Authenticator" and therefore cannot be transplanted into "Exported Authenticator"; therefore, they cannot be transplanted into
other protocols. other protocols.
In TLS 1.3 the client can not explicitly learn from the TLS layer In TLS 1.3, the client cannot explicitly learn from the TLS layer
whether its Finished message was accepted. Because the application whether its Finished message was accepted. Because the application
traffic keys are not dependent on the client's final flight, traffic keys are not dependent on the client's final flight,
receiving messages from the server does not prove that the server receiving messages from the server does not prove that the server
received the client's Finished. To avoid disagreement between the received the client's Finished message. To avoid disagreement
client and server on the authentication status of EAs, servers MUST between the client and server on the authentication status of
verify the client Finished before sending an EA or processing a Exported Authenticators, servers MUST verify the client Finished
received EA. message before sending an EA or processing a received Exported
Authenticator.
10. Acknowledgements
Comments on this proposal were provided by Martin Thomson.
Suggestions for Section 9 were provided by Karthikeyan Bhargavan.
11. References 10. References
11.1. Normative References 10.1. Normative References
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [HMAC] 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/info/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/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 15, line 36 skipping to change at line 680
May 2017, <https://www.rfc-editor.org/info/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/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS [RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018, and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>. <https://www.rfc-editor.org/info/rfc8447>.
11.2. Informative References 10.2. Informative References
[IANA-EXPORT]
IANA, "TLS Exporter Labels",
<https://www.iana.org/assignments/tls-parameters/>.
[IANA-HANDSHAKE]
IANA, "TLS HandshakeType",
<https://www.iana.org/assignments/tls-parameters/>.
[IANA-TLS] IANA, "TLS ExtensionType Values",
<https://www.iana.org/assignments/tls-extensiontype-
values/>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP", Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013, RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>. <https://www.rfc-editor.org/info/rfc6960>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>. <https://www.rfc-editor.org/info/rfc6962>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>. June 2014, <https://www.rfc-editor.org/info/rfc7250>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[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/info/rfc9001>. <https://www.rfc-editor.org/info/rfc9001>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://www.rfc-editor.org/info/rfc9162>.
[SIGMAC] Krawczyk, H., "A Unilateral-to-Mutual Authentication [SIGMAC] Krawczyk, H., "A Unilateral-to-Mutual Authentication
Compiler for Key Exchange (with Applications to Client Compiler for Key Exchange (with Applications to Client
Authentication in TLS 1.3)", 2016, Authentication in TLS 1.3)", Proceedings of the 2016 ACM
SIGSAC Conference on Computer and Communications Security,
DOI 10.1145/2976749.2978325, August 2016,
<https://eprint.iacr.org/2016/711.pdf>. <https://eprint.iacr.org/2016/711.pdf>.
Acknowledgements
Comments on this proposal were provided by Martin Thomson.
Suggestions for Section 9 were provided by Karthikeyan Bhargavan.
Author's Address Author's Address
Nick Sullivan Nick Sullivan
Cloudflare Inc. Cloudflare Inc.
Email: nick@cloudflare.com Email: nick@cloudflare.com
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