rfc9334.original   rfc9334.txt 
RATS Working Group H. Birkholz Internet Engineering Task Force (IETF) H. Birkholz
Internet-Draft Fraunhofer SIT Request for Comments: 9334 Fraunhofer SIT
Intended status: Informational D. Thaler Category: Informational D. Thaler
Expires: 1 April 2023 Microsoft ISSN: 2070-1721 Microsoft
M. Richardson M. Richardson
Sandelman Software Works Sandelman Software Works
N. Smith N. Smith
Intel Intel
W. Pan W. Pan
Huawei Technologies Huawei
28 September 2022 January 2023
Remote Attestation Procedures Architecture Remote ATtestation procedureS (RATS) Architecture
draft-ietf-rats-architecture-22
Abstract Abstract
In network protocol exchanges it is often useful for one end of a In network protocol exchanges, it is often useful for one end of a
communication to know whether the other end is in an intended communication to know whether the other end is in an intended
operating state. This document provides an architectural overview of operating state. This document provides an architectural overview of
the entities involved that make such tests possible through the the entities involved that make such tests possible through the
process of generating, conveying, and evaluating evidentiary claims. process of generating, conveying, and evaluating evidentiary Claims.
An attempt is made to provide for a model that is neutral toward It provides a model that is neutral toward processor architectures,
processor architectures, the content of claims, and protocols. the content of Claims, and protocols.
Note to Readers
Discussion of this document takes place on the RATS Working Group
mailing list (rats@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/rats/
(https://mailarchive.ietf.org/arch/browse/rats/).
Source for this draft and an issue tracker can be found at
https://github.com/ietf-rats-wg/architecture (https://github.com/
ietf-rats-wg/architecture).
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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
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approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
This Internet-Draft will expire on 1 April 2023. 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/rfc9334.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
2. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5 2. Reference Use Cases
2.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5 2.1. Network Endpoint Assessment
2.2. Confidential Machine Learning Model Protection . . . . . 6 2.2. Confidential Machine Learning Model Protection
2.3. Confidential Data Protection . . . . . . . . . . . . . . 6 2.3. Confidential Data Protection
2.4. Critical Infrastructure Control . . . . . . . . . . . . . 6 2.4. Critical Infrastructure Control
2.5. Trusted Execution Environment Provisioning . . . . . . . 7 2.5. Trusted Execution Environment Provisioning
2.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7 2.6. Hardware Watchdog
2.7. FIDO Biometric Authentication . . . . . . . . . . . . . . 8 2.7. FIDO Biometric Authentication
3. Architectural Overview . . . . . . . . . . . . . . . . . . . 8 3. Architectural Overview
3.1. Two Types of Environments of an Attester . . . . . . . . 10 3.1. Two Types of Environments of an Attester
3.2. Layered Attestation Environments . . . . . . . . . . . . 12 3.2. Layered Attestation Environments
3.3. Composite Device . . . . . . . . . . . . . . . . . . . . 14 3.3. Composite Device
3.4. Implementation Considerations . . . . . . . . . . . . . . 16 3.4. Implementation Considerations
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 17 4. Terminology
4.1. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.1. Roles
4.2. Artifacts . . . . . . . . . . . . . . . . . . . . . . . . 18 4.2. Artifacts
5. Topological Patterns . . . . . . . . . . . . . . . . . . . . 20 5. Topological Patterns
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 20 5.1. Passport Model
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 21 5.2. Background-Check Model
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 22 5.3. Combinations
6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 24 6. Roles and Entities
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 24 7. Trust Model
7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 24 7.1. Relying Party
7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 26 7.2. Attester
7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 26 7.3. Relying Party Owner
7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 26 7.4. Verifier
7.5. Endorser, Reference Value Provider, and Verifier Owner . 28 7.5. Endorser, Reference Value Provider, and Verifier Owner
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 29 8. Conceptual Messages
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 29 8.1. Evidence
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 29 8.2. Endorsements
8.3. Reference Values . . . . . . . . . . . . . . . . . . . . 30 8.3. Reference Values
8.4. Attestation Results . . . . . . . . . . . . . . . . . . . 30 8.4. Attestation Results
8.5. Appraisal Policies . . . . . . . . . . . . . . . . . . . 31 8.5. Appraisal Policies
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 32 9. Claims Encoding Formats
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 33 10. Freshness
10.1. Explicit Timekeeping using Synchronized Clocks . . . . . 34 10.1. Explicit Timekeeping Using Synchronized Clocks
10.2. Implicit Timekeeping using Nonces . . . . . . . . . . . 35 10.2. Implicit Timekeeping Using Nonces
10.3. Implicit Timekeeping using Epoch IDs . . . . . . . . . . 35 10.3. Implicit Timekeeping Using Epoch IDs
10.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 36 10.4. Discussion
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 37 11. Privacy Considerations
12. Security Considerations . . . . . . . . . . . . . . . . . . . 38 12. Security Considerations
12.1. Attester and Attestation Key Protection . . . . . . . . 38 12.1. Attester and Attestation Key Protection
12.1.1. On-Device Attester and Key Protection . . . . . . . 39 12.1.1. On-Device Attester and Key Protection
12.1.2. Attestation Key Provisioning Processes . . . . . . . 39 12.1.2. Attestation Key Provisioning Processes
12.2. Conceptual Message Protection . . . . . . . . . . . . . 41 12.2. Conceptual Message Protection
12.3. Epoch ID-based Attestation . . . . . . . . . . . . . . . 42 12.3. Attestation Based on Epoch ID
12.4. Trust Anchor Protection . . . . . . . . . . . . . . . . 42 12.4. Trust Anchor Protection
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43 13. IANA Considerations
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 43 14. References
15. Notable Contributions . . . . . . . . . . . . . . . . . . . . 43 14.1. Normative References
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 14.2. Informative References
16.1. Normative References . . . . . . . . . . . . . . . . . . 43 Appendix A. Time Considerations
16.2. Informative References . . . . . . . . . . . . . . . . . 44 A.1. Example 1: Timestamp-Based Passport Model
Appendix A. Time Considerations . . . . . . . . . . . . . . . . 46 A.2. Example 2: Nonce-Based Passport Model
A.1. Example 1: Timestamp-based Passport Model Example . . . . 48 A.3. Example 3: Passport Model Based on Epoch ID
A.2. Example 2: Nonce-based Passport Model Example . . . . . . 50 A.4. Example 4: Timestamp-Based Background-Check Model
A.3. Example 3: Epoch ID-based Passport Model Example . . . . 52 A.5. Example 5: Nonce-Based Background-Check Model
A.4. Example 4: Timestamp-based Background-Check Model Acknowledgments
Example . . . . . . . . . . . . . . . . . . . . . . . . . 53 Contributors
A.5. Example 5: Nonce-based Background-Check Model Example . . 54 Authors' Addresses
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57
1. Introduction 1. Introduction
The question of how one system can know that another system can be The question of how one system can know that another system can be
trusted has found new interest and relevance in a world where trusted trusted has found new interest and relevance in a world where trusted
computing elements are maturing in processor architectures. computing elements are maturing in processor architectures.
Systems that have been attested and verified to be in a good state Systems that have been attested and verified to be in a good state
(for some value of "good") can improve overall system posture. (for some value of "good") can improve overall system posture.
Conversely, systems that cannot be attested and verified to be in a Conversely, systems that cannot be attested and verified to be in a
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service, or otherwise flagged for repair. service, or otherwise flagged for repair.
For example: For example:
* A bank backend system might refuse to transact with another system * A bank backend system might refuse to transact with another system
that is not known to be in a good state. that is not known to be in a good state.
* A healthcare system might refuse to transmit electronic healthcare * A healthcare system might refuse to transmit electronic healthcare
records to a system that is not known to be in a good state. records to a system that is not known to be in a good state.
In Remote Attestation Procedures (RATS), one peer (the "Attester") In Remote ATtestation procedureS (RATS), one peer (the "Attester")
produces believable information about itself - Evidence - to enable a produces believable information about itself ("Evidence") to enable a
remote peer (the "Relying Party") to decide whether to consider that remote peer (the "Relying Party") to decide whether or not to
Attester a trustworthy peer or not. RATS are facilitated by an consider that Attester a trustworthy peer. Remote attestation
additional vital party, the Verifier. procedures are facilitated by an additional vital party (the
"Verifier").
The Verifier appraises Evidence via appraisal policies and creates The Verifier appraises Evidence via appraisal policies and creates
the Attestation Results to support Relying Parties in their decision the Attestation Results to support Relying Parties in their decision
process. This document defines a flexible architecture consisting of process. This document defines a flexible architecture consisting of
attestation roles and their interactions via conceptual messages. attestation roles and their interactions via conceptual messages.
Additionally, this document defines a universal set of terms that can Additionally, this document defines a universal set of terms that can
be mapped to various existing and emerging Remote Attestation be mapped to various existing and emerging remote attestation
Procedures. Common topological patterns and the sequence of data procedures. Common topological patterns and the sequence of data
flows associated with them, such as the "Passport Model" and the flows associated with them, such as the "Passport Model" and the
"Background-Check Model", are illustrated. The purpose is to define "Background-Check Model", are illustrated. The purpose is to define
useful terminology for remote attestation and enable readers to map useful terminology for remote attestation and enable readers to map
their solution architecture to the canonical attestation architecture their solution architecture to the canonical attestation architecture
provided here. Having a common terminology that provides well- provided here. Having a common terminology that provides well-
understood meanings for common themes such as roles, device understood meanings for common themes, such as roles, device
composition, topological patterns, and appraisal procedures is vital composition, topological patterns, and appraisal procedures, is vital
for semantic interoperability across solutions and platforms for semantic interoperability across solutions and platforms
involving multiple vendors and providers. involving multiple vendors and providers.
Amongst other things, this document is about trust and Amongst other things, this document is about trust and
trustworthiness. Trust is a choice one makes about another system. trustworthiness. Trust is a choice one makes about another system.
Trustworthiness is a quality about the other system that can be used Trustworthiness is a quality about the other system that can be used
in making one's decision to trust it or not. This is a subtle in making one's decision to trust it or not. This is a subtle
difference and being familiar with the difference is crucial for difference; being familiar with the difference is crucial for using
using this document. Additionally, the concepts of freshness and this document. Additionally, the concepts of freshness and trust
trust relationships with respect to RATS are elaborated on to enable relationships are specified to enable implementers to choose
implementers to choose appropriate solutions to compose their Remote appropriate solutions to compose their remote attestation procedures.
Attestation Procedures.
2. Reference Use Cases 2. Reference Use Cases
This section covers a number of representative and generic use cases This section covers a number of representative and generic use cases
for remote attestation, independent of specific solutions. The for remote attestation, independent of specific solutions. The
purpose is to provide motivation for various aspects of the purpose is to provide motivation for various aspects of the
architecture presented in this document. Many other use cases exist, architecture presented in this document. Many other use cases exist;
and this document does not intend to have a complete list, only to this document does not contain a complete list. It only illustrates
illustrate a set of use cases that collectively cover all the a set of use cases that collectively cover all the functionality
functionality required in the architecture. required in the architecture.
Each use case includes a description followed by an additional Each use case includes a description followed by an additional
summary of the Attester and Relying Party roles derived from the use summary of the Attester and Relying Party roles derived from the use
case. case.
2.1. Network Endpoint Assessment 2.1. Network Endpoint Assessment
Network operators want trustworthy reports that include identity and Network operators want trustworthy reports that include identity and
version information about the hardware and software on the machines version information about the hardware and software on the machines
attached to their network. Examples of reports include purposes, attached to their network. Examples of reports include purposes
such as inventory summaries, audit results, anomaly notifications, (such as inventory summaries), audit results, and anomaly
typically including the maintenance of log records or trend reports. notifications (which typically include the maintenance of log records
The network operator may also want a policy by which full access is or trend reports). The network operator may also want a policy by
only granted to devices that meet some definition of hygiene, and so which full access is only granted to devices that meet some
wants to get Claims about such information and verify its validity. definition of hygiene, and so wants to get Claims about such
Remote attestation is desired to prevent vulnerable or compromised information and verify its validity. Remote attestation is desired
devices from getting access to the network and potentially harming to prevent vulnerable or compromised devices from getting access to
others. the network and potentially harming others.
Typically, a solution starts with a specific component (sometimes Typically, a solution starts with a specific component (sometimes
referred to as a root of trust) that often provides trustworthy referred to as a "root of trust") that often provides a trustworthy
device identity, and performs a series of operations that enables device identity and performs a series of operations that enables
trustworthiness appraisals for other components. Such components trustworthiness appraisals for other components. Such components
perform operations that help determine the trustworthiness of yet perform operations that help determine the trustworthiness of yet
other components, by collecting, protecting or signing measurements. other components by collecting, protecting, or signing measurements.
Measurements that have been signed by such components are comprised Measurements that have been signed by such components are comprised
of Evidence that when evaluated either supports or refutes a claim of of Evidence that either supports or refutes a claim of
trustworthiness. Measurements can describe a variety of attributes trustworthiness when evaluated. Measurements can describe a variety
of system components, such as hardware, firmware, BIOS, software, of attributes of system components, such as hardware, firmware, BIOS,
etc. software, etc., and how they are hardened.
Attester: A device desiring access to a network. Attester: A device desiring access to a network.
Relying Party: Network equipment such as a router, switch, or access Relying Party: Network equipment (such as a router, switch, or
point, responsible for admission of the device into the network. access point) that is responsible for admission of the device into
the network.
2.2. Confidential Machine Learning Model Protection 2.2. Confidential Machine Learning Model Protection
A device manufacturer wants to protect its intellectual property. A device manufacturer wants to protect its intellectual property.
The intellectual property's scope primarily encompasses the machine The intellectual property's scope primarily encompasses the machine
learning (ML) model that is deployed in the devices purchased by its learning (ML) model that is deployed in the devices purchased by its
customers. The protection goals include preventing attackers, customers. The protection goals include preventing attackers,
potentially the customer themselves, from seeing the details of the potentially the customer themselves, from seeing the details of the
model. model.
This typically works by having some protected environment in the Typically, this works by having some protected environment in the
device go through a remote attestation with some manufacturer service device go through a remote attestation with some manufacturer service
that can assess its trustworthiness. If remote attestation succeeds, that can assess its trustworthiness. If remote attestation succeeds,
then the manufacturer service releases either the model, or a key to then the manufacturer service releases either the model or a key to
decrypt a model already deployed on the Attester in encrypted form, decrypt a model already deployed on the Attester in encrypted form to
to the requester. the requester.
Attester: A device desiring to run an ML model. Attester: A device desiring to run an ML model.
Relying Party: A server or service holding ML models it desires to Relying Party: A server or service holding ML models it desires to
protect. protect.
2.3. Confidential Data Protection 2.3. Confidential Data Protection
This is a generalization of the ML model use case above, where the This is a generalization of the ML model use case above where the
data can be any highly confidential data, such as health data about data can be any highly confidential data, such as health data about
customers, payroll data about employees, future business plans, etc. customers, payroll data about employees, future business plans, etc.
As part of the attestation procedure, an assessment is made against a As part of the attestation procedure, an assessment is made against a
set of policies to evaluate the state of the system that is set of policies to evaluate the state of the system that is
requesting the confidential data. Attestation is desired to prevent requesting the confidential data. Attestation is desired to prevent
leaking data via compromised devices. leaking data via compromised devices.
Attester: An entity desiring to retrieve confidential data. Attester: An entity desiring to retrieve confidential data.
Relying Party: An entity that holds confidential data for release to Relying Party: An entity that holds confidential data for release to
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Potentially harmful physical equipment (e.g., power grid, traffic Potentially harmful physical equipment (e.g., power grid, traffic
control, hazardous chemical processing, etc.) is connected to a control, hazardous chemical processing, etc.) is connected to a
network in support of critical infrastructure. The organization network in support of critical infrastructure. The organization
managing such infrastructure needs to ensure that only authorized managing such infrastructure needs to ensure that only authorized
code and users can control corresponding critical processes, and that code and users can control corresponding critical processes, and that
these processes are protected from unauthorized manipulation or other these processes are protected from unauthorized manipulation or other
threats. When a protocol operation can affect a critical system threats. When a protocol operation can affect a critical system
component of the infrastructure, devices attached to that critical component of the infrastructure, devices attached to that critical
component require some assurances depending on the security context, component require some assurances depending on the security context,
including that: a requesting device or application has not been including assurances that a requesting device or application has not
compromised, and the requesters and actors act on applicable been compromised and the requesters and actors act on applicable
policies. As such, remote attestation can be used to only accept policies. As such, remote attestation can be used to only accept
commands from requesters that are within policy. commands from requesters that are within policy.
Attester: A device or application wishing to control physical Attester: A device or application wishing to control physical
equipment. equipment.
Relying Party: A device or application connected to potentially Relying Party: A device or application connected to potentially
dangerous physical equipment (hazardous chemical processing, dangerous physical equipment (hazardous chemical processing,
traffic control, power grid, etc.). traffic control, power grid, etc.).
2.5. Trusted Execution Environment Provisioning 2.5. Trusted Execution Environment Provisioning
A Trusted Application Manager (TAM) server is responsible for A Trusted Application Manager (TAM) server is responsible for
managing the applications running in a Trusted Execution Environment managing the applications running in a Trusted Execution Environment
(TEE) of a client device, as described in (TEE) of a client device, as described in [TEEP-ARCH]. To achieve
[I-D.ietf-teep-architecture]. To achieve its purpose, the TAM needs its purpose, the TAM needs to assess the state of a TEE or
to assess the state of a TEE, or of applications in the TEE, of a applications in the TEE of a client device. The TEE conducts remote
client device. The TEE conducts Remote Attestation Procedures with attestation procedures with the TAM, which can then decide whether
the TAM, which can then decide whether the TEE is already in the TEE is already in compliance with the TAM's latest policy. If
compliance with the TAM's latest policy. If not, the TAM has to not, the TAM has to uninstall, update, or install approved
uninstall, update, or install approved applications in the TEE to applications in the TEE to bring it back into compliance with the
bring it back into compliance with the TAM's policy. TAM's policy.
Attester: A device with a TEE capable of running trusted Attester: A device with a TEE capable of running trusted
applications that can be updated. applications that can be updated.
Relying Party: A TAM. Relying Party: A TAM.
2.6. Hardware Watchdog 2.6. Hardware Watchdog
There is a class of malware that holds a device hostage and does not There is a class of malware that holds a device hostage and does not
allow it to reboot to prevent updates from being applied. This can allow it to reboot to prevent updates from being applied. This can
be a significant problem, because it allows a fleet of devices to be be a significant problem because it allows a fleet of devices to be
held hostage for ransom. held hostage for ransom.
A solution to this problem is a watchdog timer implemented in a A solution to this problem is a watchdog timer implemented in a
protected environment such as a Trusted Platform Module (TPM), as protected environment, such as a Trusted Platform Module (TPM), as
described in [TCGarch] section 43.3. If the watchdog does not described in Section 43.3 of [TCGarch]. If the watchdog does not
receive regular, and fresh, Attestation Results as to the system's receive regular and fresh Attestation Results regarding the system's
health, then it forces a reboot. health, then it forces a reboot.
Attester: The device that should be protected from being held Attester: The device that should be protected from being held
hostage for a long period of time. hostage for a long period of time.
Relying Party: A watchdog capable of triggering a procedure that Relying Party: A watchdog capable of triggering a procedure that
resets a device into a known, good operational state. resets a device into a known, good operational state.
2.7. FIDO Biometric Authentication 2.7. FIDO Biometric Authentication
In the Fast IDentity Online (FIDO) protocol [WebAuthN], [CTAP], the In the Fast IDentity Online (FIDO) protocol [WebAuthN] [CTAP], the
device in the user's hand authenticates the human user, whether by device in the user's hand authenticates the human user, whether by
biometrics (such as fingerprints), or by PIN and password. FIDO biometrics (such as fingerprints) or by PIN and password. FIDO
authentication puts a large amount of trust in the device compared to authentication puts a large amount of trust in the device compared to
typical password authentication because it is the device that typical password authentication because it is the device that
verifies the biometric, PIN and password inputs from the user, not verifies the biometric, PIN, and password inputs from the user, not
the server. For the Relying Party to know that the authentication is the server. For the Relying Party to know that the authentication is
trustworthy, the Relying Party needs to know that the Authenticator trustworthy, the Relying Party needs to know that the Authenticator
part of the device is trustworthy. The FIDO protocol employs remote part of the device is trustworthy. The FIDO protocol employs remote
attestation for this. attestation for this.
The FIDO protocol supports several remote attestation protocols and a The FIDO protocol supports several remote attestation protocols and a
mechanism by which new ones can be registered and added. Remote mechanism by which new ones can be registered and added; thus, remote
attestation defined by RATS is thus a candidate for use in the FIDO attestation defined by the RATS architecture is a candidate for use
protocol. in the FIDO protocol.
Attester: FIDO Authenticator. Attester: FIDO Authenticator.
Relying Party: Any web site, mobile application backend, or service Relying Party: Any website, mobile application backend, or service
that relies on authentication data based on biometric information. that relies on authentication data based on biometric information.
3. Architectural Overview 3. Architectural Overview
Figure 1 depicts the data that flows between different roles, Figure 1 depicts the data that flows between different roles,
independent of protocol or use case. independent of protocol or use case.
.--------. .---------. .--------. .-------------. .--------. .---------. .--------. .-------------.
| Endorser | | Reference | | Verifier | | Relying Party | | Endorser | | Reference | | Verifier | | Relying Party |
'-+------' | Value | | Owner | | Owner | '-+------' | Value | | Owner | | Owner |
skipping to change at page 9, line 34 skipping to change at line 373
| v v | v v
.-----+----. .---------------. .-----+----. .---------------.
| Attester | | Relying Party | | Attester | | Relying Party |
'----------' '---------------' '----------' '---------------'
Figure 1: Conceptual Data Flow Figure 1: Conceptual Data Flow
The text below summarizes the activities conducted by the roles The text below summarizes the activities conducted by the roles
illustrated in Figure 1. Roles are assigned to entities. Entities illustrated in Figure 1. Roles are assigned to entities. Entities
are often system components [RFC4949], such as devices. As the term are often system components [RFC4949], such as devices. As the term
device is typically more intuitive than the term entity or system "device" is typically more intuitive than the term "entity" or
component, device is often used as an illustrative synonym throughout "system component", device is often used as an illustrative synonym
this document. throughout this document.
The Attester role is assigned to entities that create Evidence that The Attester role is assigned to entities that create Evidence that
is conveyed to a Verifier. is conveyed to a Verifier.
The Verifier role is assigned to entities that use the Evidence, any The Verifier role is assigned to entities that use the Evidence, any
Reference Values from Reference Value Providers, and any Endorsements Reference Values from Reference Value Providers, and any Endorsements
from Endorsers, by applying an Appraisal Policy for Evidence to from Endorsers by applying an Appraisal Policy for Evidence to assess
assess the trustworthiness of the Attester. This procedure is called the trustworthiness of the Attester. This procedure is called the
the appraisal of Evidence. "appraisal of Evidence".
Subsequently, the Verifier role generates Attestation Results for use Subsequently, the Verifier role generates Attestation Results for use
by Relying Parties. by Relying Parties.
The Appraisal Policy for Evidence might be obtained from the Verifier The Appraisal Policy for Evidence might be obtained from the Verifier
Owner via some protocol mechanism, or might be configured into the Owner via some protocol mechanism, configured into the Verifier by
Verifier by the Verifier Owner, or might be programmed into the the Verifier Owner, programmed into the Verifier, or obtained via
Verifier, or might be obtained via some other mechanism. some other mechanism.
The Relying Party role is assigned to an entity that uses Attestation The Relying Party role is assigned to an entity that uses Attestation
Results by applying its own appraisal policy to make application- Results by applying its own appraisal policy to make application-
specific decisions, such as authorization decisions. This procedure specific decisions, such as authorization decisions. This procedure
is called the appraisal of Attestation Results. is called the "appraisal of Attestation Results".
The Appraisal Policy for Attestation Results might be obtained from The Appraisal Policy for Attestation Results might be obtained from
the Relying Party Owner via some protocol mechanism, or might be the Relying Party Owner via some protocol mechanism, configured into
configured into the Relying Party by the Relying Party Owner, or the Relying Party by the Relying Party Owner, programmed into the
might be programmed into the Relying Party, or might be obtained via Relying Party, or obtained via some other mechanism.
some other mechanism.
See Section 8 for further discussion of the conceptual messages shown See Section 8 for further discussion of the conceptual messages shown
in Figure 1. Section 4 provides a more complete definition of all in Figure 1. Section 4 provides a more complete definition of all
RATS roles. RATS roles.
3.1. Two Types of Environments of an Attester 3.1. Two Types of Environments of an Attester
As shown in Figure 2, an Attester consists of at least one Attesting As shown in Figure 2, an Attester consists of at least one Attesting
Environment and at least one Target Environment co-located in one Environment and at least one Target Environment co-located in one
entity. In some implementations, the Attesting and Target entity. In some implementations, the Attesting and Target
Environments might be combined into one environment. Other Environments might be combined into one environment. Other
implementations might have multiple Attesting and Target implementations might have multiple Attesting and Target
Environments, such as in the examples described in more detail in Environments, such as in the examples described in more detail in
Section 3.2 and Section 3.3. Other examples may exist. All Sections 3.2 and 3.3. Other examples may exist. All compositions of
compositions of Attesting and Target Environments discussed in this Attesting and Target Environments discussed in this architecture can
architecture can be combined into more complex implementations. be combined into more complex implementations.
.--------------------------------. .--------------------------------.
| | | |
| Verifier | | Verifier |
| | | |
'--------------------------------' '--------------------------------'
^ ^
| |
.-------------------------|----------. .-------------------------|----------.
| | | | | |
skipping to change at page 11, line 33 skipping to change at line 448
| | | | | | | |
| v | | | v | |
| .-------+-----. | | .-------+-----. |
| | Attesting | | | | Attesting | |
| | Environment | | | | Environment | |
| | | | | | | |
| '-------------' | | '-------------' |
| Attester | | Attester |
'------------------------------------' '------------------------------------'
Figure 2: Two Types of Environments Figure 2: Two Types of Environments within an Attester
Claims are collected from Target Environments. That is, Attesting Claims are collected from Target Environments. That is, Attesting
Environments collect the values and the information to be represented Environments collect the values and the information to be represented
in Claims, by reading system registers and variables, calling into in Claims by reading system registers and variables, calling into
subsystems, taking measurements on code, memory, or other security subsystems, and taking measurements on code, memory, or other
related assets of the Target Environment. Attesting Environments relevant assets of the Target Environment. Attesting Environments
then format the Claims appropriately, and typically use key material then format the Claims appropriately; typically, they use key
and cryptographic functions, such as signing or cipher algorithms, to material and cryptographic functions, such as signing or cipher
generate Evidence. There is no limit to or requirement on the types algorithms, to generate Evidence. There is no limit or requirement
of hardware or software environments that can be used to implement an on the types of hardware or software environments that can be used to
Attesting Environment, for example: Trusted Execution Environments implement an Attesting Environment. For example, TEEs, embedded
(TEEs), embedded Secure Elements (eSEs), Trusted Platform Modules Secure Elements (eSEs), TPMs [TCGarch], or BIOS firmware.
(TPMs) [TCGarch], or BIOS firmware.
An arbitrary execution environment may not, by default, be capable of An arbitrary execution environment may not, by default, be capable of
Claims collection for a given Target Environment. Execution Claims collection for a given Target Environment. Execution
environments that are designed specifically to be capable of Claims environments that are designed specifically to be capable of Claims
collection are referred to in this document as Attesting collection are referred to in this document as "Attesting
Environments. For example, a TPM doesn't actively collect Claims Environments". For example, a TPM doesn't actively collect Claims
itself, it instead requires another component to feed various values itself. Instead, it requires another component to feed various
to the TPM. Thus, an Attesting Environment in such a case would be values to the TPM. Thus, an Attesting Environment in such a case
the combination of the TPM together with whatever component is would be the combination of the TPM together with whatever component
feeding it the measurements. is feeding it the measurements.
3.2. Layered Attestation Environments 3.2. Layered Attestation Environments
By definition, the Attester role generates Evidence. An Attester may By definition, the Attester role generates Evidence. An Attester may
consist of one or more nested environments (layers). The bottom consist of one or more nested environments (layers). The bottom
layer of an Attester has an Attesting Environment that is typically layer of an Attester has an Attesting Environment that is typically
designed to be immutable or difficult to modify by malicious code. designed to be immutable or difficult to modify by malicious code.
In order to appraise Evidence generated by an Attester, the Verifier In order to appraise Evidence generated by an Attester, the Verifier
needs to trust various layers, including the bottom Attesting needs to trust various layers, including the bottom Attesting
Environment. Trust in the Attester's layers, including the bottom Environment. Trust in the Attester's layers, including the bottom
layer, can be established in various ways as discussed in layer, can be established in various ways, as discussed in
Section 7.4. Section 7.4.
In layered attestation, Claims can be collected from or about each In layered attestation, Claims can be collected from or about each
layer beginning with an initial layer. The corresponding Claims can layer beginning with an initial layer. The corresponding Claims can
be structured in a nested fashion that reflects the nesting of the be structured in a nested fashion that reflects the nesting of the
Attester's layers. Normally, Claims are not self-asserted, rather a Attester's layers. Normally, Claims are not self-asserted. Rather,
previous layer acts as the Attesting Environment for the next layer. a previous layer acts as the Attesting Environment for the next
Claims about an initial layer typically are asserted by an Endorser. layer. Claims about an initial layer are typically asserted by an
Endorser.
The example device illustrated in Figure 3 includes (A) a BIOS stored The example device illustrated in Figure 3 includes (A) a BIOS stored
in read-only memory, (B) a bootloader, and (C) an operating system in read-only memory, (B) a bootloader, and (C) an operating system
kernel. kernel.
.-------------. Endorsement for ROM .-------------. Endorsement for ROM
| Endorser +-----------------------. | Endorser +-----------------------.
'-------------' | '-------------' |
v v
.-------------. Reference .----------. .-------------. Reference .----------.
| Reference | Values for | | | Reference | Values for | |
| Value +----------------->| Verifier | | Value +----------------->| Verifier |
| Provider(s) | ROM, bootloader, | | | Provider(s) | ROM, bootloader, | |
'-------------' and kernel '----------' '-------------' and kernel '----------'
^ ^
.------------------------------------. | .------------------------------------. |
| | | | | |
| .---------------------------. | | | .---------------------------. | |
| | Kernel | | | | | Kernel(C) | | |
| | | | | Layered | | | | | Layered
| | Target | | | Evidence | | Target | | | Evidence
| | Environment | | | for | | Environment | | | for
| '---------------+-----------' | | bootloader | '---------------+-----------' | | bootloader
| Collect | | | and | Collect | | | and
| Claims | | | kernel | Claims | | | kernel
| .---------------|-----------. | | | .---------------|-----------. | |
| | Bootloader v | | | | | Bootloader(B) v | | |
| | .-----------. | | | | | .-----------. | | |
| | Target | Attesting | | | | | | Target | Attesting | | | |
| | Environment |Environment+-----------' | | Environment |Environment+-----------'
| | | | | | | | | | | |
| | '-----------' | | | | '-----------' | |
| | ^ | | | | ^ | |
| '--------------+--|---------' | | '--------------+--|---------' |
| Collect | | Evidence for | | Collect | | Evidence for |
| Claims v | bootloader | | Claims v | bootloader |
| .-----------------+---------. | | .-----------------+---------. |
| | ROM | | | | ROM(A) | |
| | | | | | | |
| | Attesting | | | | Attesting | |
| | Environment | | | | Environment | |
| '---------------------------' | | '---------------------------' |
| | | |
'------------------------------------' '------------------------------------'
Figure 3: Layered Attester Figure 3: Layered Attester
The first Attesting Environment, the ROM in this example, has to The first Attesting Environment (the ROM in this example) has to
ensure the integrity of the bootloader (the first Target ensure the integrity of the bootloader (the first Target
Environment). There are potentially multiple kernels to boot, and Environment). There are potentially multiple kernels to boot; the
the decision is up to the bootloader. Only a bootloader with intact decision is up to the bootloader. Only a bootloader with intact
integrity will make an appropriate decision. Therefore, the Claims integrity will make an appropriate decision. Therefore, the Claims
relating to the integrity of the bootloader have to be measured relating to the integrity of the bootloader have to be measured
securely. At this stage of the boot-cycle of the device, the Claims securely. At this stage of the boot cycle of the device, the Claims
collected typically cannot be composed into Evidence. collected typically cannot be composed into Evidence.
After the boot sequence is started, the BIOS conducts the most After the boot sequence is started, the BIOS conducts the most
important and defining feature of layered attestation, which is that important and defining feature of layered attestation: the
the successfully measured bootloader now becomes (or contains) an successfully measured bootloader now becomes (or contains) an
Attesting Environment for the next layer. This procedure in layered Attesting Environment for the next layer. This procedure in layered
attestation is sometimes called "staging". It is important that the attestation is sometimes called "staging". It is important that the
bootloader not be able to alter any Claims about itself that were bootloader not be able to alter any Claims about itself that were
collected by the BIOS. This can be ensured having those Claims be collected by the BIOS. This can be ensured having those Claims be
either signed by the BIOS or stored in a tamper-proof manner by the either signed by the BIOS or stored in a tamper-proof manner by the
BIOS. BIOS.
Continuing with this example, the bootloader's Attesting Environment Continuing with this example, the bootloader's Attesting Environment
is now in charge of collecting Claims about the next Target is now in charge of collecting Claims about the next Target
Environment, which in this example is the kernel to be booted. The Environment. In this example, it is the kernel to be booted. The
final Evidence thus contains two sets of Claims: one set about the final Evidence thus contains two sets of Claims: one set about the
bootloader as measured and signed by the BIOS, plus a set of Claims bootloader as measured and signed by the BIOS and another set of
about the kernel as measured and signed by the bootloader. Claims about the kernel as measured and signed by the bootloader.
This example could be extended further by making the kernel become This example could be extended further by making the kernel become
another Attesting Environment for an application as another Target another Attesting Environment for an application as another Target
Environment. This would result in a third set of Claims in the Environment. This would result in a third set of Claims in the
Evidence pertaining to that application. Evidence pertaining to that application.
The essence of this example is a cascade of staged environments. The essence of this example is a cascade of staged environments.
Each environment has the responsibility of measuring the next Each environment has the responsibility of measuring the next
environment before the next environment is started. In general, the environment before the next environment is started. In general, the
number of layers may vary by device or implementation, and an number of layers may vary by device or implementation, and an
skipping to change at page 14, line 51 skipping to change at line 584
that it measures, rather than only one as shown by example in that it measures, rather than only one as shown by example in
Figure 3. Figure 3.
3.3. Composite Device 3.3. Composite Device
A composite device is an entity composed of multiple sub-entities A composite device is an entity composed of multiple sub-entities
such that its trustworthiness has to be determined by the appraisal such that its trustworthiness has to be determined by the appraisal
of all these sub-entities. of all these sub-entities.
Each sub-entity has at least one Attesting Environment collecting the Each sub-entity has at least one Attesting Environment collecting the
Claims from at least one Target Environment, then this sub-entity Claims from at least one Target Environment. Then, this sub-entity
generates Evidence about its trustworthiness. Therefore, each sub- generates Evidence about its trustworthiness; therefore, each sub-
entity can be called an Attester. Among all the Attesters, there may entity can be called an "Attester". Among all the Attesters, there
be only some which have the ability to communicate with the Verifier may be only some that have the ability to communicate with the
while others do not. Verifier while others do not.
For example, a carrier-grade router consists of a chassis and For example, a carrier-grade router consists of a chassis and
multiple slots. The trustworthiness of the router depends on all its multiple slots. The trustworthiness of the router depends on all its
slots' trustworthiness. Each slot has an Attesting Environment, such slots' trustworthiness. Each slot has an Attesting Environment, such
as a TEE, collecting the Claims of its boot process, after which it as a TEE, collecting the Claims of its boot process, after which it
generates Evidence from the Claims. generates Evidence from the Claims.
Among these slots, only a "main" slot can communicate with the Among these slots, only a "main" slot can communicate with the
Verifier while other slots cannot. But other slots can communicate Verifier while other slots cannot. However, other slots can
with the main slot by the links between them inside the router. So communicate with the main slot by the links between them inside the
the main slot collects the Evidence of other slots, produces the router. The main slot collects the Evidence of other slots, produces
final Evidence of the whole router and conveys the final Evidence to the final Evidence of the whole router, and conveys the final
the Verifier. Therefore, the router is a composite device, each slot Evidence to the Verifier. Therefore, the router is a composite
is an Attester, and the main slot is the lead Attester. device, each slot is an Attester, and the main slot is the lead
Attester.
Another example is a multi-chassis router composed of multiple single Another example is a multi-chassis router composed of multiple single
carrier-grade routers. Multi-chassis router setups create redundancy carrier-grade routers. Multi-chassis router setups create redundancy
groups that provide higher throughput by interconnecting multiple groups that provide higher throughput by interconnecting multiple
routers in these groups, which can be treated as one logical router routers in these groups, which can be treated as one logical router
for simpler management. A multi-chassis router setup provides a for simpler management. A multi-chassis router setup provides a
management point that connects to the Verifier. Typically, one management point that connects to the Verifier. Typically, one
router in the group is designated as the main router. Other routers router in the group is designated as the main router. Other routers
in the multi-chassis setup are connected to the main router only via in the multi-chassis setup are connected to the main router only via
physical network links and are therefore managed and appraised via physical network links; therefore, they are managed and appraised via
the main router's help. Consequently, a multi-chassis router setup the main router's help. Consequently, a multi-chassis router setup
is a composite device, each router is an Attester, and the main is a composite device, each router is an Attester, and the main
router is the lead Attester. router is the lead Attester.
Figure 4 depicts the conceptual data flow for a composite device. Figure 4 depicts the conceptual data flow for a composite device.
.-----------------------------. .-----------------------------.
| Verifier | | Verifier |
'-----------------------------' '-----------------------------'
^ ^
| |
| Evidence of | Evidence of
| Composite Device | Composite Device
| |
.----------------------------------|-------------------------------. .----------------------------------|-------------------------------.
| .--------------------------------|-----. .------------. | | .--------------------------------|-----. .------------. |
| | Collect .---------+--. | | | | | | Collect .---------+--. | | | |
| | Claims .--------->| Attesting |<--------+ Attester B +-. | | | Claims .--------->| Attesting |<--------+ Attester B +-. |
| | | |Environment | | '------------' | | | | | |Environment | | '-+----------' | |
| | .--------+-------. | |<----------+ Attester C +-. | | | .--------+-------. | |<----------+ Attester C +-. |
| | | Target | | | | '------------' | | | | | Target | | | | '-+----------' | |
| | | Environment(s) | | |<------------+ ... | | | | | Environment(s) | | |<------------+ ... | |
| | | | '------------' | Evidence '------------' | | | | | '------------' | Evidence '------------' |
| | '----------------' | of | | | '----------------' | of |
| | | Attesters | | | | Attesters |
| | lead Attester A | (via Internal Links or | | | lead Attester A | (via Internal Links or |
| '--------------------------------------' Network Connections) | | '--------------------------------------' Network Connections) |
| | | |
| Composite Device | | Composite Device |
'------------------------------------------------------------------' '------------------------------------------------------------------'
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In this scenario, the trust model described in Section 7 can also be In this scenario, the trust model described in Section 7 can also be
applied to an inside Verifier. applied to an inside Verifier.
3.4. Implementation Considerations 3.4. Implementation Considerations
An entity can take on multiple RATS roles (e.g., Attester, Verifier, An entity can take on multiple RATS roles (e.g., Attester, Verifier,
Relying Party, etc.) at the same time. Multiple entities can Relying Party, etc.) at the same time. Multiple entities can
cooperate to implement a single RATS role as well. In essence, the cooperate to implement a single RATS role as well. In essence, the
combination of roles and entities can be arbitrary. For example, in combination of roles and entities can be arbitrary. For example, in
the composite device scenario, the entity inside the lead Attester the composite device scenario, the entity inside the lead Attester
can also take on the role of a Verifier, and the outer entity of can also take on the role of a Verifier and the outer entity of
Verifier can take on the role of a Relying Party. After collecting Verifier can take on the role of a Relying Party. After collecting
the Evidence of other Attesters, this inside Verifier uses the Evidence of other Attesters, this inside Verifier uses
Endorsements and appraisal policies (obtained the same way as by any Endorsements and appraisal policies (obtained the same way as by any
other Verifier) as part of the appraisal procedures that generate other Verifier) as part of the appraisal procedures that generate
Attestation Results. The inside Verifier then conveys the Attestation Results. The inside Verifier then conveys the
Attestation Results of other Attesters to the outside Verifier, Attestation Results of other Attesters to the outside Verifier,
whether in the same conveyance protocol as part of the Evidence or whether in the same conveyance protocol as part of the Evidence or
not. not.
As explained in Section 4, there are a variety of roles in the RATS As explained in Section 4, there are a variety of roles in the RATS
architecture and they are defined by a unique combination of architecture; they are defined by a unique combination of artifacts
artifacts they produce and consume. Conversely, artifacts are also they produce and consume. Conversely, artifacts are also defined by
defined by the roles that produce or consume them. To produce an the roles that produce or consume them. To produce an artifact means
artifact means that a given role introduces it into the RATS that a given role introduces it into the RATS architecture. To
architecture. To consume an artifact means that a given role has consume an artifact means that a given role has responsibility for
responsibility for processing it in the RATS architecture. Roles processing it in the RATS architecture. Roles also have the ability
also have the ability to perform additional actions such as caching to perform additional actions, such as caching or forwarding
or forwarding artifacts as opaque data. As depicted in Section 5, artifacts as opaque data. As depicted in Section 5, these additional
these additional actions can be performed by several roles. actions can be performed by several roles.
4. Terminology 4. Terminology
[RFC4949] has defined a number of terms that are also used in this [RFC4949] has defined a number of terms that are also used in this
document. Some of the terms are close to, but not exactly the same. document. Some of the terms are close to, but not exactly the same.
Where the terms are similar, they are noted below with references. Where the terms are similar, they are noted below with references.
As explained in [RFC4949], Section 2.6 when this document says As explained in Section 2.6 of [RFC4949], when this document says
"Compare:", the terminology used in this document differs "Compare:", the terminology used in this document differs
significantly from the definition in the reference. significantly from the definition in the reference.
This document uses the following terms. This document uses the terms in the subsections that follow.
4.1. Roles 4.1. Roles
Attester: A role performed by an entity (typically a device) whose Attester: A role performed by an entity (typically a device) whose
Evidence must be appraised in order to infer the extent to which Evidence must be appraised in order to infer the extent to which
the Attester is considered trustworthy, such as when deciding the Attester is considered trustworthy, such as when deciding
whether it is authorized to perform some operation. whether it is authorized to perform some operation.
Produces: Evidence Produces: Evidence
Relying Party: A role performed by an entity that depends on the Relying Party: A role performed by an entity that depends on the
validity of information about an Attester, for purposes of validity of information about an Attester for purposes of reliably
reliably applying application specific actions. Compare: /relying applying application-specific actions. Compare: relying party
party/ in [RFC4949]. [RFC4949].
Consumes: Attestation Results, Appraisal Policy for Attestation Consumes: Attestation Results, Appraisal Policy for Attestation
Results Results
Verifier: A role performed by an entity that appraises the validity Verifier: A role performed by an entity that appraises the validity
of Evidence about an Attester and produces Attestation Results to of Evidence about an Attester and produces Attestation Results to
be used by a Relying Party. be used by a Relying Party.
Consumes: Evidence, Reference Values, Endorsements, Appraisal Consumes: Evidence, Reference Values, Endorsements, Appraisal
Policy for Evidence Policy for Evidence
Produces: Attestation Results Produces: Attestation Results
Relying Party Owner: A role performed by an entity (typically an Relying Party Owner: A role performed by an entity (typically an
administrator), that is authorized to configure Appraisal Policy administrator) that is authorized to configure an Appraisal Policy
for Attestation Results in a Relying Party. for Attestation Results in a Relying Party.
Produces: Appraisal Policy for Attestation Results Produces: Appraisal Policy for Attestation Results
Verifier Owner: A role performed by an entity (typically an Verifier Owner: A role performed by an entity (typically an
administrator), that is authorized to configure Appraisal Policy administrator) that is authorized to configure an Appraisal Policy
for Evidence in a Verifier. for Evidence in a Verifier.
Produces: Appraisal Policy for Evidence Produces: Appraisal Policy for Evidence
Endorser: A role performed by an entity (typically a manufacturer) Endorser: A role performed by an entity (typically a manufacturer)
whose Endorsements may help Verifiers appraise the authenticity of whose Endorsements may help Verifiers appraise the authenticity of
Evidence and infer further capabilities of the Attester. Evidence and infer further capabilities of the Attester.
Produces: Endorsements Produces: Endorsements
Reference Value Provider: A role performed by an entity (typically a Reference Value Provider: A role performed by an entity (typically a
manufacturer) whose Reference Values help Verifiers appraise manufacturer) whose Reference Values help Verifiers appraise
Evidence to determine if acceptable known Claims have been Evidence to determine if acceptable known Claims have been
recorded by the Attester. recorded by the Attester.
Produces: Reference Values Produces: Reference Values
4.2. Artifacts 4.2. Artifacts
Claim: A piece of asserted information, often in the form of a name/ Claim: A piece of asserted information, often in the form of a name/
value pair. Claims make up the usual structure of Evidence and value pair. Claims make up the usual structure of Evidence and
other RATS artifacts. Compare: /claim/ in [RFC7519]. other RATS conceptual messages. Compare: claim [RFC7519].
Endorsement: A secure statement that an Endorser vouches for the Endorsement: A secure statement that an Endorser vouches for the
integrity of an Attester's various capabilities such as Claims integrity of an Attester's various capabilities, such as Claims
collection and Evidence signing. collection and Evidence signing.
Consumed By: Verifier Consumed By: Verifier
Produced By: Endorser Produced By: Endorser
Evidence: A set of Claims generated by an Attester to be appraised Evidence: A set of Claims generated by an Attester to be appraised
by a Verifier. Evidence may include configuration data, by a Verifier. Evidence may include configuration data,
measurements, telemetry, or inferences. measurements, telemetry, or inferences.
Consumed By: Verifier Consumed By: Verifier
Produced By: Attester Produced By: Attester
Attestation Result: The output generated by a Verifier, typically Attestation Result: The output generated by a Verifier, typically
including information about an Attester, where the Verifier including information about an Attester, where the Verifier
vouches for the validity of the results. vouches for the validity of the results.
Consumed By: Relying Party Consumed By: Relying Party
Produced By: Verifier Produced By: Verifier
Appraisal Policy for Evidence: A set of rules that informs how a Appraisal Policy for Evidence: A set of rules that a Verifier uses
Verifier evaluates the validity of information about an Attester. to evaluate the validity of information about an Attester.
Compare: /security policy/ in [RFC4949]. Compare: security policy [RFC4949].
Consumed By: Verifier Consumed By: Verifier
Produced By: Verifier Owner Produced By: Verifier Owner
Appraisal Policy for Attestation Results: A set of rules that direct Appraisal Policy for Attestation Results: A set of rules that direct
how a Relying Party uses the Attestation Results regarding an how a Relying Party uses the Attestation Results regarding an
Attester generated by the Verifiers. Compare: /security policy/ Attester generated by the Verifiers. Compare: security policy
in [RFC4949]. [RFC4949].
Consumed by: Relying Party Consumed by: Relying Party
Produced by: Relying Party Owner Produced by: Relying Party Owner
Reference Values: A set of values against which values of Claims can Reference Values: A set of values against which values of Claims can
be compared as part of applying an Appraisal Policy for Evidence. be compared as part of applying an Appraisal Policy for Evidence.
Reference Values are sometimes referred to in other documents as Reference Values are sometimes referred to in other documents as
known-good values, golden measurements, or nominal values, "known-good values", "golden measurements", or "nominal values".
although those terms typically assume comparison for equality, These terms typically assume comparison for equality, whereas
whereas here Reference Values might be more general and be used in here, Reference Values might be more general and be used in any
any sort of comparison. sort of comparison.
Consumed By: Verifier Consumed By: Verifier
Produced By: Reference Value Provider Produced By: Reference Value Provider
5. Topological Patterns 5. Topological Patterns
Figure 1 shows a data-flow diagram for communication between an Figure 1 shows a data flow diagram for communication between an
Attester, a Verifier, and a Relying Party. The Attester conveys its Attester, a Verifier, and a Relying Party. The Attester conveys its
Evidence to the Verifier for appraisal, and the Relying Party Evidence to the Verifier for appraisal and the Relying Party receives
receives the Attestation Result from the Verifier. This section the Attestation Result from the Verifier. This section refines the
refines the data-flow diagram by describing two reference models, as data-flow diagram by describing two reference models, as well as one
well as one example composition thereof. The discussion that follows example composition thereof. The discussion that follows is for
is for illustrative purposes only and does not constrain the illustrative purposes only and does not constrain the interactions
interactions between RATS roles to the presented patterns. between RATS roles to the presented models.
5.1. Passport Model 5.1. Passport Model
The passport model is so named because of its resemblance to how The Passport Model is so named because of its resemblance to how
nations issue passports to their citizens. The nature of the nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority Evidence that an individual needs to provide to its local authority
is specific to the country involved. The citizen retains control of is specific to the country involved. The citizen retains control of
the resulting passport document and presents it to other entities the resulting passport document and presents it to other entities
when it needs to assert a citizenship or identity Claim, such as an when it needs to assert a citizenship or identity Claim, such as at
airport immigration desk. The passport is considered sufficient an airport immigration desk. The passport is considered sufficient
because it vouches for the citizenship and identity Claims, and it is because it vouches for the citizenship and identity Claims and it is
issued by a trusted authority. issued by a trusted authority.
Thus, in this immigration desk analogy, the citizen is the Attester, Thus, in this immigration desk analogy, the citizen is the Attester,
the passport issuing agency is a Verifier, and the passport the passport-issuing agency is a Verifier, and the passport
application and identifying information (e.g., birth certificate) is application and identifying information (e.g., birth certificate) is
the the Evidence. The passport is an Attestation Result, and the the Evidence. The passport is an Attestation Result and the
immigration desk is a Relying Party. immigration desk is a Relying Party.
In this model, an Attester conveys Evidence to a Verifier, which In this model, an Attester conveys Evidence to a Verifier that
compares the Evidence against its appraisal policy. The Verifier compares the Evidence against its appraisal policy. The Verifier
then gives back an Attestation Result which the Attester treats as then gives back an Attestation Result that the Attester treats as
opaque data. opaque data.
The Attester does not consume the Attestation Result, but might cache The Attester does not consume the Attestation Result, but it might
it. The Attester can then present the Attestation Result (and cache it. The Attester can then present the Attestation Result (and
possibly additional Claims) to a Relying Party, which then compares possibly additional Claims) to a Relying Party, which then compares
this information against its own appraisal policy. The Attester may this information against its own appraisal policy. The Attester may
also present the same Attestation Result to other Relying Parties. also present the same Attestation Result to other Relying Parties.
Three ways in which the process may fail include: There are three ways in which the process may fail:
* First, the Verifier may not issue a positive Attestation Result * First, the Verifier may not issue a positive Attestation Result
due to the Evidence not passing the Appraisal Policy for Evidence. due to the Evidence not passing the Appraisal Policy for Evidence.
* The second way in which the process may fail is when the * The second way in which the process may fail is when the
Attestation Result is examined by the Relying Party, and based Attestation Result is examined by the Relying Party, and based
upon the Appraisal Policy for Attestation Results, the result does upon the Appraisal Policy for Attestation Results, the result does
not pass the policy. not comply with the policy.
* The third way is when the Verifier is unreachable or unavailable. * The third way is when the Verifier is unreachable or unavailable.
As with any other information needed by the Relying Party to make an As with any other information needed by the Relying Party to make an
authorization decision, an Attestation Result can be carried in a authorization decision, an Attestation Result can be carried in a
resource access protocol between the Attester and Relying Party. In resource access protocol between the Attester and Relying Party. In
this model the details of the resource access protocol constrain the this model, the details of the resource access protocol constrain the
serialization format of the Attestation Result. The format of the serialization format of the Attestation Result. On the other hand,
Evidence on the other hand is only constrained by the Attester- the format of the Evidence is only constrained by the Attester-
Verifier remote attestation protocol. This implies that Verifier remote attestation protocol. This implies that
interoperability and standardization is more relevant for Attestation interoperability and standardization is more relevant for Attestation
Results than it is for Evidence. Results than it is for Evidence.
.------------. .------------.
| | Compare Evidence | | Compare Evidence
| Verifier | against appraisal policy | Verifier | against appraisal policy
| | | |
'--------+---' '--------+---'
^ | ^ |
skipping to change at page 21, line 41 skipping to change at line 890
.---+--------. .-------------. .---+--------. .-------------.
| +------------->| | Compare Attestation | +------------->| | Compare Attestation
| Attester | Attestation | Relying | Result against | Attester | Attestation | Relying | Result against
| | Result | Party | appraisal policy | | Result | Party | appraisal policy
'------------' '-------------' '------------' '-------------'
Figure 5: Passport Model Figure 5: Passport Model
5.2. Background-Check Model 5.2. Background-Check Model
The background-check model is so named because of the resemblance of The Background-Check Model is so named because of the resemblance of
how employers and volunteer organizations perform background checks. how employers and volunteer organizations perform background checks.
When a prospective employee provides Claims about education or When a prospective employee provides Claims about education or
previous experience, the employer will contact the respective previous experience, the employer will contact the respective
institutions or former employers to validate the Claim. Volunteer institutions or former employers to validate the Claim. Volunteer
organizations often perform police background checks on volunteers in organizations often perform police background checks on volunteers in
order to determine the volunteer's trustworthiness. Thus, in this order to determine the volunteer's trustworthiness. Thus, in this
analogy, a prospective volunteer is an Attester, the organization is analogy, a prospective volunteer is an Attester, the organization is
the Relying Party, and the organization that issues a report is a the Relying Party, and the organization that issues a report is a
Verifier. Verifier.
In this model, an Attester conveys Evidence to a Relying Party, which In this model, an Attester conveys Evidence to a Relying Party, which
treats it as opaque and simply forwards it on to a Verifier. The treats it as opaque and simply forwards it on to a Verifier. The
Verifier compares the Evidence against its appraisal policy, and Verifier compares the Evidence against its appraisal policy and
returns an Attestation Result to the Relying Party. The Relying returns an Attestation Result to the Relying Party. The Relying
Party then compares the Attestation Result against its own appraisal Party then compares the Attestation Result against its own appraisal
policy. policy.
The resource access protocol between the Attester and Relying Party The resource access protocol between the Attester and Relying Party
includes Evidence rather than an Attestation Result, but that includes Evidence rather than an Attestation Result, but that
Evidence is not processed by the Relying Party. Evidence is not processed by the Relying Party.
Since the Evidence is merely forwarded on to a trusted Verifier, any Since the Evidence is merely forwarded on to a trusted Verifier, any
serialization format can be used for Evidence because the Relying serialization format can be used for Evidence because the Relying
Party does not need a parser for it. The only requirement is that Party does not need a parser for it. The only requirement is that
the Evidence can be _encapsulated in_ the format required by the the Evidence can be _encapsulated_ in the format required by the
resource access protocol between the Attester and Relying Party. resource access protocol between the Attester and Relying Party.
However, like in the Passport model, an Attestation Result is still However, as seen in the Passport Model, an Attestation Result is
consumed by the Relying Party. Code footprint and attack surface still consumed by the Relying Party. Code footprint and attack
area can be minimized by using a serialization format for which the surface area can be minimized by using a serialization format for
Relying Party already needs a parser to support the protocol between which the Relying Party already needs a parser to support the
the Attester and Relying Party, which may be an existing standard or protocol between the Attester and Relying Party, which may be an
widely deployed resource access protocol. Such minimization is existing standard or widely deployed resource access protocol. Such
especially important if the Relying Party is a constrained node. minimization is especially important if the Relying Party is a
constrained node.
.-------------. .-------------.
| | Compare Evidence | | Compare Evidence
| Verifier | against appraisal | Verifier | against appraisal
| | policy | | policy
'--------+----' '--------+----'
^ | ^ |
Evidence | | Attestation Evidence | | Attestation
| | Result | | Result
| v | v
.------------. .----|--------. .------------. .----|--------.
| +-------------->|---' | Compare Attestation | +-------------->|---' | Compare Attestation
| Attester | Evidence | Relying | Result against | Attester | Evidence | Relying | Result against
| | | Party | appraisal policy | | | Party | appraisal policy
'------------' '-------------' '------------' '-------------'
Figure 6: Background-Check Model Figure 6: Background-Check Model
5.3. Combinations 5.3. Combinations
One variation of the background-check model is where the Relying One variation of the Background-Check Model is where the Relying
Party and the Verifier are on the same machine, performing both Party and the Verifier are on the same machine, performing both
functions together. In this case, there is no need for a protocol functions together. In this case, there is no need for a protocol
between the two. between the two.
It is also worth pointing out that the choice of model depends on the It is also worth pointing out that the choice of model depends on the
use case, and that different Relying Parties may use different use case and that different Relying Parties may use different
topological patterns. topological patterns.
The same device may need to create Evidence for different Relying The same device may need to create Evidence for different Relying
Parties and/or different use cases. For instance, it would use one Parties and/or different use cases. For instance, it would use one
model to provide Evidence to a network infrastructure device to gain model to provide Evidence to a network infrastructure device to gain
access to the network, and the other model to provide Evidence to a access to the network and the other model to provide Evidence to a
server holding confidential data to gain access to that data. As server holding confidential data to gain access to that data. As
such, both models may simultaneously be in use by the same device. such, both models may simultaneously be in use by the same device.
Figure 7 shows another example of a combination where Relying Party 1 Figure 7 shows another example of a combination where Relying Party 1
uses the passport model, whereas Relying Party 2 uses an extension of uses the Passport Model, whereas Relying Party 2 uses an extension of
the background-check model. Specifically, in addition to the basic the Background-Check Model. Specifically, in addition to the basic
functionality shown in Figure 6, Relying Party 2 actually provides functionality shown in Figure 6, Relying Party 2 actually provides
the Attestation Result back to the Attester, allowing the Attester to the Attestation Result back to the Attester, allowing the Attester to
use it with other Relying Parties. This is the model that the use it with other Relying Parties. This is the model that the TAM
Trusted Application Manager plans to support in the TEEP architecture plans to support in the TEEP architecture [TEEP-ARCH].
[I-D.ietf-teep-architecture].
.-------------. .-------------.
| | Compare Evidence | | Compare Evidence
| Verifier | against appraisal policy | Verifier | against appraisal policy
| | | |
'--------+----' '--------+----'
^ | ^ |
Evidence | | Attestation Evidence | | Attestation
| | Result | | Result
| v | v
skipping to change at page 24, line 11 skipping to change at line 1002
'-------------' '-------------' '-------------' '-------------'
Figure 7: Example Combination Figure 7: Example Combination
6. Roles and Entities 6. Roles and Entities
An entity in the RATS architecture includes at least one of the roles An entity in the RATS architecture includes at least one of the roles
defined in this document. defined in this document.
An entity can aggregate more than one role into itself, such as being An entity can aggregate more than one role into itself, such as being
both a Verifier and a Relying Party, or being both a Reference Value both a Verifier and a Relying Party or being both a Reference Value
Provider and an Endorser. As such, any conceptual messages (see Provider and an Endorser. As such, any conceptual messages (see
Section 8 for more discussion) originating from such roles might also Section 8 for more discussion) originating from such roles might also
be combined. For example, Reference Values might be conveyed as part be combined. For example, Reference Values might be conveyed as part
of an appraisal policy if the Verifier Owner and Reference Value of an appraisal policy if the Verifier Owner and Reference Value
Provider roles are combined. Similarly, Reference Values might be Provider roles are combined. Similarly, Reference Values might be
conveyed as part of an Endorsement if the Endorser and Reference conveyed as part of an Endorsement if the Endorser and Reference
Value Provider roles are combined. Value Provider roles are combined.
Interactions between roles aggregated into the same entity do not Interactions between roles aggregated into the same entity do not
necessarily use the Internet Protocol. Such interactions might use a necessarily use the Internet Protocol. Such interactions might use a
loopback device or other IP-based communication between separate loopback device or other IP-based communication between separate
environments, but they do not have to. Alternative channels to environments, but they do not have to. Alternative channels to
convey conceptual messages include function calls, sockets, GPIO convey conceptual messages include function calls, sockets, General-
interfaces, local busses, or hypervisor calls. This type of Purpose Input/Output (GPIO) interfaces, local buses, or hypervisor
conveyance is typically found in composite devices. Most calls. This type of conveyance is typically found in composite
importantly, these conveyance methods are out-of-scope of RATS, but devices. Most importantly, these conveyance methods are out of scope
they are presumed to exist in order to convey conceptual messages of the RATS architecture, but they are presumed to exist in order to
appropriately between roles. convey conceptual messages appropriately between roles.
In essence, an entity that combines more than one role creates and In essence, an entity that combines more than one role creates and
consumes the corresponding conceptual messages as defined in this consumes the corresponding conceptual messages as defined in this
document. document.
7. Trust Model 7. Trust Model
7.1. Relying Party 7.1. Relying Party
This document covers scenarios for which a Relying Party trusts a This document covers scenarios for which a Relying Party trusts a
Verifier that can appraise the trustworthiness of information about Verifier that can appraise the trustworthiness of information about
an Attester. Such trust is expressed by storing one or more "trust an Attester. Such trust is expressed by storing one or more "trust
anchors" in a secure location known as a trust anchor store. anchors" in a secure location known as a "trust anchor store".
As defined in [RFC6024], "A trust anchor represents an authoritative As defined in [RFC6024]:
entity via a public key and associated data. The public key is used
to verify digital signatures, and the associated data is used to | A trust anchor represents an authoritative entity via a public key
constrain the types of information for which the trust anchor is | and associated data. The public key is used to verify digital
authoritative." The trust anchor may be a certificate or it may be a | signatures, and the associated data is used to constrain the types
raw public key along with additional data if necessary such as its | of information for which the trust anchor is authoritative.
public key algorithm and parameters. In the context of this
document, a trust anchor may also be a symmetric key, as in The trust anchor may be a certificate or it may be a raw public key
[TCG-DICE-SIBDA] or the symmetric mode described in along with additional data if necessary, such as its public key
[I-D.tschofenig-rats-psa-token]. algorithm and parameters. In the context of this document, a trust
anchor may also be a symmetric key, as in [TCG-DICE-SIBDA], or the
symmetric mode described in [RATS-PSA-TOKEN].
Thus, trusting a Verifier might be expressed by having the Relying Thus, trusting a Verifier might be expressed by having the Relying
Party store the Verifier's key or certificate in its trust anchor Party store the Verifier's key or certificate in its trust anchor
store, or might be expressed by storing the public key or certificate store. It might also be expressed by storing the public key or
of an entity (e.g., a Certificate Authority) that is in the certificate of an entity (e.g., a Certificate Authority) that is in
Verifier's certificate path. For example, the Relying Party can the Verifier's certificate path. For example, the Relying Party can
verify that the Verifier is an expected one by out-of-band verify that the Verifier is an expected one by out-of-band
establishment of key material, combined with a protocol like TLS to establishment of key material combined with a protocol like TLS to
communicate. There is an assumption that between the establishment communicate. There is an assumption that the Verifier has not been
of the trusted key material and the creation of the Evidence, that compromised between the establishment of the trusted key material and
the Verifier has not been compromised. the creation of the Evidence.
For a stronger level of security, the Relying Party might require For a stronger level of security, the Relying Party might require
that the Verifier first provide information about itself that the that the Verifier first provide information about itself that the
Relying Party can use to assess the trustworthiness of the Verifier Relying Party can use to assess the trustworthiness of the Verifier
before accepting its Attestation Results. Such process would provide before accepting its Attestation Results. Such a process would
a stronger level of confidence in the correctness of the information provide a stronger level of confidence in the correctness of the
provided, such as a belief that the authentic Verifier has not been information provided, such as a belief that the authentic Verifier
compromised by malware. has not been compromised by malware.
For example, one explicit way for a Relying Party "A" to establish For example, one explicit way for a Relying Party "A" to establish
such confidence in the correctness of a Verifier "B", would be for B such confidence in the correctness of a Verifier "B" would be for B
to first act as an Attester where A acts as a combined Verifier/ to first act as an Attester where A acts as a combined Verifier/
Relying Party. If A then accepts B as trustworthy, it can choose to Relying Party. If A then accepts B as trustworthy, it can choose to
accept B as a Verifier for other Attesters. accept B as a Verifier for other Attesters.
Similarly, the Relying Party also needs to trust the Relying Party Similarly, the Relying Party also needs to trust the Relying Party
Owner for providing its Appraisal Policy for Attestation Results, and Owner for providing its Appraisal Policy for Attestation Results,
in some scenarios the Relying Party might even require that the and, in some scenarios, the Relying Party might even require that the
Relying Party Owner go through a remote attestation procedure with it Relying Party Owner go through a remote attestation procedure with it
before the Relying Party will accept an updated policy. This can be before the Relying Party will accept an updated policy. This can be
done similarly to how a Relying Party could establish trust in a done in a manner similar to how a Relying Party could establish trust
Verifier as discussed above, i.e., verifying credentials against a in a Verifier as discussed above, i.e., verifying credentials against
trust anchor store and optionally requiring Attestation Results from a trust anchor store and optionally requiring Attestation Results
the Relying Party Owner. from the Relying Party Owner.
7.2. Attester 7.2. Attester
In some scenarios, Evidence might contain sensitive information such In some scenarios, Evidence might contain sensitive information, such
as Personally Identifiable Information (PII) or system identifiable as Personally Identifiable Information (PII) or system identifiable
information. Thus, an Attester must trust entities to which it information. Thus, an Attester must trust the entities to which it
conveys Evidence, to not reveal sensitive data to unauthorized conveys Evidence to not reveal sensitive data to unauthorized
parties. The Verifier might share this information with other parties. The Verifier might share this information with other
authorized parties, according to a governing policy that address the authorized parties according to a governing policy that addresses the
handling of sensitive information (potentially included in Appraisal handling of sensitive information (potentially included in Appraisal
Policies for Evidence). In the background-check model, this Evidence Policies for Evidence). In the Background-Check Model, this Evidence
may also be revealed to Relying Party(s). may also be revealed to Relying Parties.
When Evidence contains sensitive information, an Attester typically When Evidence contains sensitive information, an Attester typically
requires that a Verifier authenticates itself (e.g., at TLS session requires that a Verifier authenticates itself (e.g., at TLS session
establishment) and might even request a remote attestation before the establishment) and might even request a remote attestation before the
Attester sends the sensitive Evidence. This can be done by having Attester sends the sensitive Evidence. This can be done by having
the Attester first act as a Verifier/Relying Party, and the Verifier the Attester first act as a Verifier/Relying Party and the Verifier
act as its own Attester, as discussed above. act as its own Attester, as discussed above.
7.3. Relying Party Owner 7.3. Relying Party Owner
The Relying Party Owner might also require that the Relying Party The Relying Party Owner might also require that the Relying Party
first act as an Attester, providing Evidence that the Owner can first act as an Attester by providing Evidence that the Owner can
appraise, before the Owner would give the Relying Party an updated appraise before the Owner would give the Relying Party an updated
policy that might contain sensitive information. In such a case, policy that might contain sensitive information. In such a case,
authentication or attestation in both directions might be needed, in authentication or attestation in both directions might be needed.
which case typically one side's Evidence must be considered safe to Typically, one side's Evidence must be considered safe to share with
share with an untrusted entity, in order to bootstrap the sequence. an untrusted entity in order to bootstrap the sequence. See
See Section 11 for more discussion. Section 11 for more discussion.
7.4. Verifier 7.4. Verifier
The Verifier trusts (or more specifically, the Verifier's security The Verifier trusts (or more specifically, the Verifier's security
policy is written in a way that configures the Verifier to trust) a policy is written in a way that configures the Verifier to trust) a
manufacturer, or the manufacturer's hardware, so as to be able to manufacturer or the manufacturer's hardware so as to be able to
appraise the trustworthiness of that manufacturer's devices. Such appraise the trustworthiness of that manufacturer's devices. Such
trust is expressed by storing one or more trust anchors in the trust is expressed by storing one or more trust anchors in the
Verifier's trust anchor store. Verifier's trust anchor store.
In a typical solution, a Verifier comes to trust an Attester In a typical solution, a Verifier comes to trust an Attester
indirectly by having an Endorser (such as a manufacturer) vouch for indirectly by having an Endorser (such as a manufacturer) vouch for
the Attester's ability to securely generate Evidence through the Attester's ability to securely generate Evidence through
Endorsements (see Section 8.2). Endorsements might describe the ways Endorsements (see Section 8.2). Endorsements might describe the ways
in which the Attester resists attack, protects secrets and measures in which the Attester resists attacks, protects secrets, and measures
Target Environments. Consequently, the Endorser's key material is Target Environments. Consequently, the Endorser's key material is
stored in the Verifier's trust anchor store so that Endorsements can stored in the Verifier's trust anchor store so that Endorsements can
be authenticated and used in the Verifier's appraisal process. be authenticated and used in the Verifier's appraisal process.
In some solutions, a Verifier might be configured to directly trust In some solutions, a Verifier might be configured to directly trust
an Attester by having the Verifier have the Attester's key material an Attester by having the Verifier possess the Attester's key
(rather than the Endorser's) in its trust anchor store. material (rather than the Endorser's) in its trust anchor store.
Such direct trust must first be established at the time of trust Such direct trust must first be established at the time of trust
anchor store configuration either by checking with an Endorser at anchor store configuration either by checking with an Endorser at
that time, or by conducting a security analysis of the specific that time or by conducting a security analysis of the specific
device. Having the Attester directly in the trust anchor store device. Having the Attester directly in the trust anchor store
narrows the Verifier's trust to only specific devices rather than all narrows the Verifier's trust to only specific devices rather than all
devices the Endorser might vouch for, such as all devices devices the Endorser might vouch for, such as all devices
manufactured by the same manufacturer in the case that the Endorser manufactured by the same manufacturer in the case that the Endorser
is a manufacturer. is a manufacturer.
Such narrowing is often important since physical possession of a Such narrowing is often important since physical possession of a
device can also be used to conduct a number of attacks, and so a device can also be used to conduct a number of attacks, and so a
device in a physically secure environment (such as one's own device in a physically secure environment (such as one's own
premises) may be considered trusted whereas devices owned by others premises) may be considered trusted, whereas devices owned by others
would not be. This often results in a desire to either have the would not be. This often results in a desire either to have the
owner run their own Endorser that would only endorse devices one owner run their own Endorser that would only endorse devices one owns
owns, or to use Attesters directly in the trust anchor store. When or to use Attesters directly in the trust anchor store. When there
there are many Attesters owned, the use of an Endorser enables better are many Attesters owned, the use of an Endorser enables better
scalability. scalability.
That is, a Verifier might appraise the trustworthiness of an That is, a Verifier might appraise the trustworthiness of an
application component, operating system component, or service under application component, operating system component, or service under
the assumption that information provided about it by the lower-layer the assumption that information provided about it by the lower-layer
firmware or software is true. A stronger level of assurance of firmware or software is true. A stronger level of assurance of
security comes when information can be vouched for by hardware or by security comes when information can be vouched for by hardware or by
ROM code, especially if such hardware is physically resistant to ROM code, especially if such hardware is physically resistant to
hardware tampering. In most cases, components that have to be hardware tampering. In most cases, components that have to be
vouched for via Endorsements because no Evidence is generated about vouched for via Endorsements (because no Evidence is generated about
them are referred to as roots of trust. them) are referred to as "roots of trust".
The manufacturer having arranged for an Attesting Environment to be The manufacturer having arranged for an Attesting Environment to be
provisioned with key material with which to sign Evidence, the provisioned with key material with which to sign Evidence, the
Verifier is then provided with some way of verifying the signature on Verifier is then provided with some way of verifying the signature on
the Evidence. This may be in the form of an appropriate trust the Evidence. This may be in the form of an appropriate trust anchor
anchor, or the Verifier may be provided with a database of public or the Verifier may be provided with a database of public keys
keys (rather than certificates) or even carefully curated and secured (rather than certificates) or even carefully curated and secured
lists of symmetric keys. lists of symmetric keys.
The nature of how the Verifier manages to validate the signatures The nature of how the Verifier manages to validate the signatures
produced by the Attester is critical to the secure operation of a produced by the Attester is critical to the secure operation of a
remote attestation system, but is not the subject of standardization remote attestation system but is not the subject of standardization
within this architecture. within this architecture.
A conveyance protocol that provides authentication and integrity A conveyance protocol that provides authentication and integrity
protection can be used to convey Evidence that is otherwise protection can be used to convey Evidence that is otherwise
unprotected (e.g., not signed). Appropriate conveyance of unprotected (e.g., not signed). Appropriate conveyance of
unprotected Evidence (e.g., [I-D.birkholz-rats-uccs]) relies on the unprotected Evidence (e.g., [RATS-UCCS]) relies on the following
following conveyance protocol's protection capabilities: conveyance protocol's protection capabilities:
1. The key material used to authenticate and integrity protect the 1. The key material used to authenticate and integrity protect the
conveyance channel is trusted by the Verifier to speak for the conveyance channel is trusted by the Verifier to speak for the
Attesting Environment(s) that collected Claims about the Target Attesting Environment(s) that collected Claims about the Target
Environment(s). Environment(s).
2. All unprotected Evidence that is conveyed is supplied exclusively 2. All unprotected Evidence that is conveyed is supplied exclusively
by the Attesting Environment that has the key material that by the Attesting Environment that has the key material that
protects the conveyance channel protects the conveyance channel.
3. A trusted environment protects the conveyance channel's key 3. A trusted environment protects the conveyance channel's key
material which may depend on other Attesting Environments with material, which may depend on other Attesting Environments with
equivalent strength protections. equivalent strength protections.
As illustrated in [I-D.birkholz-rats-uccs], an entity that receives As illustrated in [RATS-UCCS], an entity that receives unprotected
unprotected Evidence via a trusted conveyance channel always takes on Evidence via a trusted conveyance channel always takes on the
the responsibility of vouching for the Evidence's authenticity and responsibility of vouching for the Evidence's authenticity and
freshness. If protected Evidence is generated, the Attester's freshness. If protected Evidence is generated, the Attester's
Attesting Environments take on that responsibility. In cases where Attesting Environments take on that responsibility. In cases where
unprotected Evidence is processed by a Verifier, Relying Parties have unprotected Evidence is processed by a Verifier, Relying Parties have
to trust that the Verifier is capable of handling Evidence in a to trust that the Verifier is capable of handling Evidence in a
manner that preserves the Evidence's authenticity and freshness. manner that preserves the Evidence's authenticity and freshness.
Generating and conveying unprotected Evidence always creates Generating and conveying unprotected Evidence always creates
significant risk and the benefits of that approach have to be significant risk and the benefits of that approach have to be
carefully weighed against potential drawbacks. carefully weighed against potential drawbacks.
See Section 12 for discussion on security strength. See Section 12 for discussion on security strength.
7.5. Endorser, Reference Value Provider, and Verifier Owner 7.5. Endorser, Reference Value Provider, and Verifier Owner
In some scenarios, the Endorser, Reference Value Provider, and In some scenarios, the Endorser, Reference Value Provider, and
Verifier Owner may need to trust the Verifier before giving the Verifier Owner may need to trust the Verifier before giving the
Endorsement, Reference Values, or appraisal policy to it. This can Endorsement, Reference Values, or appraisal policy to it. This can
be done similarly to how a Relying Party might establish trust in a be done in a similar manner to how a Relying Party might establish
Verifier. trust in a Verifier.
As discussed in Section 7.3, authentication or attestation in both As discussed in Section 7.3, authentication or attestation in both
directions might be needed, in which case typically one side's directions might be needed. Typically, one side's identity or
identity or Evidence must be considered safe to share with an Evidence in this case must be considered safe to share with an
untrusted entity, in order to bootstrap the sequence. See Section 11 untrusted entity in order to bootstrap the sequence. See Section 11
for more discussion. for more discussion.
8. Conceptual Messages 8. Conceptual Messages
Figure 1 illustrates the flow of conceptual messages between various Figure 1 illustrates the flow of conceptual messages between various
roles. This section provides additional elaboration and roles. This section provides additional elaboration and
implementation considerations. It is the responsibility of protocol implementation considerations. It is the responsibility of protocol
specifications to define the actual data format and semantics of any specifications to define the actual data format and semantics of any
relevant conceptual messages. relevant conceptual messages.
8.1. Evidence 8.1. Evidence
Evidence is a set of Claims about the target environment that reveal Evidence is a set of Claims about the Target Environment that reveal
operational status, health, configuration or construction that have operational status, health, configuration, or construction that have
security relevance. Evidence is appraised by a Verifier to establish security relevance. Evidence is appraised by a Verifier to establish
its relevance, compliance, and timeliness. Claims need to be its relevance, compliance, and timeliness. Claims need to be
collected in a manner that is reliable such that a Target Environment collected in a manner that is reliable such that a Target Environment
cannot lie to the Attesting Environment about its trustworthiness cannot lie to the Attesting Environment about its trustworthiness
properties. Evidence needs to be securely associated with the target properties. Evidence needs to be securely associated with the Target
environment so that the Verifier cannot be tricked into accepting Environment so that the Verifier cannot be tricked into accepting
Claims originating from a different environment (that may be more Claims originating from a different environment (that may be more
trustworthy). Evidence also must be protected from an active on-path trustworthy). Evidence also must be protected from an active on-path
attacker who may observe, change or misdirect Evidence as it travels attacker who may observe, change, or misdirect Evidence as it travels
from Attester to Verifier. The timeliness of Evidence can be from the Attester to the Verifier. The timeliness of Evidence can be
captured using Claims that pinpoint the time or interval when changes captured using Claims that pinpoint the time or interval when changes
in operational status, health, and so forth occur. in operational status, health, and so forth occur.
8.2. Endorsements 8.2. Endorsements
An Endorsement is a secure statement that some entity (e.g., a An Endorsement is a secure statement that some entity (e.g., a
manufacturer) vouches for the integrity of the device's various manufacturer) vouches for the integrity of the device's various
capabilities such as claims collection, signing, launching code, capabilities, such as Claims collection, signing, launching code,
transitioning to other environments, storing secrets, and more. For transitioning to other environments, storing secrets, and more. For
example, if the device's signing capability is in hardware, then an example, if the device's signing capability is in hardware, then an
Endorsement might be a manufacturer certificate that signs a public Endorsement might be a manufacturer certificate that signs a public
key whose corresponding private key is only known inside the device's key whose corresponding private key is only known inside the device's
hardware. Thus, when Evidence and such an Endorsement are used hardware. Thus, when Evidence and such an Endorsement are used
together, an appraisal procedure can be conducted based on appraisal together, an appraisal procedure can be conducted based on appraisal
policies that may not be specific to the device instance, but merely policies that may not be specific to the device instance but are
specific to the manufacturer providing the Endorsement. For example, merely specific to the manufacturer providing the Endorsement. For
an appraisal policy might simply check that devices from a given example, an appraisal policy might simply check that devices from a
manufacturer have information matching a set of Reference Values, or given manufacturer have information matching a set of Reference
an appraisal policy might have a set of more complex logic on how to Values. An appraisal policy might also have a set of more complex
appraise the validity of information. logic on how to appraise the validity of information.
However, while an appraisal policy that treats all devices from a However, while an appraisal policy that treats all devices from a
given manufacturer the same may be appropriate for some use cases, it given manufacturer the same may be appropriate for some use cases, it
would be inappropriate to use such an appraisal policy as the sole would be inappropriate to use such an appraisal policy as the sole
means of authorization for use cases that wish to constrain _which_ means of authorization for use cases that wish to constrain _which_
compliant devices are considered authorized for some purpose. For compliant devices are considered authorized for some purpose. For
example, an enterprise using remote attestation for Network Endpoint example, an enterprise using remote attestation for Network Endpoint
Assessment [RFC5209] may not wish to let every healthy laptop from Assessment (NEA) [RFC5209] may not wish to let every healthy laptop
the same manufacturer onto the network, but instead only want to let from the same manufacturer onto the network. Instead, it may only
devices that it legally owns onto the network. Thus, an Endorsement want to let devices that it legally owns onto the network. Thus, an
may be helpful information in authenticating information about a Endorsement may be helpful information in authenticating information
device, but is not necessarily sufficient to authorize access to about a device, but is not necessarily sufficient to authorize access
resources which may need device-specific information such as a public to resources that may need device-specific information, such as a
key for the device or component or user on the device. public key for the device or component or user on the device.
8.3. Reference Values 8.3. Reference Values
Reference Values used in appraisal procedures come from a Reference Reference Values used in appraisal procedures come from a Reference
Value Provider and are then used by the Verifier to compare to Value Provider and are then used by the Verifier to compare to
Evidence. Reference Values with matching Evidence produces Evidence. Reference Values with matching Evidence produce acceptable
acceptable Claims. Additionally, appraisal policy may play a role in Claims. Additionally, an appraisal policy may play a role in
determining the acceptance of Claims. determining the acceptance of Claims.
8.4. Attestation Results 8.4. Attestation Results
Attestation Results are the input used by the Relying Party to decide Attestation Results are the input used by the Relying Party to decide
the extent to which it will trust a particular Attester, and allow it the extent to which it will trust a particular Attester and allow it
to access some data or perform some operation. to access some data or perform some operation.
Attestation Results may carry a boolean value indicating compliance Attestation Results may carry a boolean value indicating compliance
or non-compliance with a Verifier's appraisal policy, or may carry a or non-compliance with a Verifier's appraisal policy or may carry a
richer set of Claims about the Attester, against which the Relying richer set of Claims about the Attester, against which the Relying
Party applies its Appraisal Policy for Attestation Results. Party applies its Appraisal Policy for Attestation Results.
The quality of the Attestation Results depends upon the ability of The quality of the Attestation Results depends upon the ability of
the Verifier to evaluate the Attester. Different Attesters have a the Verifier to evaluate the Attester. Different Attesters have a
different _Strength of Function_ [strengthoffunction], which results different _Strength of Function_ [strengthoffunction], which results
in the Attestation Results being qualitatively different in strength. in the Attestation Results being qualitatively different in strength.
An Attestation Result that indicates non-compliance can be used by an An Attestation Result that indicates non-compliance can be used by an
Attester (in the passport model) or a Relying Party (in the Attester (in the Passport Model) or a Relying Party (in the
background-check model) to indicate that the Attester should not be Background-Check Model) to indicate that the Attester should not be
treated as authorized and may be in need of remediation. In some treated as authorized and may be in need of remediation. In some
cases, it may even indicate that the Evidence itself cannot be cases, it may even indicate that the Evidence itself cannot be
authenticated as being correct. authenticated as being correct.
By default, the Relying Party does not believe the Attester to be By default, the Relying Party does not believe the Attester to be
compliant. Upon receipt of an authentic Attestation Result and given compliant. Upon receipt of an authentic Attestation Result and given
the Appraisal Policy for Attestation Results is satisfied, the the Appraisal Policy for Attestation Results is satisfied, the
Attester is allowed to perform the prescribed actions or access. The Attester is allowed to perform the prescribed actions or access. The
simplest such appraisal policy might authorize granting the Attester simplest such appraisal policy might authorize granting the Attester
full access or control over the resources guarded by the Relying full access or control over the resources guarded by the Relying
Party. A more complex appraisal policy might involve using the Party. A more complex appraisal policy might involve using the
information provided in the Attestation Result to compare against information provided in the Attestation Result to compare against
expected values, or to apply complex analysis of other information expected values or to apply complex analysis of other information
contained in the Attestation Result. contained in the Attestation Result.
Thus, Attestation Results can contain detailed information about an Thus, Attestation Results can contain detailed information about an
Attester, which can include privacy sensitive information as Attester, which can include privacy sensitive information as
discussed in Section 11. Unlike Evidence, which is often very discussed in Section 11. Unlike Evidence, which is often very
device- and vendor-specific, Attestation Results can be vendor- device- and vendor-specific, Attestation Results can be vendor-
neutral, if the Verifier has a way to generate vendor-agnostic neutral, if the Verifier has a way to generate vendor-agnostic
information based on the appraisal of vendor-specific information in information based on the appraisal of vendor-specific information in
Evidence. This allows a Relying Party's appraisal policy to be Evidence. This allows a Relying Party's appraisal policy to be
simpler, potentially based on standard ways of expressing the simpler, potentially based on standard ways of expressing the
information, while still allowing interoperability with heterogeneous information, while still allowing interoperability with heterogeneous
devices. devices.
Finally, whereas Evidence is signed by the device (or indirectly by a Finally, whereas Evidence is signed by the device (or indirectly by a
manufacturer, if Endorsements are used), Attestation Results are manufacturer if Endorsements are used), Attestation Results are
signed by a Verifier, allowing a Relying Party to only need a trust signed by a Verifier, allowing a Relying Party to only need a trust
relationship with one entity, rather than a larger set of entities, relationship with one entity rather than a larger set of entities for
for purposes of its appraisal policy. purposes of its appraisal policy.
8.5. Appraisal Policies 8.5. Appraisal Policies
The Verifier, when appraising Evidence, or the Relying Party, when The Verifier (when appraising Evidence) or the Relying Party (when
appraising Attestation Results, checks the values of matched Claims appraising Attestation Results) checks the values of matched Claims
against constraints specified in its appraisal policy. Examples of against constraints specified in its appraisal policy. Examples of
such constraints checking include: such constraints checking include the following:
* comparison for equality against a Reference Value, or * Comparison for equality against a Reference Value.
* a check for being in a range bounded by Reference Values, or * A check for being in a range bounded by Reference Values.
* membership in a set of Reference Values, or * Membership in a set of Reference Values.
* a check against values in other Claims. * A check against values in other Claims.
Upon completing all appraisal policy constraints, the remaining Upon completing all appraisal policy constraints, the remaining
Claims are accepted as input toward determining Attestation Results, Claims are accepted as input toward determining Attestation Results
when appraising Evidence, or as input to a Relying Party, when (when appraising Evidence) or as input to a Relying Party (when
appraising Attestation Results. appraising Attestation Results).
9. Claims Encoding Formats 9. Claims Encoding Formats
The following diagram illustrates a relationship to which remote Figure 8 illustrates a relationship to which remote attestation is
attestation is desired to be added: desired to be added:
.-------------. .------------. Evaluate .-------------. .------------. Evaluate
| +-------------->| | request | +-------------->| | request
| Attester | Access some | Relying | against | Attester | Access some | Relying | against
| | resource | Party | security | | resource | Party | security
'-------------' '------------' policy '-------------' '------------' policy
Figure 8: Typical Resource Access Figure 8: Typical Resource Access
In this diagram, the protocol between Attester and a Relying Party In this diagram, the protocol between the Attester and a Relying
can be any new or existing protocol (e.g., HTTP(S), COAP(S), ROLIE Party can be any new or existing protocol (e.g., HTTP(S), CoAP(S),
[RFC8322], 802.1x, OPC UA [OPCUA], etc.), depending on the use case. Resource-Oriented Lightweight Information Exchange (ROLIE) [RFC8322],
802.1x, OPC UA [OPCUA], etc.) depending on the use case.
Typically, such protocols already have mechanisms for passing Typically, such protocols already have mechanisms for passing
security information for authentication and authorization purposes. security information for authentication and authorization purposes.
Common formats include JWTs [RFC7519], CWTs [RFC8392], and X.509 Common formats include JSON Web Tokens (JWTs) [RFC7519], CWTs
certificates. [RFC8392], and X.509 certificates.
Retrofitting already deployed protocols with remote attestation Retrofitting already-deployed protocols with remote attestation
requires adding RATS conceptual messages to the existing data flows. requires adding RATS conceptual messages to the existing data flows.
This must be done in a way that does not degrade the security This must be done in a way that does not degrade the security
properties of the systems involved and should use native extension properties of the systems involved and should use extension
mechanisms provided by the underlying protocol. For example, if a mechanisms provided by the underlying protocol. For example, if a
TLS handshake is to be extended with remote attestation capabilities, TLS handshake is to be extended with remote attestation capabilities,
attestation Evidence may be embedded in an ad-hoc X.509 certificate attestation Evidence may be embedded in an ad hoc X.509 certificate
extension (e.g., [TCG-DICE]), or into a new TLS Certificate Type extension (e.g., [TCG-DICE]) or into a new TLS Certificate Type
(e.g., [I-D.tschofenig-tls-cwt]). (e.g., [TLS-CWT]).
Especially for constrained nodes there is a desire to minimize the Especially for constrained nodes, there is a desire to minimize the
amount of parsing code needed in a Relying Party, in order to both amount of parsing code needed in a Relying Party in order to both
minimize footprint and to minimize the attack surface. While it minimize footprint and the attack surface. While it would be
would be possible to embed a CWT inside a JWT, or a JWT inside an possible to embed a CWT inside a JWT, or a JWT inside an X.509
X.509 extension, etc., there is a desire to encode the information extension, etc., there is a desire to encode the information in a
natively in a format that is already supported by the Relying Party. format that is already supported by the Relying Party.
This motivates having a common "information model" that describes the This motivates having a common "information model" that describes the
set of remote attestation related information in an encoding-agnostic set of remote attestation related information in an encoding-agnostic
way, and allowing multiple encoding formats (CWT, JWT, X.509, etc.) way and allows multiple encoding formats (CWT, JWT, X.509, etc.) that
that encode the same information into the Claims format needed by the encode the same information into the Claims format needed by the
Relying Party. Relying Party.
The following diagram illustrates that Evidence and Attestation Figure 9 illustrates that Evidence and Attestation Results might be
Results might be expressed via multiple potential encoding formats, expressed via multiple potential encoding formats so that they can be
so that they can be conveyed by various existing protocols. It also conveyed by various existing protocols. It also motivates why the
motivates why the Verifier might also be responsible for accepting Verifier might also be responsible for accepting Evidence that
Evidence that encodes Claims in one format, while issuing Attestation encodes Claims in one format while issuing Attestation Results that
Results that encode Claims in a different format. encode Claims in a different format.
Evidence Attestation Results Evidence Attestation Results
.--------------. CWT CWT .-------------------. .--------------. CWT CWT .-------------------.
| Attester-A +-----------. .---------->| Relying Party V | | Attester-A +-----------. .---------->| Relying Party V |
'--------------' | | `-------------------' '--------------' | | `-------------------'
v | v |
.--------------. JWT .---------+--. JWT .-------------------. .--------------. JWT .---------+--. JWT .-------------------.
| Attester-B +-------->| +-------->| Relying Party W | | Attester-B +-------->| +-------->| Relying Party W |
'--------------' | | `-------------------' '--------------' | | `-------------------'
| | | |
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Figure 9: Multiple Attesters and Relying Parties with Different Figure 9: Multiple Attesters and Relying Parties with Different
Formats Formats
10. Freshness 10. Freshness
A Verifier or Relying Party might need to learn the point in time A Verifier or Relying Party might need to learn the point in time
(i.e., the "epoch") an Evidence or Attestation Result has been (i.e., the "epoch") an Evidence or Attestation Result has been
produced. This is essential in deciding whether the included Claims produced. This is essential in deciding whether the included Claims
can be considered fresh, meaning they still reflect the latest state can be considered fresh, meaning they still reflect the latest state
of the Attester, and that any Attestation Result was generated using of the Attester, and that any Attestation Result was generated using
the latest Appraisal Policy for Evidence. the latest Appraisal Policy for Evidence, Endorsements, and Reference
Values.
This section provides a number of details. It does not however This section provides a number of details. However, it does not
define any protocol formats, the interactions shown are abstract. define any protocol formats and the interactions shown are abstract.
This section is intended for those creating protocols and solutions This section is intended for those creating protocols and solutions
to understand the options available to ensure freshness. The way in to understand the options available to ensure freshness. The way in
which freshness is provisioned in a protocol is an architectural which freshness is provisioned in a protocol is an architectural
decision. Provisioning of freshness has an impact on the number of decision. Provisioning of freshness has an impact on the number of
needed round trips in a protocol, and therefore must be made very needed round trips in a protocol; therefore, it must be made very
early in the design. Different decisions will have significant early in the design. Different decisions will have significant
impacts on resulting interoperability, which is why this section goes impacts on resulting interoperability, which is why this section goes
into sufficient detail such that choices in freshness will be into sufficient detail such that choices in freshness will be
compatible across interacting protocols, such as depicted in compatible across interacting protocols, such as depicted in
Figure 9. Figure 9.
Freshness is assessed based on the Appraisal Policy for Evidence or Freshness is assessed based on the Appraisal Policy for Evidence or
Attestation Results that compares the estimated epoch against an Attestation Results that compares the estimated epoch against an
"expiry" threshold defined locally to that policy. There is, "expiry" threshold defined locally to that policy. There is,
however, always a race condition possible in that the state of the however, always a race condition possible in that the state of the
Attester, and the appraisal policies might change immediately after Attester and the appraisal policies might change immediately after
the Evidence or Attestation Result was generated. The goal is merely the Evidence or Attestation Result was generated. The goal is merely
to narrow their recentness to something the Verifier (for Evidence) to narrow their recentness to something the Verifier (for Evidence)
or Relying Party (for Attestation Result) is willing to accept. Some or Relying Party (for Attestation Result) is willing to accept. Some
flexibility on the freshness requirement is a key component for flexibility on the freshness requirement is a key component for
enabling caching and reuse of both Evidence and Attestation Results, enabling caching and reuse of both Evidence and Attestation Results,
which is especially valuable in cases where their computation uses a which is especially valuable in cases where their computation uses a
substantial part of the resource budget (e.g., energy in constrained substantial part of the resource budget (e.g., energy in constrained
devices). devices).
There are three common approaches for determining the epoch of There are three common approaches for determining the epoch of
Evidence or an Attestation Result. Evidence or an Attestation Result.
10.1. Explicit Timekeeping using Synchronized Clocks 10.1. Explicit Timekeeping Using Synchronized Clocks
The first approach is to rely on synchronized and trustworthy clocks, The first approach is to rely on synchronized and trustworthy clocks
and include a signed timestamp (see [I-D.birkholz-rats-tuda]) along and include a signed timestamp (see [RATS-TUDA]) along with the
with the Claims in the Evidence or Attestation Result. Timestamps Claims in the Evidence or Attestation Result. Timestamps can also be
can also be added on a per-Claim basis to distinguish the time of added on a per-Claim basis to distinguish the time of generation of
generation of Evidence or Attestation Result from the time that a Evidence or Attestation Result from the time that a specific Claim
specific Claim was generated. The clock's trustworthiness can was generated. The clock's trustworthiness can generally be
generally be established via Endorsements and typically requires established via Endorsements and typically requires additional Claims
additional Claims about the signer's time synchronization mechanism. about the signer's time synchronization mechanism.
In some use cases, however, a trustworthy clock might not be However, a trustworthy clock might not be available in some use
available. For example, in many Trusted Execution Environments cases. For example, in many TEEs today, a clock is only available
(TEEs) today, a clock is only available outside the TEE and so cannot outside the TEE; thus, it cannot be trusted by the TEE.
be trusted by the TEE.
10.2. Implicit Timekeeping using Nonces 10.2. Implicit Timekeeping Using Nonces
A second approach places the onus of timekeeping solely on the A second approach places the onus of timekeeping solely on the
Verifier (for Evidence) or the Relying Party (for Attestation Verifier (for Evidence) or the Relying Party (for Attestation
Results), and might be suitable, for example, in case the Attester Results). For example, this approach might be suitable in case the
does not have a trustworthy clock or time synchronization is Attester does not have a trustworthy clock or time synchronization is
otherwise impaired. In this approach, a non-predictable nonce is otherwise impaired. In this approach, an unpredictable nonce is sent
sent by the appraising entity, and the nonce is then signed and by the appraising entity and the nonce is then signed and included
included along with the Claims in the Evidence or Attestation Result. along with the Claims in the Evidence or Attestation Result. After
After checking that the sent and received nonces are the same, the checking that the sent and received nonces are the same, the
appraising entity knows that the Claims were signed after the nonce appraising entity knows that the Claims were signed after the nonce
was generated. This allows associating a "rough" epoch to the was generated. This allows associating a "rough" epoch to the
Evidence or Attestation Result. In this case the epoch is said to be Evidence or Attestation Result. In this case, the epoch is said to
rough because: be rough because:
* The epoch applies to the entire Claim set instead of a more * The epoch applies to the entire Claim set instead of a more
granular association, and granular association, and
* The time between the creation of Claims and the collection of * The time between the creation of Claims and the collection of
Claims is indistinguishable. Claims is indistinguishable.
10.3. Implicit Timekeeping using Epoch IDs 10.3. Implicit Timekeeping Using Epoch IDs
A third approach relies on having epoch identifiers (or "IDs") A third approach relies on having epoch identifiers (IDs)
periodically sent to both the sender and receiver of Evidence or periodically sent to both the sender and receiver of Evidence or
Attestation Results by some "Epoch ID Distributor". Attestation Results by some "epoch ID distributor".
Epoch IDs are different from nonces as they can be used more than Epoch IDs are different from nonces as they can be used more than
once and can even be used by more than one entity at the same time. once and can even be used by more than one entity at the same time.
Epoch IDs are different from timestamps as they do not have to convey Epoch IDs are different from timestamps as they do not have to convey
information about a point in time, i.e., they are not necessarily information about a point in time, i.e., they are not necessarily
monotonically increasing integers. monotonically increasing integers.
Like the nonce approach, this allows associating a "rough" epoch Like the nonce approach, this allows associating a "rough" epoch
without requiring a trustworthy clock or time synchronization in without requiring a trustworthy clock or time synchronization in
order to generate or appraise the freshness of Evidence or order to generate or appraise the freshness of Evidence or
Attestation Results. Only the Epoch ID Distributor requires access Attestation Results. Only the epoch ID distributor requires access
to a clock so it can periodically send new epoch IDs. to a clock so it can periodically send new epoch IDs.
The most recent epoch ID is included in the produced Evidence or The most recent epoch ID is included in the produced Evidence or
Attestation Results, and the appraising entity can compare the epoch Attestation Results, and the appraising entity can compare the epoch
ID in received Evidence or Attestation Results against the latest ID in received Evidence or Attestation Results against the latest
epoch ID it received from the Epoch ID Distributor to determine if it epoch ID it received from the epoch ID distributor to determine if it
is within the current epoch. An actual solution also needs to take is within the current epoch. An actual solution also needs to take
into account race conditions when transitioning to a new epoch, such into account race conditions when transitioning to a new epoch, such
as by using a counter signed by the Epoch ID Distributor as the epoch as by using a counter signed by the epoch ID distributor as the epoch
ID, or by including both the current and previous epoch IDs in ID, by including both the current and previous epoch IDs in messages
messages and/or checks, by requiring retries in case of mismatching and/or checks by requiring retries in case of mismatching epoch IDs,
epoch IDs, or by buffering incoming messages that might be associated or by buffering incoming messages that might be associated with an
with an epoch ID that the receiver has not yet obtained. epoch ID that the receiver has not yet obtained.
More generally, in order to prevent an appraising entity from More generally, in order to prevent an appraising entity from
generating false negatives (e.g., discarding Evidence that is deemed generating false negatives (e.g., discarding Evidence that is deemed
stale even if it is not), the appraising entity should keep an "epoch stale even if it is not), the appraising entity should keep an "epoch
window" consisting of the most recently received epoch IDs. The window" consisting of the most recently received epoch IDs. The
depth of such epoch window is directly proportional to the maximum depth of such epoch window is directly proportional to the maximum
network propagation delay between the first to receive the epoch ID network propagation delay between the first to receive the epoch ID
and the last to receive the epoch ID, and it is inversely and the last to receive the epoch ID and it is inversely proportional
proportional to the epoch duration. The appraising entity shall to the epoch duration. The appraising entity shall compare the epoch
compare the epoch ID carried in the received Evidence or Attestation ID carried in the received Evidence or Attestation Result with the
Result with the epoch IDs in its epoch window to find a suitable epoch IDs in its epoch window to find a suitable match.
match.
Whereas the nonce approach typically requires the appraising entity Whereas the nonce approach typically requires the appraising entity
to keep state for each nonce generated, the epoch ID approach to keep state for each nonce generated, the epoch ID approach
minimizes the state kept to be independent of the number of Attesters minimizes the state kept to be independent of the number of Attesters
or Verifiers from which it expects to receive Evidence or Attestation or Verifiers from which it expects to receive Evidence or Attestation
Results, as long as all use the same Epoch ID Distributor. Results as long as all use the same epoch ID distributor.
10.4. Discussion 10.4. Discussion
Implicit and explicit timekeeping can be combined into hybrid Implicit and explicit timekeeping can be combined into hybrid
mechanisms. For example, if clocks exist within the Attesting mechanisms. For example, if clocks exist within the Attesting
Environment and are considered trustworthy (tamper-proof) but are not Environment and are considered trustworthy (tamper-proof) but are not
synchronized, a nonce-based exchange may be used to determine the synchronized, a nonce-based exchange may be used to determine the
(relative) time offset between the involved peers, followed by any (relative) time offset between the involved peers followed by any
number of timestamp based exchanges. number of timestamp based exchanges.
It is important to note that the actual values in Claims might have It is important to note that the actual values in Claims might have
been generated long before the Claims are signed. If so, it is the been generated long before the Claims are signed. If so, it is the
signer's responsibility to ensure that the values are still correct signer's responsibility to ensure that the values are still fresh
when they are signed. For example, values generated at boot time when they are signed. For example, values generated at boot time
might have been saved to secure storage until network connectivity is might have been saved to secure storage until network connectivity is
established to the remote Verifier and a nonce is obtained. established to the remote Verifier and a nonce is obtained.
A more detailed discussion with examples appears in Appendix A. A more detailed discussion with examples appears in Appendix A.
For a discussion on the security of epoch IDs see Section 12.3. For a discussion on the security of epoch IDs see Section 12.3.
11. Privacy Considerations 11. Privacy Considerations
skipping to change at page 37, line 27 skipping to change at line 1600
This information might be particularly interesting to many attackers. This information might be particularly interesting to many attackers.
For example, knowing that a device is running a weak version of For example, knowing that a device is running a weak version of
firmware provides a way to aim attacks better. firmware provides a way to aim attacks better.
In some circumstances, if an attacker can become aware of In some circumstances, if an attacker can become aware of
Endorsements, Reference Values, or appraisal policies, it could Endorsements, Reference Values, or appraisal policies, it could
potentially provide an attacker with insight into defensive potentially provide an attacker with insight into defensive
mitigations. It is recommended that attention be paid to mitigations. It is recommended that attention be paid to
confidentiality of such information. confidentiality of such information.
Additionally, many Claims in Evidence, many Claims in Attestation Additionally, many Evidence, Attestation Results, and appraisal
Results, and appraisal policies potentially contain Personally policies potentially contain Personally Identifying Information (PII)
Identifying Information (PII) depending on the end-to-end use case of depending on the end-to-end use case of the remote attestation
the remote attestation procedure. Remote attestation that includes procedure. Remote attestation that includes containers and
containers and applications, e.g., a blood pressure monitor, may applications, e.g., a blood pressure monitor, may further reveal
further reveal details about specific systems or users. details about specific systems or users.
In some cases, an attacker may be able to make inferences about the In some cases, an attacker may be able to make inferences about the
contents of Evidence from the resulting effects or timing of the contents of Evidence from the resulting effects or timing of the
processing. For example, an attacker might be able to infer the processing. For example, an attacker might be able to infer the
value of specific Claims if it knew that only certain values were value of specific Claims if it knew that only certain values were
accepted by the Relying Party. accepted by the Relying Party.
Conceptual messages (see Section 8) carrying sensitive or Conceptual messages (see Section 8) carrying sensitive or
confidential information are expected to be integrity protected confidential information are expected to be integrity protected
(i.e., either via signing or a secure channel) and optionally might (i.e., either via signing or a secure channel) and optionally might
be confidentiality protected via encryption. If there isn't be confidentiality protected via encryption. If there isn't
confidentiality protection of conceptual messages themselves, the confidentiality protection of conceptual messages themselves, the
underlying conveyance protocol should provide these protections. underlying conveyance protocol should provide these protections.
As Evidence might contain sensitive or confidential information, As Evidence might contain sensitive or confidential information,
Attesters are responsible for only sending such Evidence to trusted Attesters are responsible for only sending such Evidence to trusted
Verifiers. Some Attesters might want a stronger level of assurance Verifiers. Some Attesters might want a stronger level of assurance
of the trustworthiness of a Verifier before sending Evidence to it. of the trustworthiness of a Verifier before sending Evidence to it.
In such cases, an Attester can first act as a Relying Party and ask In such cases, an Attester can first act as a Relying Party and ask
for the Verifier's own Attestation Result, and appraising it just as for the Verifier's own Attestation Result. Appraising it just as a
a Relying Party would appraise an Attestation Result for any other Relying Party would appraise an Attestation Result for any other
purpose. purpose.
Another approach to deal with Evidence is to remove PII from the Another approach to deal with Evidence is to remove PII from the
Evidence while still being able to verify that the Attester is one of Evidence while still being able to verify that the Attester is one of
a large set. This approach is often called "Direct Anonymous a large set. This approach is often called "Direct Anonymous
Attestation". See [CCC-DeepDive] section 6.2 and [I-D.ietf-rats-daa] Attestation". See Section 6.2 of [CCC-DeepDive] and [RATS-DAA] for
for more discussion. more discussion.
12. Security Considerations 12. Security Considerations
This document provides an architecture for doing remote attestation. This document provides an architecture for doing remote attestation.
No specific wire protocol is documented here. Without a specific No specific wire protocol is documented here. Without a specific
proposal to compare against, it is impossible to know if the security proposal to compare against, it is impossible to know if the security
threats listed below have been mitigated well. threats listed below have been mitigated well.
The security considerations below should be read as being essentially The security considerations below should be read as being,
requirements against realizations of the RATS Architecture. Some essentially, requirements against realizations of the RATS
threats apply to protocols, some are against implementations (code), architecture. Some threats apply to protocols and some are against
and some threats are against physical infrastructure (such as implementations (code) and physical infrastructure (such as
factories). factories).
The fundamental purpose of the RATS architecture is to allow a The fundamental purpose of the RATS architecture is to allow a
Relying Party to establish a basis for trusting the Attester. Relying Party to establish a basis for trusting the Attester.
12.1. Attester and Attestation Key Protection 12.1. Attester and Attestation Key Protection
Implementers need to pay close attention to the protection of the Implementers need to pay close attention to the protection of the
Attester and the manufacturing processes for provisioning attestation Attester and the manufacturing processes for provisioning attestation
key material. If either of these are compromised, intended levels of key material. If either of these are compromised, intended levels of
assurance for RATS are compromised because attackers can forge assurance for remote attestation procedures are compromised because
Evidence or manipulate the Attesting Environment. For example, a attackers can forge Evidence or manipulate the Attesting Environment.
Target Environment should not be able to tamper with the Attesting For example, a Target Environment should not be able to tamper with
Environment that measures it, by isolating the two environments from the Attesting Environment that measures it by isolating the two
each other in some way. environments from each other in some way.
Remote attestation applies to use cases with a range of security Remote attestation applies to use cases with a range of security
requirements, so the protections discussed here range from low to requirements. The protections discussed here range from low to high
high security where low security may be limited to application or security: low security may be limited to application or process
process isolation by the device's operating system, and high security isolation by the device's operating system and high security may
may involve specialized hardware to defend against physical attacks involve specialized hardware to defend against physical attacks on a
on a chip. chip.
12.1.1. On-Device Attester and Key Protection 12.1.1. On-Device Attester and Key Protection
It is assumed that an Attesting Environment is sufficiently isolated It is assumed that an Attesting Environment is sufficiently isolated
from the Target Environment it collects Claims about and that it from the Target Environment it collects Claims about and that it
signs the resulting Claims set with an attestation key, so that the signs the resulting Claims set with an attestation key so that the
Target Environment cannot forge Evidence about itself. Such an Target Environment cannot forge Evidence about itself. Such an
isolated environment might be provided by a process, a dedicated isolated environment might be provided by a process, a dedicated
chip, a TEE, a virtual machine, or another secure mode of operation. chip, a TEE, a virtual machine, or another secure mode of operation.
The Attesting Environment must be protected from unauthorized The Attesting Environment must be protected from unauthorized
modification to ensure it behaves correctly. Confidentiality modification to ensure it behaves correctly. Confidentiality
protection of the Attesting Environment's signing key is vital so it protection of the Attesting Environment's signing key is vital so it
cannot be misused to forge Evidence. cannot be misused to forge Evidence.
In many cases the user or owner of a device that includes the role of In many cases, the user or owner of a device that includes the role
Attester must not be able to modify or extract keys from the of Attester must not be able to modify or extract keys from the
Attesting Environments, to prevent creating forged Evidence. Some Attesting Environments to prevent creating forged Evidence. Some
common examples include the user of a mobile phone or FIDO common examples include the user of a mobile phone or FIDO
authenticator. authenticator.
Measures for a minimally protected system might include process or Measures for a minimally protected system might include process or
application isolation provided by a high-level operating system, and application isolation provided by a high-level operating system and
restricted access to root or system privileges. In contrast, For restricted access to root or system privileges. In contrast, for
really simple single-use devices that don't use a protected mode really simple single-use devices that don't use a protected mode
operating system, like a Bluetooth speaker, the only factual operating system (like a Bluetooth speaker), the only factual
isolation might be the sturdy housing of the device. isolation might be the sturdy housing of the device.
Measures for a moderately protected system could include a special Measures for a moderately protected system could include a special
restricted operating environment, such as a TEE. In this case, only restricted operating environment, such as a TEE. In this case, only
security-oriented software has access to the Attester and key security-oriented software has access to the Attester and key
material. material.
Measures for a highly protected system could include specialized Measures for a highly protected system could include specialized
hardware that is used to provide protection against chip decapping hardware that is used to provide protection against chip decapping
attacks, power supply and clock glitching, faulting injection and RF attacks, power supply and clock glitching, faulting injection and RF,
and power side channel attacks. and power side channel attacks.
12.1.2. Attestation Key Provisioning Processes 12.1.2. Attestation Key Provisioning Processes
Attestation key provisioning is the process that occurs in the Attestation key provisioning is the process that occurs in the
factory or elsewhere to establish signing key material on the device factory or elsewhere to establish signing key material on the device
and the validation key material off the device. Sometimes this and the validation key material off the device. Sometimes, this
procedure is referred to as personalization or customization. procedure is referred to as "personalization" or "customization".
When generating keys off-device in the factory or in the device, the When generating keys off-device in the factory or in the device, the
use of a Cryptographically Strong Sequence ([RFC4086], Section 6.2) use of a cryptographically strong sequence ([RFC4086], Section 6.2)
needs consideration. needs consideration.
12.1.2.1. Off-Device Key Generation 12.1.2.1. Off-Device Key Generation
One way to provision key material is to first generate it external to One way to provision key material is to first generate it external to
the device and then copy the key onto the device. In this case, the device and then copy the key onto the device. In this case,
confidentiality protection of the generator, as well as for the path confidentiality protection of the generator and the path over which
over which the key is provisioned, is necessary. The manufacturer the key is provisioned is necessary. The manufacturer needs to take
needs to take care to protect corresponding key material with care to protect corresponding key material with measures appropriate
measures appropriate for its value. for its value.
The degree of protection afforded to this key material can vary by The degree of protection afforded to this key material can vary by
the intended function of the device and the specific practices of the the intended function of the device and the specific practices of the
device manufacturer or integrator. The confidentiality protection is device manufacturer or integrator. The confidentiality protection is
fundamentally based upon some amount of physical protection: while fundamentally based upon some amount of physical protection. While
encryption is often used to provide confidentiality when a key is encryption is often used to provide confidentiality when a key is
conveyed across a factory, where the attestation key is created or conveyed across a factory where the attestation key is created or
applied, it must be available in an unencrypted form. The physical applied, it must be available in an unencrypted form. The physical
protection can therefore vary from situations where the key is protection can therefore vary from situations where the key is
unencrypted only within carefully controlled secure enclaves within unencrypted only within carefully controlled secure enclaves within
silicon, to situations where an entire facility is considered secure, silicon to situations where an entire facility is considered secure
by the simple means of locked doors and limited access. by the simple means of locked doors and limited access.
The cryptography that is used to enable confidentiality protection of The cryptography that is used to enable confidentiality protection of
the attestation key comes with its own requirements to be secured. the attestation key comes with its own requirements to be secured.
This results in recursive problems, as the key material used to This results in recursive problems, as the key material used to
provision attestation keys must again somehow have been provisioned provision attestation keys must again somehow have been provisioned
securely beforehand (requiring an additional level of protection, and securely beforehand (requiring an additional level of protection and
so on). so on).
Commonly, a combination of some physical security measures and some Commonly, a combination of some physical security measures and some
cryptographic measures are used to establish confidentiality cryptographic measures are used to establish confidentiality
protection. protection.
12.1.2.2. On-Device Key Generation 12.1.2.2. On-Device Key Generation
When key material is generated within a device and the secret part of When key material is generated within a device and the secret part of
it never leaves the device, then the problem may lessen. For public- it never leaves the device, the problem may lessen. For public-key
key cryptography, it is, by definition, not necessary to maintain cryptography, it is not necessary to maintain confidentiality of the
confidentiality of the public key: however integrity of the chain of public key. However, integrity of the chain of custody of the public
custody of the public key is necessary in order to avoid attacks key is necessary in order to avoid attacks where an attacker is able
where an attacker is able to get a key they control endorsed. to get a key endorsed that the attacker controls.
To summarize: attestation key provisioning must ensure that only To summarize, attestation key provisioning must ensure that only
valid attestation key material is established in Attesters. valid attestation key material is established in Attesters.
12.2. Conceptual Message Protection 12.2. Conceptual Message Protection
Any solution that conveys information in any conceptual message (see Any solution that conveys information in any conceptual message (see
Section 8) must support end-to-end integrity protection and replay Section 8) must support end-to-end integrity protection and replay
attack prevention, and often also needs to support additional attack prevention. It often also needs to support additional
security properties, including: security properties, including:
* end-to-end encryption, * end-to-end encryption,
* denial of service protection, * denial-of-service protection,
* authentication, * authentication,
* auditing, * auditing,
* fine grained access controls, and * fine-grained access controls, and
* logging. * logging.
Section 10 discusses ways in which freshness can be used in this Section 10 discusses ways in which freshness can be used in this
architecture to protect against replay attacks. architecture to protect against replay attacks.
To assess the security provided by a particular appraisal policy, it To assess the security provided by a particular appraisal policy, it
is important to understand the strength of the root of trust, e.g., is important to understand the strength of the root of trust, e.g.,
whether it is mutable software, or firmware that is read-only after whether it is mutable software or firmware that is read-only after
boot, or immutable hardware/ROM. boot or immutable hardware/ROM.
It is also important that the appraisal policy was itself obtained It is also important that the appraisal policy was obtained securely
securely. If an attacker can configure or modify appraisal policies, itself. If an attacker can configure or modify appraisal policies
Endorsements or Reference Values for a Relying Party or for a and Endorsements or Reference Values for a Relying Party or a
Verifier, then integrity of the process is compromised. Verifier, then integrity of the process is compromised.
Security protections in RATS may be applied at different layers, Security protections in the RATS architecture may be applied at
whether by a conveyance protocol, or an information encoding format. different layers, whether by a conveyance protocol or an information
This architecture expects conceptual messages to be end-to-end encoding format. This architecture expects conceptual messages to be
protected based on the role interaction context. For example, if an end-to-end protected based on the role interaction context. For
Attester produces Evidence that is relayed through some other entity example, if an Attester produces Evidence that is relayed through
that doesn't implement the Attester or the intended Verifier roles, some other entity that doesn't implement the Attester or the intended
then the relaying entity should not expect to have access to the Verifier roles, then the relaying entity should not expect to have
Evidence. access to the Evidence.
The RATS architecture allows for an entity to function in multiple The RATS architecture allows for an entity to function in multiple
roles (Section 6) and for composite devices (Section 3.3). roles (Section 6) and for composite devices (Section 3.3).
Implementers need to evaluate their designs to ensure that the Implementers need to evaluate their designs to ensure that the
assumed security properties of the individual components and roles assumed security properties of the individual components and roles
still hold despite the lack of separation, and that emergent risk is still hold despite the lack of separation and that emergent risk is
not introduced. The specifics of this evaluation will depend on the not introduced. The specifics of this evaluation will depend on the
implementation and the use case and hence is out of scope for this implementation and the use case; hence, they are out of scope for
document. Isolation mechanisms in software or hardware that separate this document. Isolation mechanisms in software or hardware that
Attesting Environments and Target Environments Section 3.1 can separate Attesting Environments and Target Environments (Section 3.1)
support an implementer's evaluation and resulting design decisions. can support an implementer's evaluation and resulting design
decisions.
12.3. Epoch ID-based Attestation 12.3. Attestation Based on Epoch ID
Epoch IDs, described in Section 10.3, can be tampered with, replayed, Epoch IDs, described in Section 10.3, can be tampered with, replayed,
dropped, delayed, and reordered by an attacker. dropped, delayed, and reordered by an attacker.
An attacker could be either external or belong to the distribution An attacker could either be external or belong to the distribution
group, for example, if one of the Attester entities have been group (for example, if one of the Attester entities have been
compromised. compromised).
An attacker who is able to tamper with epoch IDs can potentially lock An attacker who is able to tamper with epoch IDs can potentially lock
all the participants in a certain epoch of choice forever, all the participants in a certain epoch of choice forever,
effectively freezing time. This is problematic since it destroys the effectively freezing time. This is problematic since it destroys the
ability to ascertain freshness of Evidence and Attestation Results. ability to ascertain freshness of Evidence and Attestation Results.
To mitigate this threat, the transport should be at least integrity To mitigate this threat, the transport should be at least integrity
protected and provide origin authentication. protected and provide origin authentication.
Selective dropping of epoch IDs is equivalent to pinning the victim Selective dropping of epoch IDs is equivalent to pinning the victim
node to a past epoch. An attacker could drop epoch IDs to only some node to a past epoch. An attacker could drop epoch IDs to only some
entities and not others, which will typically result in a denial of entities and not others, which will typically result in a denial of
service due to the permanent staleness of the Attestation Result or service due to the permanent staleness of the Attestation Result or
Evidence. Evidence.
Delaying or reordering epoch IDs is equivalent to manipulating the Delaying or reordering epoch IDs is equivalent to manipulating the
victim's timeline at will. This ability could be used by a malicious victim's timeline at will. This ability could be used by a malicious
actor (e.g., a compromised router) to mount a confusion attack where, actor (e.g., a compromised router) to mount a confusion attack. For
for example, a Verifier is tricked into accepting Evidence coming example, a Verifier can be tricked into accepting Evidence coming
from a past epoch as fresh, while in the meantime the Attester has from a past epoch as fresh, while, in the meantime, the Attester has
been compromised. been compromised.
Reordering and dropping attacks are mitigated if the transport Reordering and dropping attacks are mitigated if the transport
provides the ability to detect reordering and drop. However, the provides the ability to detect reordering and drop. However, the
delay attack described above can't be thwarted in this manner. delay attack described above can't be thwarted in this manner.
12.4. Trust Anchor Protection 12.4. Trust Anchor Protection
As noted in Section 7, Verifiers and Relying Parties have trust As noted in Section 7, Verifiers and Relying Parties have trust
anchor stores that must be secured. [RFC6024] contains more anchor stores that must be secured. [RFC6024] contains more
skipping to change at page 43, line 9 skipping to change at line 1862
keys. Section 6 of [NIST-800-57-p1] contains a comprehensive keys. Section 6 of [NIST-800-57-p1] contains a comprehensive
treatment of the topic, including the protection of symmetric key treatment of the topic, including the protection of symmetric key
material. Specifically, a trust anchor store must resist material. Specifically, a trust anchor store must resist
modification against unauthorized insertion, deletion, and modification against unauthorized insertion, deletion, and
modification. Additionally, if the trust anchor is a symmetric key, modification. Additionally, if the trust anchor is a symmetric key,
the trust anchor store must not allow unauthorized read. the trust anchor store must not allow unauthorized read.
If certificates are used as trust anchors, Verifiers and Relying If certificates are used as trust anchors, Verifiers and Relying
Parties are also responsible for validating the entire certificate Parties are also responsible for validating the entire certificate
path up to the trust anchor, which includes checking for certificate path up to the trust anchor, which includes checking for certificate
revocation. For an example of such a proceedure see Section 6 of revocation. For an example of such a procedure, see Section 6 of
[RFC5280]. [RFC5280].
13. IANA Considerations 13. IANA Considerations
This document does not require any actions by IANA. This document has no IANA actions.
14. Acknowledgments
Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica
Fitzgerald-McKay, Diego Lopez, Laurence Lundblade, Paul Rowe, Hannes
Tschofenig, Frank Xia, and David Wooten.
15. Notable Contributions
Thomas Hardjono created initial versions of the terminology section
in collaboration with Ned Smith. Eric Voit provided the conceptual
separation between Attestation Provision Flows and Attestation
Evidence Flows. Monty Wisemen created the content structure of the
first three architecture drafts. Carsten Bormann provided many of
the motivational building blocks with respect to the Internet Threat
Model.
Peter Loscocco contributed critical review feedback as part of the
weekly design team meetings that added precision and depth to several
sections.
16. References 14. References
16.1. Normative References 14.1. Normative References
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., Polk, W., and RFC Publisher, "Internet X.509
Infrastructure Certificate and Certificate Revocation List Public Key Infrastructure Certificate and Certificate
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, Revocation List (CRL) Profile", RFC 5280,
<https://www.rfc-editor.org/rfc/rfc5280>. DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token [RFC7519] Jones, M., Bradley, J., Sakimura, N., and RFC Publisher,
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519,
<https://www.rfc-editor.org/rfc/rfc7519>. May 2015, <https://www.rfc-editor.org/info/rfc7519>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., Tschofenig, H.,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, and RFC Publisher, "CBOR Web Token (CWT)", RFC 8392,
May 2018, <https://www.rfc-editor.org/rfc/rfc8392>. DOI 10.17487/RFC8392, May 2018,
<https://www.rfc-editor.org/info/rfc8392>.
16.2. Informative References 14.2. Informative References
[CCC-DeepDive] [CCC-DeepDive]
Confidential Computing Consortium, "Confidential Computing Confidential Computing Consortium, "A Technical Analysis
Deep Dive", n.d., of Confidential Computing", Version 1.3, November 2022,
<https://confidentialcomputing.io/whitepaper-02-latest>. <https://confidentialcomputing.io/white-papers-reports>.
[CTAP] FIDO Alliance, "Client to Authenticator Protocol", n.d., [CTAP] FIDO Alliance, "Client to Authenticator Protocol (CTAP)",
<https://fidoalliance.org/specs/fido-v2.0-id-20180227/ February 2018, <https://fidoalliance.org/specs/fido-v2.0-
fido-client-to-authenticator-protocol-v2.0-id- id-20180227/fido-client-to-authenticator-protocol-v2.0-id-
20180227.html>. 20180227.html>.
[I-D.birkholz-rats-tuda] [NIST-800-57-p1]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann, Barker, E., "Recommendation for Key Management: Part 1 -
"Time-Based Uni-Directional Attestation", Work in General", DOI 10.6028/NIST.SP.800-57pt1r5, May 2020,
Progress, Internet-Draft, draft-birkholz-rats-tuda-07, 10 <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
July 2022, <https://datatracker.ietf.org/doc/html/draft- NIST.SP.800-57pt1r5.pdf>.
birkholz-rats-tuda-07>.
[I-D.birkholz-rats-uccs] [OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C. Part 2: Security Model, Release 1.03", OPC 10000-2 ,
Bormann, "A CBOR Tag for Unprotected CWT Claims Sets", November 2015, <https://opcfoundation.org/developer-tools/
Work in Progress, Internet-Draft, draft-birkholz-rats- specifications-unified-architecture/part-2-security-
uccs-03, 8 March 2021, model/>.
<https://datatracker.ietf.org/doc/html/draft-birkholz-
rats-uccs-03>.
[I-D.ietf-rats-daa] [RATS-DAA] Birkholz, H., Newton, C., Chen, L., and D. Thaler, "Direct
Birkholz, H., Newton, C., Chen, L., and D. Thaler, "Direct
Anonymous Attestation for the Remote Attestation Anonymous Attestation for the Remote Attestation
Procedures Architecture", Work in Progress, Internet- Procedures Architecture", Work in Progress, Internet-
Draft, draft-ietf-rats-daa-02, 7 September 2022, Draft, draft-ietf-rats-daa-02, 7 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats- <https://datatracker.ietf.org/doc/html/draft-ietf-rats-
daa-02>. daa-02>.
[I-D.ietf-teep-architecture] [RATS-PSA-TOKEN]
Pei, M., Tschofenig, H., Thaler, D., and D. M. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-18, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-teep-
architecture-18>.
[I-D.tschofenig-rats-psa-token]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A. L., and Tschofenig, H., Frost, S., Brossard, M., Shaw, A. L., and
T. Fossati, "Arm's Platform Security Architecture (PSA) T. Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft, Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-10, 6 September 2022, draft-tschofenig-rats-psa-token-10, 6 September 2022,
<https://datatracker.ietf.org/doc/html/draft-tschofenig- <https://datatracker.ietf.org/doc/html/draft-tschofenig-
rats-psa-token-10>. rats-psa-token-10>.
[I-D.tschofenig-tls-cwt] [RATS-TUDA]
Tschofenig, H. and M. Brossard, "Using CBOR Web Tokens Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
(CWTs) in Transport Layer Security (TLS) and Datagram "Time-Based Uni-Directional Attestation", Work in
Transport Layer Security (DTLS)", Work in Progress, Progress, Internet-Draft, draft-birkholz-rats-tuda-07, 10
Internet-Draft, draft-tschofenig-tls-cwt-02, 13 July 2020, July 2022, <https://datatracker.ietf.org/doc/html/draft-
<https://datatracker.ietf.org/doc/html/draft-tschofenig- birkholz-rats-tuda-07>.
tls-cwt-02>.
[NIST-800-57-p1]
Barker, E., "Recommendation for Key Managemement: Part 1 -
General", May 2020,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-57pt1r5.pdf>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification, [RATS-UCCS]
Part 2: Security Model, Release 1.03", OPC 10000-2 , 25 Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C.
November 2015, <https://opcfoundation.org/developer-tools/ Bormann, "A CBOR Tag for Unprotected CWT Claims Sets",
specifications-unified-architecture/part-2-security- Work in Progress, Internet-Draft, draft-ietf-rats-uccs-03,
model/>. 11 July 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-uccs-03>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, [RFC4086] Eastlake 3rd, D., Schiller, J., Crocker, S., and RFC
"Randomness Requirements for Security", BCP 106, RFC 4086, Publisher, "Randomness Requirements for Security",
DOI 10.17487/RFC4086, June 2005, BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/rfc/rfc4086>. <https://www.rfc-editor.org/info/rfc4086>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", [RFC4949] Shirey, R. and RFC Publisher, "Internet Security Glossary,
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August
<https://www.rfc-editor.org/rfc/rfc4949>. 2007, <https://www.rfc-editor.org/info/rfc4949>.
[RFC5209] Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J. [RFC5209] Sangster, P., Khosravi, H., Mani, M., Narayan, K., Tardo,
Tardo, "Network Endpoint Assessment (NEA): Overview and J., and RFC Publisher, "Network Endpoint Assessment (NEA):
Requirements", RFC 5209, DOI 10.17487/RFC5209, June 2008, Overview and Requirements", RFC 5209,
<https://www.rfc-editor.org/rfc/rfc5209>. DOI 10.17487/RFC5209, June 2008,
<https://www.rfc-editor.org/info/rfc5209>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management [RFC6024] Reddy, R., Wallace, C., and RFC Publisher, "Trust Anchor
Requirements", RFC 6024, DOI 10.17487/RFC6024, October Management Requirements", RFC 6024, DOI 10.17487/RFC6024,
2010, <https://www.rfc-editor.org/rfc/rfc6024>. October 2010, <https://www.rfc-editor.org/info/rfc6024>.
[RFC8322] Field, J., Banghart, S., and D. Waltermire, "Resource- [RFC8322] Field, J., Banghart, S., Waltermire, D., and RFC
Oriented Lightweight Information Exchange (ROLIE)", Publisher, "Resource-Oriented Lightweight Information
RFC 8322, DOI 10.17487/RFC8322, February 2018, Exchange (ROLIE)", RFC 8322, DOI 10.17487/RFC8322,
<https://www.rfc-editor.org/rfc/rfc8322>. February 2018, <https://www.rfc-editor.org/info/rfc8322>.
[strengthoffunction] [strengthoffunction]
NISC, "Strength of Function", n.d., NIST, "Strength of Function",
<https://csrc.nist.gov/glossary/term/ <https://csrc.nist.gov/glossary/term/
strength_of_function>. strength_of_function>.
[TCG-DICE] Trusted Computing Group, "DICE Certificate Profiles", [TCG-DICE] Trusted Computing Group, "DICE Attestation Architecture",
n.d., <https://trustedcomputinggroup.org/wp- Version 1.00, Revision 0.23, March 2021,
content/uploads/DICE-Certificate-Profiles- <https://trustedcomputinggroup.org/wp-content/uploads/
r01_3june2020-1.pdf>. DICE-Attestation-Architecture-r23-final.pdf>.
[TCG-DICE-SIBDA] [TCG-DICE-SIBDA]
Trusted Computing Group, "Symmetric Identity Based Device Trusted Computing Group, "Symmetric Identity Based Device
Attestation for DICE", 24 July 2019, Attestation", Version 1.0, Revision 0.95, January 2020,
<https://trustedcomputinggroup.org/wp-content/uploads/ <https://trustedcomputinggroup.org/wp-content/uploads/
TCG_DICE_SymIDAttest_v1_r0p94_pubrev.pdf>. TCG_DICE_SymIDAttest_v1_r0p95_pub-1.pdf>.
[TCGarch] Trusted Computing Group, "Trusted Platform Module Library [TCGarch] Trusted Computing Group, "Trusted Platform Module Library,
- Part 1: Architecture", 8 November 2019, Part 1: Architecture", November 2019,
<https://trustedcomputinggroup.org/wp-content/uploads/ <https://trustedcomputinggroup.org/wp-content/uploads/
TCG_TPM2_r1p59_Part1_Architecture_pub.pdf>. TCG_TPM2_r1p59_Part1_Architecture_pub.pdf>.
[TEEP-ARCH]
Pei, M., Tschofenig, H., Thaler, D., and D. M. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-19, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-teep-
architecture-19>.
[TLS-CWT] Tschofenig, H. and M. Brossard, "Using CBOR Web Tokens
(CWTs) in Transport Layer Security (TLS) and Datagram
Transport Layer Security (DTLS)", Work in Progress,
Internet-Draft, draft-tschofenig-tls-cwt-02, 13 July 2020,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
tls-cwt-02>.
[WebAuthN] W3C, "Web Authentication: An API for accessing Public Key [WebAuthN] W3C, "Web Authentication: An API for accessing Public Key
Credentials", n.d., <https://www.w3.org/TR/webauthn-1/>. Credentials Level 1", March 2019,
<https://www.w3.org/TR/webauthn-1/>.
Appendix A. Time Considerations Appendix A. Time Considerations
Section 10 discussed various issues and requirements around freshness Section 10 discussed various issues and requirements around freshness
of evidence, and summarized three approaches that might be used by of Evidence and summarized three approaches that might be used by
different solutions to address them. This appendix provides more different solutions to address them. This appendix provides more
details with examples to help illustrate potential approaches, to details with examples to help illustrate potential approaches and
inform those creating specific solutions. inform those creating specific solutions.
The table below defines a number of relevant events, with an ID that The table below defines a number of relevant events with an ID that
is used in subsequent diagrams. The times of said events might be is used in subsequent diagrams. The times of said events might be
defined in terms of an absolute clock time, such as the Coordinated defined in terms of an absolute clock time, such as the Coordinated
Universal Time timescale, or might be defined relative to some other Universal Time timescale, or might be defined relative to some other
timestamp or timeticks counter, such as a clock resetting its epoch timestamp or timeticks counter, such as a clock resetting its epoch
each time it is powered on. each time it is powered on.
+====+============+=================================================+ +====+============+=================================================+
| ID | Event | Explanation of event | | ID | Event | Explanation of event |
+====+============+=================================================+ +====+============+=================================================+
| VG | Value | A value to appear in a Claim was created. | | VG | Value | A value to appear in a Claim was created. |
| | generated | In some cases, a value may have technically | | | generated | In some cases, a value may have technically |
| | | existed before an Attester became aware of | | | | existed before an Attester became aware of |
| | | it but the Attester might have no idea how | | | | it, but the Attester might have no idea how |
| | | long it has had that value. In such a | | | | long it has had that value. In such a |
| | | case, the Value created time is the time at | | | | case, the value created time is the time at |
| | | which the Claim containing the copy of the | | | | which the Claim containing the copy of the |
| | | value was created. | | | | value was created. |
+----+------------+-------------------------------------------------+ +----+------------+-------------------------------------------------+
| NS | Nonce sent | A nonce not predictable to an Attester | | NS | Nonce sent | A nonce not predictable to an Attester |
| | | (recentness & uniqueness) is sent to an | | | | (recentness & uniqueness) is sent to an |
| | | Attester. | | | | Attester. |
+----+------------+-------------------------------------------------+ +----+------------+-------------------------------------------------+
| NR | Nonce | A nonce is relayed to an Attester by | | NR | Nonce | A nonce is relayed to an Attester by |
| | relayed | another entity. | | | relayed | another entity. |
+----+------------+-------------------------------------------------+ +----+------------+-------------------------------------------------+
skipping to change at page 48, line 4 skipping to change at line 2068
| OP | Operation | The Relying Party performs some operation | | OP | Operation | The Relying Party performs some operation |
| | performed | requested by the Attester via a resource | | | performed | requested by the Attester via a resource |
| | | access protocol as depicted in Figure 8, | | | | access protocol as depicted in Figure 8, |
| | | e.g., across a session created earlier at | | | | e.g., across a session created earlier at |
| | | time(RA). | | | | time(RA). |
+----+------------+-------------------------------------------------+ +----+------------+-------------------------------------------------+
| RX | Result | An Attestation Result should no longer be | | RX | Result | An Attestation Result should no longer be |
| | expiry | accepted, according to the Verifier that | | | expiry | accepted, according to the Verifier that |
| | | generated it. | | | | generated it. |
+----+------------+-------------------------------------------------+ +----+------------+-------------------------------------------------+
Table 1
Table 1: Relevant Events over Time
Using the table above, a number of hypothetical examples of how a Using the table above, a number of hypothetical examples of how a
solution might be built are illustrated below. This list is not solution might be built are illustrated below. This list is not
intended to be complete, but is just representative enough to intended to be complete; it is just representative enough to
highlight various timing considerations. highlight various timing considerations.
All times are relative to the local clocks, indicated by an "_a" All times are relative to the local clocks, indicated by an "_a"
(Attester), "_v" (Verifier), or "_r" (Relying Party) suffix. (Attester), "_v" (Verifier), or "_r" (Relying Party) suffix.
Times with an appended Prime (') indicate a second instance of the Times with an appended Prime (') indicate a second instance of the
same event. same event.
How and if clocks are synchronized depends upon the model. How and if clocks are synchronized depends upon the model.
In the figures below, curly braces indicate containment. For In the figures below, curly braces indicate containment. For
example, the notation Evidence{foo} indicates that 'foo' is contained example, the notation Evidence{foo} indicates that 'foo' is contained
in the Evidence and is thus covered by its signature. in the Evidence; thus, it is covered by its signature.
A.1. Example 1: Timestamp-based Passport Model Example A.1. Example 1: Timestamp-Based Passport Model
The following example illustrates a hypothetical Passport Model Figure 10 illustrates a hypothetical Passport Model solution that
solution that uses timestamps and requires roughly synchronized uses timestamps and requires roughly synchronized clocks between the
clocks between the Attester, Verifier, and Relying Party, which Attester, Verifier, and Relying Party, which depends on using a
depends on using a secure clock synchronization mechanism. As a secure clock synchronization mechanism. As a result, the receiver of
result, the receiver of a conceptual message containing a timestamp a conceptual message containing a timestamp can directly compare it
can directly compare it to its own clock and timestamps. to its own clock and timestamps.
.----------. .----------. .---------------. .----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party | | Attester | | Verifier | | Relying Party |
'----+-----' '-----+----' '-------+-------' '----+-----' '-----+----' '-------+-------'
| | | | | |
time(VG_a) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
time(EG_a) | | time(EG_a) | |
skipping to change at page 49, line 29 skipping to change at line 2121
|<-----Attestation Result---------+ | |<-----Attestation Result---------+ |
| {time(RG_v),time(RX_v)} | | | {time(RG_v),time(RX_v)} | |
~ ~ ~ ~
| | | |
+--Attestation Result{time(RG_v),time(RX_v)}--> time(RA_r) +--Attestation Result{time(RG_v),time(RX_v)}--> time(RA_r)
| | | |
~ ~ ~ ~
| | | |
| time(OP_r) | time(OP_r)
Figure 10: Timestamp-Based Passport Model
The Verifier can check whether the Evidence is fresh when appraising The Verifier can check whether the Evidence is fresh when appraising
it at time(RG_v) by checking time(RG_v) - time(EG_a) < Threshold, it at time(RG_v) by checking time(RG_v) - time(EG_a) < Threshold,
where the Verifier's threshold is large enough to account for the where the Verifier's threshold is large enough to account for the
maximum permitted clock skew between the Verifier and the Attester. maximum permitted clock skew between the Verifier and the Attester.
If time(VG_a) is also included in the Evidence along with the Claim If time(VG_a) is included in the Evidence along with the Claim value
value generated at that time, and the Verifier decides that it can generated at that time, and the Verifier decides that it can trust
trust the time(VG_a) value, the Verifier can also determine whether the time(VG_a) value, the Verifier can also determine whether the
the Claim value is recent by checking time(RG_v) - time(VG_a) < Claim value is recent by checking time(RG_v) - time(VG_a) <
Threshold. The threshold is decided by the Appraisal Policy for Threshold. The threshold is decided by the Appraisal Policy for
Evidence, and again needs to take into account the maximum permitted Evidence and, again, needs to take into account the maximum permitted
clock skew between the Verifier and the Attester. clock skew between the Verifier and the Attester.
The Attester does not consume the Attestation Result, but might cache The Attester does not consume the Attestation Result but might cache
it. it.
The Relying Party can check whether the Attestation Result is fresh The Relying Party can check whether the Attestation Result is fresh
when appraising it at time(RA_r) by checking time(RA_r) - time(RG_v) when appraising it at time(RA_r) by checking the time(RA_r) -
< Threshold, where the Relying Party's threshold is large enough to time(RG_v) < Threshold, where the Relying Party's threshold is large
account for the maximum permitted clock skew between the Relying enough to account for the maximum permitted clock skew between the
Party and the Verifier. The result might then be used for some time Relying Party and the Verifier. The result might then be used for
(e.g., throughout the lifetime of a connection established at some time (e.g., throughout the lifetime of a connection established
time(RA_r)). The Relying Party must be careful, however, to not at time(RA_r)). However, the Relying Party must be careful not to
allow continued use beyond the period for which it deems the allow continued use beyond the period for which it deems the
Attestation Result to remain fresh enough. Thus, it might allow use Attestation Result to remain fresh enough. Thus, it might allow use
(at time(OP_r)) as long as time(OP_r) - time(RG_v) < Threshold. (at time(OP_r)) as long as time(OP_r) - time(RG_v) < Threshold.
However, if the Attestation Result contains an expiry time time(RX_v) However, if the Attestation Result contains an expiry time
then it could explicitly check time(OP_r) < time(RX_v). time(RX_v), then it could explicitly check time(OP_r) < time(RX_v).
A.2. Example 2: Nonce-based Passport Model Example A.2. Example 2: Nonce-Based Passport Model
The following example illustrates a hypothetical Passport Model Figure 11 illustrates a hypothetical Passport Model solution that
solution that uses nonces instead of timestamps. Compared to the uses nonces instead of timestamps. Compared to the timestamp-based
timestamp-based example, it requires an extra round trip to retrieve example, it requires an extra round trip to retrieve a nonce and
a nonce, and requires that the Verifier and Relying Party track state requires that the Verifier and Relying Party track state to remember
to remember the nonce for some period of time. the nonce for some period of time.
The advantage is that it does not require that any clocks are The advantage is that it does not require that any clocks are
synchronized. As a result, the receiver of a conceptual message synchronized. As a result, the receiver of a conceptual message
containing a timestamp cannot directly compare it to its own clock or containing a timestamp cannot directly compare it to its own clock or
timestamps. Thus, we use a suffix ("a" for Attester, "v" for timestamps. Thus, we use a suffix ("a" for Attester, "v" for
Verifier, and "r" for Relying Party) on the IDs below indicating Verifier, and "r" for Relying Party) on the IDs below indicating
which clock generated them, since times from different clocks cannot which clock generated them since times from different clocks cannot
be compared. Only the delta between two events from the sender can be compared. Only the delta between two events from the sender can
be used by the receiver. be used by the receiver.
.----------. .----------. .---------------. .----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party | | Attester | | Verifier | | Relying Party |
'----+-----' '-----+----' '-------+-------' '----+-----' '-----+----' '-------+-------'
| | | | | |
time(VG_a) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
skipping to change at page 51, line 37 skipping to change at line 2201
| | | |
time(RR_a) | time(RR_a) |
| | | |
+--[Attestation Result{time(RX_v)-time(RG_v)}, -->|time(RA_r) +--[Attestation Result{time(RX_v)-time(RG_v)}, -->|time(RA_r)
| Nonce2, time(RR_a)-time(EG_a)] | | Nonce2, time(RR_a)-time(EG_a)] |
| | | |
~ ~ ~ ~
| | | |
| time(OP_r) | time(OP_r)
Figure 11: Nonce-Based Passport Model
In this example solution, the Verifier can check whether the Evidence In this example solution, the Verifier can check whether the Evidence
is fresh at time(RG_v) by verifying that time(RG_v)-time(NS_v) < is fresh at time(RG_v) by verifying that time(RG_v)-time(NS_v) <
Threshold. Threshold.
The Verifier cannot, however, simply rely on a Nonce to determine However, the Verifier cannot simply rely on a Nonce to determine
whether the value of a Claim is recent, since the Claim value might whether the value of a Claim is recent since the Claim value might
have been generated long before the nonce was sent by the Verifier. have been generated long before the nonce was sent by the Verifier.
However, if the Verifier decides that the Attester can be trusted to Nevertheless, if the Verifier decides that the Attester can be
correctly provide the delta time(EG_a)-time(VG_a), then it can trusted to correctly provide the delta time(EG_a)-time(VG_a), then it
determine recency by checking time(RG_v)-time(NS_v) + time(EG_a)- can determine recency by checking time(RG_v)-time(NS_v) + time(EG_a)-
time(VG_a) < Threshold. time(VG_a) < Threshold.
Similarly if, based on an Attestation Result from a Verifier it Similarly if, based on an Attestation Result from a Verifier it
trusts, the Relying Party decides that the Attester can be trusted to trusts, the Relying Party decides that the Attester can be trusted to
correctly provide time deltas, then it can determine whether the correctly provide time deltas, then it can determine whether the
Attestation Result is fresh by checking time(OP_r)-time(NS_r) + Attestation Result is fresh by checking time(OP_r)-time(NS_r) +
time(RR_a)-time(EG_a) < Threshold. Although the Nonce2 and time(RR_a)-time(EG_a) < Threshold. Although the Nonce2 and
time(RR_a)-time(EG_a) values cannot be inside the Attestation Result, time(RR_a)-time(EG_a) values cannot be inside the Attestation Result,
they might be signed by the Attester such that the Attestation Result they might be signed by the Attester such that the Attestation Result
vouches for the Attester's signing capability. vouches for the Attester's signing capability.
The Relying Party must still be careful, however, to not allow However, the Relying Party must still be careful not to allow
continued use beyond the period for which it deems the Attestation continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX_v)-time(RG_v), then the Relying validity lifetime in terms of time(RX_v)-time(RG_v), then the Relying
Party can check time(OP_r)-time(NS_r) < time(RX_v)-time(RG_v). Party can check time(OP_r)-time(NS_r) < time(RX_v)-time(RG_v).
A.3. Example 3: Epoch ID-based Passport Model Example A.3. Example 3: Passport Model Based on Epoch ID
The example in Figure 10 illustrates a hypothetical Passport Model The example in Figure 12 illustrates a hypothetical Passport Model
solution that uses epoch IDs instead of nonces or timestamps. solution that uses epoch IDs instead of nonces or timestamps.
The Epoch ID Distributor broadcasts epoch ID I which starts a new The epoch ID distributor broadcasts epoch ID I, which starts a new
epoch E for a protocol participant upon reception at time(IR). epoch E for a protocol participant upon reception at time(IR).
The Attester generates Evidence incorporating epoch ID I and conveys The Attester generates Evidence incorporating epoch ID I and conveys
it to the Verifier. it to the Verifier.
The Verifier appraises that the received epoch ID I is "fresh" The Verifier appraises that the received epoch ID I is "fresh"
according to the definition provided in Section 10.3 whereby retries according to the definition provided in Section 10.3 whereby retries
are required in the case of mismatching epoch IDs, and generates an are required in the case of mismatching epoch IDs; then the Verifier
Attestation Result. The Attestation Result is conveyed to the generates an Attestation Result. The Attestation Result is conveyed
Attester. to the Attester.
After the transmission of epoch ID I' a new epoch E' is established After the transmission of epoch ID I' a new epoch E' is established
when I' is received by each protocol participant. The Attester when I' is received by each protocol participant. The Attester
relays the Attestation Result obtained during epoch E (associated relays the Attestation Result obtained during epoch E (associated
with epoch ID I) to the Relying Party using the epoch ID for the with epoch ID I) to the Relying Party using the epoch ID for the
current epoch I'. If the Relying Party had not yet received I', then current epoch I'. If the Relying Party had not yet received I', then
the Attestation Result would be rejected, but in this example, it is the Attestation Result would be rejected. The Attestation Result is
received. received in this example.
In the illustrated scenario, the epoch ID for relaying an Attestation In Figure 12, the epoch ID for relaying an Attestation Result to the
Result to the Relying Party is current, while a previous epoch ID was Relying Party is current while a previous epoch ID was used to
used to generate Verifier evaluated evidence. This indicates that at generate Verifier evaluated Evidence. This indicates that at least
least one epoch transition has occurred, and the Attestation Results one epoch transition has occurred and the Attestation Results may
may only be as fresh as the previous epoch. If the Relying Party only be as fresh as the previous epoch. If the Relying Party
remembers the previous epoch ID I during an epoch window as discussed remembers the previous epoch ID I during an epoch window as discussed
in Section 10.3, and the message is received during that window, the in Section 10.3, and the message is received during that window, the
Attestation Result is accepted as fresh, and otherwise it is rejected Attestation Result is accepted as fresh; otherwise, it is rejected as
as stale. stale.
.-------------. .-------------.
.----------. | Epoch ID | .----------. .---------------. .----------. | Epoch ID | .----------. .---------------.
| Attester | | Distributor | | Verifier | | Relying Party | | Attester | | Distributor | | Verifier | | Relying Party |
'----+-----' '------+------' '-----+----' '-------+-------' '----+-----' '------+------' '-----+----' '-------+-------'
| | | | | | | |
time(VG_a) | | | time(VG_a) | | |
| | | | | | | |
~ | ~ ~ ~ | ~ ~
| | | | | | | |
skipping to change at page 53, line 35 skipping to change at line 2295
| | | | | | | |
time(IR'_a) <----I'-o--I' ----> time(IR'_v) --> time(IR'_r) time(IR'_a) <----I'-o--I' ----> time(IR'_v) --> time(IR'_r)
| | | | | |
+---[Attestation Result--------------------> time(RA_r) +---[Attestation Result--------------------> time(RA_r)
| {I,time(RX_v)-time(RG_v)},I'] | | | {I,time(RX_v)-time(RG_v)},I'] | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
| | time(OP_r) | | time(OP_r)
Figure 10: Epoch ID-based Passport Model Figure 12: Epoch ID-Based Passport Model
A.4. Example 4: Timestamp-based Background-Check Model Example A.4. Example 4: Timestamp-Based Background-Check Model
The following example illustrates a hypothetical Background-Check Figure 13 illustrates a hypothetical Background-Check Model solution
Model solution that uses timestamps and requires roughly synchronized that uses timestamps and requires roughly synchronized clocks between
clocks between the Attester, Verifier, and Relying Party. The the Attester, Verifier, and Relying Party. The Attester conveys
Attester conveys Evidence to the Relying Party, which treats it as Evidence to the Relying Party, which treats it as opaque and simply
opaque and simply forwards it on to the Verifier. forwards it on to the Verifier.
.----------. .---------------. .----------. .----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier | | Attester | | Relying Party | | Verifier |
'-------+--' '-------+-------' '----+-----' '-------+--' '-------+-------' '----+-----'
| | | | | |
time(VG_a) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
time(EG_a) | | time(EG_a) | |
skipping to change at page 54, line 27 skipping to change at line 2327
| time(ER_r) ---Evidence{time(EG_a)}---->| | time(ER_r) ---Evidence{time(EG_a)}---->|
| | | | | |
| | time(RG_v) | | time(RG_v)
| | | | | |
| time(RA_r) <---Attestation Result------+ | time(RA_r) <---Attestation Result------+
| | {time(RX_v)} | | | {time(RX_v)} |
~ ~ ~ ~ ~ ~
| | | | | |
| time(OP_r) | | time(OP_r) |
Figure 13: Timestamp-Based Background-Check Model
The time considerations in this example are equivalent to those The time considerations in this example are equivalent to those
discussed under Example 1 above. discussed under Example 1.
A.5. Example 5: Nonce-based Background-Check Model Example A.5. Example 5: Nonce-Based Background-Check Model
The following example illustrates a hypothetical Background-Check Figure 14 illustrates a hypothetical Background-Check Model solution
Model solution that uses nonces and thus does not require that any that uses nonces; thus, it does not require that any clocks be
clocks are synchronized. In this example solution, a nonce is synchronized. In this example solution, a nonce is generated by a
generated by a Verifier at the request of a Relying Party, when the Verifier at the request of a Relying Party when the Relying Party
Relying Party needs to send one to an Attester. needs to send one to an Attester.
.----------. .---------------. .----------. .----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier | | Attester | | Relying Party | | Verifier |
'----+-----' '-------+-------' '----+-----' '----+-----' '-------+-------' '----+-----'
| | | | | |
time(VG_a) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
| |<-------Nonce-----------time(NS_v) | |<-------Nonce-----------time(NS_v)
skipping to change at page 55, line 31 skipping to change at line 2366
| time(ER_r) ---Evidence{Nonce}--->| | time(ER_r) ---Evidence{Nonce}--->|
| | | | | |
| | time(RG_v) | | time(RG_v)
| | | | | |
| ime(RA_r) <---Attestation Result--+ | ime(RA_r) <---Attestation Result--+
| | {time(RX_v)-time(RG_v)} | | | {time(RX_v)-time(RG_v)} |
~ ~ ~ ~ ~ ~
| | | | | |
| time(OP_r) | | time(OP_r) |
The Verifier can check whether the Evidence is fresh, and whether a Figure 14: Nonce-Based Background-Check Model
Claim value is recent, the same as in Example 2 above.
The Verifier can check whether the Evidence is fresh and a Claim
value is recent, which is the same as Example 2.
However, unlike in Example 2, the Relying Party can use the Nonce to However, unlike in Example 2, the Relying Party can use the Nonce to
determine whether the Attestation Result is fresh, by verifying that determine whether the Attestation Result is fresh by verifying that
time(OP_r)-time(NR_r) < Threshold. time(OP_r)-time(NR_r) < Threshold.
The Relying Party must still be careful, however, to not allow However, the Relying Party must still be careful not to allow
continued use beyond the period for which it deems the Attestation continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX_v)-time(RG_v), then the Relying validity lifetime in terms of time(RX_v)-time(RG_v), then the Relying
Party can check time(OP_r)-time(ER_r) < time(RX_v)-time(RG_v). Party can check time(OP_r)-time(ER_r) < time(RX_v)-time(RG_v).
Contributors Acknowledgments
Monty Wiseman
Email: montywiseman32@gmail.com
Liang Xia
Email: frank.xialiang@huawei.com
Laurence Lundblade
Email: lgl@island-resort.com
Eliot Lear
Email: elear@cisco.com
Jessica Fitzgerald-McKay
Sarah C. Helbe
Andrew Guinn
Peter Loscocco
Email: pete.loscocco@gmail.com
Eric Voit
Thomas Fossati
Email: thomas.fossati@arm.com
Paul Rowe
Carsten Bormann The authors would like to thank the following people for their input:
Email: cabo@tzi.org
Giri Mandyam Joerg Borchert, Carsten Bormann, Nancy Cam-Winget, Guy Fedorkow,
Email: mandyam@qti.qualcomm.com Jessica Fitzgerald-McKay, Thomas Fossati, Simon Frost, Andrew Guinn,
Thomas Hardjano, Eliot Lear, Diego Lopez, Peter Loscocco, Laurence
Lundblade, Giri Mandyam, Daniel Migault, Kathleen Moriarty, Paul
Rowe, Hannes Tschofenig, Eric Voit, Monty Wiseman, David Wooten, and
Liang Xia.
Kathleen Moriarty Contributors
Email: kathleen.moriarty.ietf@gmail.com
Guy Fedorkow Thomas Hardjono created initial versions of the terminology section
Email: gfedorkow@juniper.net in collaboration with Ned Smith. Eric Voit provided the conceptual
separation between Attestation Provision Flows and Attestation
Evidence Flows. Monty Wisemen was a key author of a document that
was merged to create this document. Carsten Bormann provided many of
the motivational building blocks with respect to the Internet Threat
Model.
Simon Frost Peter Loscocco contributed critical review feedback as part of the
Email: Simon.Frost@arm.com weekly design team meetings that added precision and depth to several
sections.
Authors' Addresses Authors' Addresses
Henk Birkholz Henk Birkholz
Fraunhofer SIT Fraunhofer SIT
Rheinstrasse 75 Rheinstrasse 75
64295 Darmstadt 64295 Darmstadt
Germany Germany
Email: henk.birkholz@sit.fraunhofer.de Email: henk.birkholz@sit.fraunhofer.de
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