RATS Endorsements
draft-ietf-rats-endorsements-09

RATS Working Group                                             D. Thaler
Internet-Draft                                       Armidale Consulting
Updates: 9334 (if approved)                                  H. Birkholz
Intended status: Informational                            Fraunhofer SIT
Expires: 3 September 2026                                     T. Fossati
                                                                  Linaro
                                                            2 March 2026

                           RATS Endorsements
                    draft-ietf-rats-endorsements-09

Abstract

   In the IETF Remote Attestation Procedures (RATS) architecture, a
   Verifier accepts Evidence and uses Appraisal Policy for Evidence,
   typically with additional input from Endorsements and Reference
   Values, to generate Attestation Results in formats that are useful
   for Relying Parties.  This document illustrates the purpose and role
   of Endorsements and discusses some considerations in the choice of
   message format for Endorsements in the scope of the RATS
   architecture.

   This document does not aim to define a conceptual message format for
   Endorsements and Reference Values.  Instead, it extends RFC9334 to
   provide further details on Reference Values and Endorsements, as
   these topics were outside the scope of the RATS charter when RFC9334
   was developed.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Remote ATtestation
   ProcedureS Working Group mailing list (rats@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/rats/.

   Source for this draft and an issue tracker can be found at
   https://github.com/dthaler/rats-endorsements.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Actual State vs Reference State . . . . . . . . . . . . . . .   3
     2.1.  RATS Conceptual Messages  . . . . . . . . . . . . . . . .   4
   3.  Conditionally Endorsed Values . . . . . . . . . . . . . . . .   6
   4.  Endorsing Verification Keys . . . . . . . . . . . . . . . . .   7
   5.  Timeliness  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Multiple Endorsements . . . . . . . . . . . . . . . . . . . .   9
   7.  Endorsement Format Considerations . . . . . . . . . . . . . .  11
     7.1.  Security Considerations for Formats . . . . . . . . . . .  11
     7.2.  Scalability Considerations for Formats  . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   11. Informative References  . . . . . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   Section 3 of [RFC9334] provides an overview of the roles and
   conceptual messages in the IETF RATS architecture.  As discussed in
   that document, a Verifier accepts a well-defined set of RATS
   conceptual messages: Evidence, Endorsements and Reference Values, as
   well as Appraisal Policy for Evidence.  A Verifier appraises Evidence
   using Appraisal Policy for Evidence, typically against a set of
   Reference Values.

   When [RFC9334] was developed, providing details of Reference Values
   and Endorsements were outside the scope of the RATS Working Group's
   charter.  However, this has since changed, and the purpose of this
   document is to update [RFC9334] to provide further details on
   Reference Values and Endorsements.

2.  Actual State vs Reference State

   Appraisal policies (Appraisal Policy for Evidence, and Appraisal
   Policy for Attestation Results) involve comparing the actual state of
   an Attester against desired or undesired states, in order to
   determine how trustworthy the Attester is for its purposes.  The
   state of an Attester represents the Attester's "shape" as the
   arrangement of its various execution environments, which are
   typically organized hierarchically.  The state of an Attester also
   encompasses the combination of static and dynamic composition (e.g.,
   provisioned and deployed software, firmware, and micro-code), static
   and dynamic configuration, and the resulting operational state of its
   components at a certain point in time.  Thus, a Verifier needs to
   receive conceptual messages with information about actual state, and
   information about desired/undesired states, and an appraisal policy
   that controls how the two are compared.

   Each Attester in general has at least one Attesting Environment and
   one Target Environment (e.g., hardware, firmware, Operating System,
   etc.).  Typically, each Attester has multiple Target Environments,
   each with their own "claims sets" representing their actual state.
   Additionally, the number of Target Environments and Attesting
   Environments that are components of an Attester are not limited.

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   "Actual state" is a group of claims sets about the actual state of
   the Attester at a given point in time.  Each claims set holds claims
   about a specific Target Environment that is essential to determining
   trustworthiness.  Generally speaking, each claim has a name and a
   singleton value, being a value collected from a Target Environment of
   a specific Attester at a given point in time.  Some claims may
   inherently have multiple values, such as a list of files in a given
   location on the device, but in the context of this document such a
   list is treated as a single unit, representing one Attester at one
   point in time.

   "Reference state" is a group of claims sets about the desired or
   undesired state of an Attester.  Typically, each claim has a name and
   a set of potential values, being the values that are allowed/
   disallowed when determining the trustworthiness of the Attester.
   Generally, there may be varying degrees of gradation beyond just
   "allowed" or "disallowed."  Reference state can have a set of values
   per claim per Target Environment.  This is contrasted with actual
   state, which has a single value per claim per Target Environment.
   Actual state applies to one device at one point in time.  Appraisal
   policy then specifies how to match the actual state values against a
   set of Reference Values.

   Some examples of such matching include:

   *  An actual value must be in the set of allowed Reference Values.

   *  An actual value must not be in the set of disallowed Reference
      Values.

   *  An actual value must be in a range where two Reference Values are
      the min and max.

2.1.  RATS Conceptual Messages

   RATS conceptual messages in [RFC9334] fall into the above categories
   as follows:

   *  Actual state: Evidence, Endorsements, Attestation Results

   *  Reference state: Reference Values

   *  Appraisal policy: Appraisal Policy for Evidence, Appraisal Policy
      for Attestation Results

   Hints or suggestions for how to do a comparison might be supplied by
   a Reference Value Provider (as part of Reference Values), an Endorser
   (in an Endorsement), and/or an Attester (in Evidence), but the

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   Verifier Owner is authoritative for Appraisal Policy for Evidence,
   and the Relying Party Owner is authoritative for Appraisal Policy for
   Attestation Results as depicted in Section 3 of [RFC9334].

   Figure 1 below shows an example of Verifier input for a layered
   Attester as discussed in Section 3.2 of [RFC9334].

             .-- .------------.   Appraisal    .-----------------. --.
             |   |Actual state|   Policy for   | Reference state |   |
             |   |  (layer N) |   Evidence     |    (layer N)    |   | R
             |   '------------'       |        '-----------------'   | e
             |                        |                              | f
             |   .------------.       |        .-----------------.   | e
    Evidence |   |Actual state|       |        | Reference state |   | r
             |   |  (layer 2) |       |        |    (layer 2)    |   | e
             |   '------------'       |        '-----------------'   | n
             |                        v                              | c
             |   .------------.  <==========>  .-----------------.   | e
             |   |Actual state|   Comparison   | Reference state |   |
             |   |  (layer 1) |   Rules        |    (layer 1)    |   | V
             '-- '------------'                '-----------------'   | a
                                                                     | l
             .-- .------------.                .-----------------.   | u
Endorsements |   |Actual state|                | Reference state |   | e
             |   |  (layer 0) |                |    (layer 0)    |   | s
             '-- '------------'                '-----------------' --'

                   Figure 1: Example Verifier Input

   While the above example only shows one layer within Endorsements as
   the typical case, there could be multiple layers (see Section 6),
   such as a chip added to a hardware board potentially from a different
   vendor.

   A Trust Anchor Store is a special case of state above, where the
   Reference State would be the set of trust anchors accepted (or
   rejected) by the Verifier, and the Actual State would be a trust
   anchor used to appraise Evidence or Endorsements.

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   In layered attestation using DICE [TCG-DICE] for example, the actual
   state of each layer is signed by a key held by the next lower layer.
   Thus in the example diagram above, the layer 2 actual state (e.g., OS
   state) is signed by a layer 1 key (e.g., a signing key used by the
   firmware), the layer 1 actual state (e.g., firmware state) is signed
   by a layer 0 key (e.g., a hardware key stored in ROM), and the layer
   0 actual state (hardware specs and key ID) is signed by a layer 0 key
   (e.g., a vendor key) which is matched against the Verifier's trust
   anchor store, which is part of the layer 0 reference state depicted
   above.

3.  Conditionally Endorsed Values

   The example in Figure 1 shows Evidence containing actual state for
   layers 1 through N, and an Endorsement containing actual state for
   layer 0.  However, some claims in Endorsements might be conditional
   and so are only treated as actual state if a condition is met.

   A claim is conditional if it only applies if other actual state
   matches Reference Values, according to some matching policy.  For
   example, an Endorser for a given CPU might provide additional
   information about the CPU's supported features based on the current
   firmware configuration.  Alternatively, an Endorser might provide
   information indicating that a specific security certification (e.g.,
   [GP-Cert] and [OCP-SAFE]) is associated with the device if the serial
   number falls within a given range and the firmware is configured in a
   specific way.

                                         Conditional Endorsements
                                        .-----------------------.
                                       /                         \
         .--------------.              .-----------+-------------.
         | Actual State | <==========> | Condition | claims-sets |
         '--------------'  Endorsement '-----------+---+---------'
                 ^         Matching                    |
                 |         Rules                [condition is met]
                 |                                     |
                 |           .-------------.           |
                  '----------+ claims-sets +----------'
                             '-------------'

                     Figure 2: Conditional Endorsements

   Thus, actual state is determined by starting with a collection of
   unconditional claims and adding any conditional claims whose
   conditions are met based on the actual state (Figure 2).  This
   process is then repeated until no more conditional claims are added.

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   Verifier policies around matching actual state against reference
   state are normally expressed in Appraisal Policy for Evidence.
   Similarly, reference state is normally expressed in the Reference
   Values conceptual message.  Such policies allow a Verifier and
   Relying Parties to make their decisions about the trustworthiness of
   an Attester.

   The use of conditionally endorsed values, however, is different in
   that a matching policy is not about trustworthiness (and hence not
   "appraisal" per se) but rather about whether an Endorser's claim is
   applicable or not, and thus usable as input to trustworthiness
   appraisal or not.

   As such the matching policy for conditionally endorsed values must be
   up to the Endorser not the Appraisal Policy Provider.  Thus, an
   Endorsement format that supports conditionally endorsed values (e.g.,
   [I-D.ietf-rats-corim]) would probably include some minimal matching
   policy (e.g., exact match against a singleton reference value).  This
   unfortunately complicates the Verifier design as it may need multiple
   parsers for matching policies.

4.  Endorsing Verification Keys

   Attesting Environments have cryptographic keys that allow
   authenticating the Evidence that they produce.

   Typically, the bottom-most Attesting Environment in an Attester will
   sign claims about one or more Target Environments (see also the DICE
   example at the end of Section 2.1) with a private key that the
   Attesting Environment possesses, and the Verifier will appraise the
   resulting Evidence with a public key it possesses, called a
   verification key below.  While use of public key cryptography is
   typical for a verification key, cryptography other than public key
   may also be used.

   Endorsing the linkage between such verification keys and their
   associated Attesting Environments is crucial to the verification
   process.

   The Verifier must have access to a verification key for each
   Attesting Environment.  Such a key could be provisioned directly in
   the Verifier.  However, for scalability the Verifier typically is
   provisioned with a trusted root CA certificate (a trust anchor).
   This means that an Endorsement from an Endorser includes the
   Attesting Enviroment's verification key material in the form of a
   certificate that chains up to that trust anchor (a certification
   path).  Such a certificate might be stored in the Verifier, or might
   be resolved on demand via some protocol, or might be passed to the

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   Verifier along with the Evidence to appraise, depending on the
   protocol or general remote attestation procedure.  Details are out of
   scope of this document and left to protocol or procedure
   specifications.

   Specific protocol documents are also responsible for documenting what
   particular algorithm or cryptographic protocol is used for the
   verification of the Attester.  The verification key (i.e., a key with
   the purpose of signature checking) could be, typically, a symmetric
   key, a raw public key, or a certified public key.

   Evidence can contain an identifier for the Attester (e.g., [RFC9711]
   ueid) in a dedicated "identity claim" that can be used by the
   Verifier to look up its verification key for the Attester.

   While identity claims are just another type of claims that may be
   endorsed, some implementations might treat them differently.  For
   example, a Verifier might perform a first step to cryptographically
   appraise that the Evidence has been generated by the Attester that
   has the key material associated with the identifier in the identity
   claim(s) before spending effort on another step to appraise other
   claims for determining trustworthiness.

   This document treats identity claims as with any other claims but
   allows Appraisal Policy for Evidence to have multiple phases if
   desired.

5.  Timeliness

   Specific protocol documents are also responsible for documenting how
   Timeliness of the Endorsement itself (e.g., using a certificate
   lifetime) is provided.

   Section 8.1 of [RFC9334] discusses timeliness of claims in Evidence.
   When additional "static" claims (i.e., claims representing invariant
   properties of the Environment) are provided in Endorsements, no
   additional steps are needed for timeliness of those claims since they
   are static rather than dynamically varying over time.  Once
   timeliness of Evidence is appraised, any matching conditionally
   endorsed values can be applied.

   If Endorsements ever carry dynamic claims in the future (e.g.,
   whether any vulnerabilities in the version of firmware are currently
   known), then the same timeliness considerations as for claims in
   Evidence would apply, and would be the responsibility of specific
   protocol documents.  See Section 10 of [RFC9334] and Appendix A of
   [RFC9334] for further discussion.

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6.  Multiple Endorsements

   Figure 1 shows an example with an Endorsement at layer 0, such as a
   hardware manufacturer providing claims about the hardware.  However,
   the same could be done at other layers in addition.  For example, an
   OS vendor might provide additional static claims about the OS
   software it provides, and application developers might provide
   additional static claims about the applications they release.

   Figure 3 depicts an example with an Attester consisting of an
   application, OS, firmware, and hardware, each from a different vendor
   that provides an Endorsement for their own Target Environment,
   containing additional claims about that Target Environment.  Thus
   each Target Environment (application, OS, firmware, and hardware) has
   one set of claims ("claims set 1") in the Evidence, and an additional
   set of claims ("claims set 2") in the Endorsement from its
   manufacturer.  A Verifier that trusts each Endorser would thus use
   the claims sets from both conceptual messages (Endorsements and
   Evidence) when comparing against reference state for a given Target
   Environment.

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                    .---------------------------. .-----------------.
     App            |            .------------. | | .------------.  |
     Endorser ----> |Endorsement |    app     | | | |    app     |  |
                    |            |claims set 2| | | |claims set 1|  |
                    |            '------------' | | '------------' E|
                    '---------------------------' |                v|
                                                  |                i|
                    .---------------------------. |                d|
     OS             |            .------------. | | .------------. e|
     Endorser ----> |Endorsement |     OS     | | | |     OS     | n|
                    |            |claims set 2| | | |claims set 1| c|
                    |            '------------' | | '------------' e|
                    '---------------------------' |                 |
                                                  |                 |
                    .---------------------------. |                 |
     Firmware       |            .------------. | | .------------.  |
     Endorser ----> |Endorsement |  firmware  | | | |  firmware  |  |
                    |            |claims set 2| | | |claims set 1|  |
                    |            '------------' | | '------------'  |
                    '---------------------------' |                 |
                                                  |                 |
                    .---------------------------. |                 |
     Hardware       |            .------------. | | .------------.  |
     Endorser ----> |Endorsement |  hardware  | | | |  hardware  |  |
                    |            |claims set 2| | | |claims set 1|  |
                    |            '------------' | | '------------'  |
                    '---------------------------' '-----------------'
                                                          ^
                                                          |
     Attester -------------------------------------------'

                      Figure 3: Multiple Endorsements

   When Target Environments from different vendors each have their own
   Endorser, a Verifier must be able to distinguish which Endorser is
   allowed to provide an Endorsement about which Target Environment.
   For example, the OS Endorser might be trusted to provide additional
   claims about the OS, but not about the hardware.  Thus, it is not as
   simple as saying that a Verifier has a trusted set of Endorsers.  The
   binding between Target Environment and Endorser might be part of the
   Appraisal Policy for Evidence, or might be specified as part of the
   Evidence itself (e.g., claims from a Target Environment might include
   an identifier of what Endorser can provide additional claims about
   it), or some combination of the two.  An Endorsement format
   specification should explain how this concern is addressed.

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7.  Endorsement Format Considerations

   This section discusses considerations around formats for
   Endorsements.

7.1.  Security Considerations for Formats

   In many scenarios, a Verifier can also support a variety of different
   formats, and while code size may not be a huge concern, simplicity
   and correctness of code is essential to security.  "Complexity is the
   enemy of security" is a popular security mantra and hence to increase
   security, any decrease in complexity helps.  As such, using the same
   format for both Evidence and Endorsements can reduce complexity and
   hence increase security.

7.2.  Scalability Considerations for Formats

   We currently assume that Reference Value Providers typically provide
   the same information to a potentially large number of clients
   (Verifiers, or potentially to other entities for later relay to a
   Verifier), and are generally on devices that are not constrained
   nodes, and hence additional scalability, including code size, is not
   a significant concern.  We also assume the same is true of Endorsers.

   The scenario where scalability in terms of code size is strongest,
   however, is when a Verifier is embedded into a constrained node.  For
   example, when a constrained node is a Relying Party for most
   purposes, but still needs a way to establish trust in the Verifier it
   will use.  In such a case, the Relying Party may have a constrained
   Verifier embedded in it that is only capable of appraising Evidence
   provided by its desired Verifier.  Thus, the Relying Party uses its
   embedded Verifier for purposes of appraising its desired Verifier
   which it treats as only an Attester, and once appraised, then uses it
   for appraisal of all other Attesters.  In this scenario, the embedded
   Verifier may have code and data size constraints, and a very simple
   (by comparison) Appraisal Policy for Evidence and desired state
   (e.g., a required trust anchor that Evidence must be signed with and
   little else).

   Using the same message format for Evidence, Endorsements, and (later)
   Attestation Results received from the later Verifier, can provide
   code size savings due to having only a single parser in this limited
   case.

   Similarly, an embedded constrained Verifier can choose to not support
   conditionally endorsed values, in order to avoid the complexity
   introduced by such.

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8.  Security Considerations

   Section 8.4 of [RFC9334] discusses how a Verifier stores one or more
   trust anchors in its trust anchor store.  A Verifier expresses its
   trust in an Endorser by storing a trust anchor for that Endorser.
   The binding from an Endorsement to a given Target Environment is done
   as discussed in Section 4 of this document.

   [RFC9334] (especially Sections 3.2 and 12) also discusses security
   considerations around the remote attestation of layers, and sources
   of appraisal policies.  Section 4 of this document covers additional
   considerations in these areas, while Section 7.1 covers additional
   considerations around Endorsement formats.

   The integrity of public and private key material and the secrecy of
   private key material must be ensured at all times.  This includes
   public keys that identify trusted supply chain actors.  For more
   detailed information on protecting Trust Anchors, refer to
   Section 12.4 of [RFC9334].

   A Verifier can use cryptographically protected, mutually
   authenticated secure channels to all its trusted input sources,
   particularly, Endorsers and Reference Value Providers.  These links
   should reach as deep as possible into the Verifier, potentially
   terminating within the appraisal session context, to avoid man-in-
   the-middle attacks.  Minimizing the use of intermediaries is also
   vital, as each intermediary is another party that might need to be
   trusted.  Refer to Section 12.2 of [RFC9334] for information on
   conceptual message protection.

9.  Privacy Considerations

   The privacy considerations regarding conceptual messages, as
   discussed in Section 11 of [RFC9334], apply.  In particular, since
   Endorsements and Reference Values can contain personally identifiable
   information (PII) about a large number of devices, strong
   confidentiality protection is required at the time of conveyance.

   Utilizing the public part of an asymmetric key pair that is used for
   Evidence generation to identify an Attesting Environment raises
   privacy considerations that must be carefully considered.

10.  IANA Considerations

   This document does not require any actions by IANA.

11.  Informative References

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   [GP-Cert]  GlobalPlatform, "Security Certification", March 2026,
              <https://globalplatform.org/certifications/security-
              certification/>.

   [I-D.ietf-rats-corim]
              Birkholz, H., Fossati, T., Deshpande, Y., Smith, N., and
              W. Pan, "Concise Reference Integrity Manifest", Work in
              Progress, Internet-Draft, draft-ietf-rats-corim-09, 20
              October 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rats-corim-09>.

   [OCP-SAFE] Open Compute Project, "The OCP Security Appraisal
              Framework and Enablement (S.A.F.E.) Program", March 2026,
              <https://www.opencompute.org/projects/ocp-safe-program>.

   [RFC9334]  Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote ATtestation procedureS (RATS)
              Architecture", RFC 9334, DOI 10.17487/RFC9334, January
              2023, <https://www.rfc-editor.org/rfc/rfc9334>.

   [RFC9711]  Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
              Wallace, "The Entity Attestation Token (EAT)", RFC 9711,
              DOI 10.17487/RFC9711, April 2025,
              <https://www.rfc-editor.org/rfc/rfc9711>.

   [TCG-DICE] Trusted Computing Group, "DICE Attestation Architecture",
              April 2025, <https://trustedcomputinggroup.org/wp-
              content/uploads/DICE-Attestation-Architecture-
              v1.2_pub.pdf>.

Acknowledgements

   The authors wish to thank the following individuals for feedback and
   ideas that contributed to this document: Yogesh Deshpande, Thomas
   Hardjono, Laurence Lundblade, Kathleen Moriarty, Michael Richardson,
   Ned Smith, and Carl Wallace

Authors' Addresses

   Dave Thaler
   Armidale Consulting
   United States of America
   Email: dave.thaler.ietf@gmail.com

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   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany
   Email: henk.birkholz@sit.fraunhofer.de

   Thomas Fossati
   Linaro
   Email: Thomas.Fossati@linaro.org

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