Remote ATtestation ProcedureS                                   S. Frost
Internet-Draft                                                       Arm
Intended status: Standards Track                            27 June 2022
Expires: 29 December 2022

             Entity Attestation Token (EAT) Collection Type


   The default top-level definitions for an EAT [I-D.ietf-rats-eat]
   assume a hierarchy involving a leading signer within the Attester.
   Some token use cases do not match that model.  This specification
   defines an extension to EAT allowing the top-level of the token to
   consist of a collection of otherwise defined tokens.

About This Document

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

   Status information for this document may be found at

   Discussion of this document takes place on the Remote ATtestation
   ProcedureS Working Group mailing list (, which
   is archived at

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   This Internet-Draft will expire on 29 December 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Design Considerations / Use Cases . . . . . . . . . . . . . .   3
   3.  Token Collection  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  CDDL . . . . . . . . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   An Attestation Token conforming to EAT [I-D.ietf-rats-eat] has a
   default top level definition for a token to be constructed
   principally as a claim set within a CBOR Web Token (CWT) [RFC8392]
   with the associated COSE envelope [RFC8152] providing at least
   integrity and authentication.  An equivalent JSON encoding for a JWT
   [RFC7519] in a JWS envelope [RFC7515] is supported as an alternative
   at the top-level definition.  The top level token can be augmented
   with related claims in a Detached Bundle (DEB).

   For the use case of transmitting a claim set through a secure
   channel, the top-level definition can be extended to use an
   Unprotected CWT Claim Set (UCCS) [I-D.ietf-rats-uccs].

   This document outlines an additional top-level extension for which
   neither of the above top level definitions match exactly: the
   attestation token consists of a collection of objects, each with
   their own integrity and some internally defined relationship through
   which the integrity of the whole collection can be determined. i.e.

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   there is no top-level signer for the set.  The objects may all share
   the same logical hierarchy in a device or have a hierarchy which is
   internally defined within the object set.

2.  Design Considerations / Use Cases

   Take a device with an attestation system consisting of a platform
   claim set and a workload claim set, each controlled by different
   components and with an underlying hardware Root of Trust.  The two
   claim sets are delivered together to make up the overall attestation
   token.  Depending upon the implementation and deployment use case,
   the signing system can either be entirely centric to the platform RoT
   or can have separate signers for the two claim sets.  In either case,
   a cryptographic binding is established between the two parts of the

   A specific manifestation of such a device is one incorporating the
   Arm Confidential Compute Architecture (CCA) attestation token
   [Arm-CCA].  In trying to prepare the attestation token using EAT,
   there were no issues constructing the claim sets or incorporating
   them into individual CWTs where appropriate.  However, in trying to
   design an 'envelope structure' to convey the two parts as a single
   report it was found that maintaining EAT compatibility would require
   very different shaped compound tokens for different models, for
   example one based on a submod arrangement and another based on a DEB,
   though with different 'leading' objects.  This would create extra
   code and explanation in areas where keeping things simple is
   desirable.  There was an alternative approach considered, which stays
   close to existing thinking on EAT, which would be to create the
   wrapper from the UCCS EAT extension containing only submods for the
   respective components.  This however stretches the current use case
   for UCCS beyond its existing description.  The RATS WG approach of
   separating UCCS from the core EAT specification to be an extension
   also encourages proposing this further extension.

   To support the CCA use case, it is also relevant to consider current
   attestation technologies which are based on certificate chains (e.g.
   SPDM, DICE, several key attestation systems).  Here also are multiple
   objects with their own integrity and an internally defined
   relationship.  If attempting to move such a technology to the EAT
   world, the same challenges apply.

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3.  Token Collection

   The proposed extension for the top-level definition is to add a
   'Token Collection' type.  The contents of the type are a map of CWTs
   (JWTs).  The DEB top-level entry for EAT is included for
   completeness, and the UCCS extension can also be embraced, though the
   use cases for these have not been explored.  The identification of
   collection members and the intra collection integrity mechanism is
   considered usage specific.  A verifier will be expected to extract
   each of the members of the collection and check their validity both
   individually and as a set.

   A map was chosen rather than an unbounded array to give the
   opportunity to add identifying map tags to each entry.  The
   interpretation of the tags will be usage specific, but may correspond
   to registered identities of specific token types.  To assist a
   verifier correlate the expected contents a profile entry can be added
   as the 'profile-label' identity in the map.

   See Appendix A for a CDDL [RFC8610] description of the proposed

   While most of the use cases for collections are for scenarios where
   there will be at least two entries in a collection, the CDDL allows
   for >= 1 entries in a collection to allow for the scenario where only
   one entry is currently available even though the normal set is

4.  Security Considerations

   A verifier for an attestation token must apply a verification process
   for the full set of entries contained within the Token Collection.
   This process will be custom to the relevant profile for the Token
   Collection and take into account any individual verification per
   entry and/or verification for the objects considered collectively,
   including the intra token integrity scheme.  As there is no overall
   signature for the Collection, protection against malicious
   modification must be contained within the entries.  It is expected
   that there exists a cryptographic binding between entries, this can
   for example be one to many or one to one in a (chain) series.
   Depending upon the use case and associated threat model, the
   freshness of entries may need extra consideration.

5.  IANA Considerations

   In the registry [IANA.cbor-tags], IANA is requested to allocate the
   tag in Table 1 from the FCFS space, with the present document as the
   specification reference.

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   | Tag    | Data Item | Semantics              |
   | TBD399 | map       | EAT Collection RFCthis |

               Table 1: EAT Collection

6.  References

6.1.  Normative References

              Lundblade, L., Mandyam, G., and J. O'Donoghue, "The Entity
              Attestation Token (EAT)", Work in Progress, Internet-
              Draft, draft-ietf-rats-eat-13, 20 May 2022,

              IANA, "Concise Binary Object Representation (CBOR) Tags",

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <>.

6.2.  Informative References

   [Arm-CCA]  Arm Ltd, "Confidential Compute Architecture", n.d..

              Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C.
              Bormann, "A CBOR Tag for Unprotected CWT Claims Sets",
              Work in Progress, Internet-Draft, draft-ietf-rats-uccs-02,
              12 January 2022, <

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

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   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <>.

Appendix A.  CDDL

   $$EAT-CBOR-Tagged-Token /= Tagged-Collection
   $$EAT-CBOR-Untagged-Token /= TL-Collection

   Tagged-Collection =  #6.TBD399(TL-Collection)

   ; Note that although the common use cases for collections are for at least two entries in a collection,
   ; the CDDL below allows for >= 1 entry to allow the scenario where only one entry is currently available even
   ; though the normal set is larger
   TL-Collection = {
       ? eat-collection-identifier,
       + cwt-collection-entries // jwt-collection-entries // DEB-collection-entries

   eat-collection-identifier = (
       profile-label => general-uri / general-oid

   cwt-collection-entries = (
       collection-entry-label => CWT-Messages

   jwt-collection-entries = (
       collection-entry-label => JWT-Messages

   DEB-collection-entries = (
       collection-entry-label => DEB-Messages

   collection-entry-label = JC<text, int>


   Thomas Fossati and Yogesh Deshpande provided insightful comments and
   review for this proposal.

Author's Address

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   Simon Frost

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