A CBOR Tag for Unprotected CWT Claims Sets
draft-ietf-rats-uccs-12

RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                           J. O'Donoghue
Expires: 7 May 2025                           Qualcomm Technologies Inc.
                                                           N. Cam-Winget
                                                           Cisco Systems
                                                              C. Bormann
                                                  Universität Bremen TZI
                                                         3 November 2024

               A CBOR Tag for Unprotected CWT Claims Sets
                        draft-ietf-rats-uccs-12

Abstract

   This document defines the Unprotected CWT Claims Set (UCCS), a data
   format for representing a CBOR Web Token (CWT) Claims Set without
   protecting it by a signature, message authentication code (MAC), or
   encryption.  UCCS enables the use of CWT claims in environments where
   protection is provided by other means, such as secure communication
   channels or trusted execution environments.  This specification
   defines a CBOR tag for UCCS and describes the UCCS format, its
   encoding, and processing considerations, and discusses security
   implications of using unprotected claims sets.

   // (This editors' note will be removed by the RFC editor:) The
   // present revision (–12) contains remaining document changes based
   // on feedback from the IESG evaluation and has been submitted as
   // input to IETF 121.

About This Document

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

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-rats-uccs/.

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

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Structure of this document  . . . . . . . . . . . . . . .   5
   2.  Deployment and Usage of UCCS  . . . . . . . . . . . . . . . .   5
   3.  Characteristics of a Secure Channel . . . . . . . . . . . . .   6
   4.  UCCS in RATS Conceptual Message Conveyance  . . . . . . . . .   7
   5.  Considerations for Using UCCS in Other RATS Contexts  . . . .   8
     5.1.  Delegated Attestation . . . . . . . . . . . . . . . . . .   8
     5.2.  Privacy Preservation  . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  CBOR Tag registration . . . . . . . . . . . . . . . . . .   9
     6.2.  Media-Type application/uccs+cbor Registration . . . . . .  10
     6.3.  Media-Type application/ujcs+json Registration . . . . . .  10
     6.4.  Content-Format registration . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12

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     7.1.  General Considerations  . . . . . . . . . . . . . . . . .  12
     7.2.  Algorithm-specific Security Considerations  . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  CDDL . . . . . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Example  . . . . . . . . . . . . . . . . . . . . . .  18
   Appendix C.  EAT  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in
   a CBOR Object Signing and Encryption (COSE, [STD96]) envelope.  COSE
   provides -- among other things -- end-to-end data origin
   authentication and integrity protection employed by RFC 8392 as well
   as optional encryption for CWTs.  Under the right circumstances
   (Section 3), though, a signature providing proof for authenticity and
   integrity can be provided through the transfer protocol and thus
   omitted from the information in a CWT without compromising the
   intended goal of authenticity and integrity.  In other words, if
   communicating parties have a preexisting security association, they
   can reuse it to provide authenticity and integrity for their
   messages, enabling the basic principle of using resources
   parsimoniously.  Specifically, if a mutually secured channel is
   established between two remote peers, and if that secure channel
   provides the required properties (as discussed below), it is possible
   to omit the protection provided by COSE, creating a use case for
   unprotected CWT Claims Sets.  Similarly, if there is one-way
   authentication, the party that did not authenticate may be in a
   position to send authentication information through this channel that
   allows the already authenticated party to authenticate the other
   party; this effectively turns the channel into a mutually secured
   channel.

   This specification allocates a CBOR tag to mark Unprotected CWT
   Claims Sets (UCCS) as such and discusses conditions for its proper
   use in the scope of Remote Attestation Procedures (RATS [RFC9334])
   for the conveyance of RATS Conceptual Messages.

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   This specification does not change [RFC8392]: An actual RFC 8392 CWT
   does not make use of the tag allocated here; the UCCS tag is an
   alternative to using COSE protection and a CWT tag.  Consequently,
   within the well-defined scope of a secure channel, it can be
   acceptable and economic to use the contents of a CWT without its COSE
   container and tag it with a UCCS CBOR tag for further processing
   within that scope -- or to use the contents of a UCCS CBOR tag for
   building a CWT to be signed by some entity that can vouch for those
   contents.

1.1.  Terminology

   The term Claim is used as in [RFC7519].

   The terms Claim Key, Claim Value, and CWT Claims Set are used as in
   [RFC8392].

   The terms Attester, Attesting Environment, Evidence, Relying Party
   and Verifier are used as in [RFC9334].

   UCCS:  Unprotected CWT Claims Set(s); CBOR map(s) of Claims as
      defined by the CWT Claims Registry that are composed of pairs of
      Claim Keys and Claim Values.

   Secure Channel:  [NIST-SP800-90Ar1] defines a Secure Channel as
      follows:

         |  "A path for transferring data between two entities or
         |  components that ensures confidentiality, integrity and
         |  replay protection, as well as mutual authentication between
         |  the entities or components.  The secure channel may be
         |  provided using approved cryptographic, physical or
         |  procedural methods, or a combination thereof."

      For the purposes of the present document, we focus on a protected
      communication channel used for conveyance that can ensure the same
      qualities as CWT without having the COSE protection available:
      mutual authentication, integrity protection, confidentiality.
      (Replay protection can be added by including a nonce claim such as
      Nonce (claim 10 [IANA.cwt]).)  Examples include conveyance via
      PCIe (Peripheral Component Interconnect Express) IDE (Integrity
      and Data Encryption) or a TLS tunnel.

   All terms referenced or defined in this section are capitalized in
   the remainder of this document.

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [BCP14] (RFC2119) (RFC8174) when, and only when, they appear in all
   capitals, as shown here.

1.2.  Structure of this document

   Section 2 briefly discusses use cases for UCCS.  Section 3 addresses
   general characteristics of secure channels, followed by a specific
   discussion of using them in the context of RATS Conceptual Message
   Conveyance in Section 4, and finally some more forward-looking
   considerations for using UCCS in other RATS contexts in Section 5.
   Conventional sections (IANA Considerations, Security Considerations,
   Normative References, and Informative References) follow.  The
   normative Appendix A provides a formal definition of the structure of
   UCCS as no formal definition of CWT Claims Sets was provided in
   [RFC8392].  This employs the Concise Data Definition Language (CDDL)
   [RFC8610], using its ability to also describe the structurally
   similar Unprotected JWT Claims Sets [RFC7519] (UJCS) in the same
   definition.  Appendix B provides an (informative) example for CBOR-
   Tagged UCCS.  The normative Appendix C provides CDDL rules that add
   UCCS-format tokens to Entity Attestation Tokens (EATs, see
   [I-D.ietf-rats-eat]) using its predefined extension points.

2.  Deployment and Usage of UCCS

   Usage scenarios involving the conveyance of Claims, in particular
   RATS, require a standardized data definition and encoding format that
   can be transferred and transported using different communication
   channels.  As these are Claims, the Claims Sets defined in [RFC8392]
   are a suitable format.  However, the way these Claims are secured
   depends on the deployment, the security capabilities of the device,
   as well as their software stack.  For example, a Claim may be
   securely stored and conveyed using a device's Trusted Execution
   Environment (TEE, see [RFC9397]) or a Trusted Platform Module (TPM,
   see [TPM2]).  Especially in some resource-constrained environments,
   the same process that provides the secure communication transport is
   also the delegate to compose the Claim to be conveyed.  Whether it is
   a transfer or transport, a Secure Channel is presumed to be used for
   conveying such UCCS.  The following sections elaborate on Secure
   Channel characteristics in general and further describe RATS usage
   scenarios and corresponding requirements for UCCS deployment.

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3.  Characteristics of a Secure Channel

   A Secure Channel for the conveyance of UCCS needs to provide the
   security properties that would otherwise be provided by COSE for a
   CWT.  In this regard, UCCS is similar in security considerations to
   JWTs [BCP225] using the algorithm "none".  Section 3.2 of RFC 8725
   [BCP225] states:

   |  [...] if a JWT is cryptographically protected end-to-end by a
   |  transport layer, such as TLS using cryptographically current
   |  algorithms, there may be no need to apply another layer of
   |  cryptographic protections to the JWT.  In such cases, the use of
   |  the "none" algorithm can be perfectly acceptable.

   The security considerations discussed, e.g., in Sections 2.1, 3.1,
   and 3.2 of RFC 8725 [BCP225] apply in an analogous way to the use of
   UCCS as elaborated on in this document.  In particular, the need to
   "Use Appropriate Algorithms" (Section 3.2 of RFC 8725 [BCP225])
   includes choosing appropriate cryptographic algorithms for setting up
   and protecting the Secure Channel.  For instance, their cryptographic
   strength should be at least as strong as any cryptographic keys the
   Secure Channel will be used for to protect in transport.  Table 5 in
   Section 7.2 provides references to some more security considerations
   for specific cryptography choices that are discussed in the COSE
   initial algorithms specification [RFC9053].

   Secure Channels are often set up in a handshake protocol that
   mutually derives a session key, where the handshake protocol
   establishes the (identity and thus) authenticity of one or both ends
   of the communication.  The session key can then be used to provide
   confidentiality and integrity of the transfer of information inside
   the Secure Channel.  (Where the handshake did not provide a mutually
   secure channel, further authentication information can be conveyed by
   the party not yet authenticated, leading to a mutually secured
   channel.)  A well-known example of a such a Secure Channel setup
   protocol is the TLS [RFC8446] handshake; the TLS record protocol can
   then be used for secure conveyance.

   As UCCS were initially created for use in RATS Secure Channels, the
   following section provides a discussion of their use in these
   channels.  Where other environments are intended to be used to convey
   UCCS, similar considerations need to be documented before UCCS can be
   used.

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4.  UCCS in RATS Conceptual Message Conveyance

   This section describes a detailed usage scenario for UCCS in the
   context of RATS in conjunction with its attendant security
   requirements.  The use of UCCS tag CPA601 outside of the RATS context
   MUST come with additional instruction leaflets and security
   considerations.

   For the purposes of this section, any RATS role can be the sender or
   the receiver of the UCCS.

   Secure Channels can be transient in nature.  For the purposes of this
   specification, the mechanisms used to establish a Secure Channel are
   out of scope.

   In the scope of RATS Claims, the receiver MUST authenticate the
   sender as part of the establishment of the Secure Channel.
   Furthermore, the channel MUST provide integrity of the communication
   between the communicating RATS roles.  For data confidentiality
   [RFC4949], the receiving side MUST be authenticated as well; this is
   achieved if the sender and receiver mutually authenticate when
   establishing the Secure Channel.  The quality of the receiver's
   authentication and authorization will influence whether the sender
   can disclose the UCCS.

   The extent to which a Secure Channel can provide assurances that UCCS
   originate from a trustworthy Attesting Environment depends on the
   characteristics of both the cryptographic mechanisms used to
   establish the channel and the characteristics of the Attesting
   Environment itself.  The assurance provided to a Relying Party
   depends on the authenticity and integrity properties of the Secure
   Channel used for conveying the UCCS to it.

   Ultimately, it is up to the receiver's policy to determine whether to
   accept a UCCS from the sender and to determine the type of Secure
   Channel it must negotiate.  While the security considerations of the
   cryptographic algorithms used are similar to COSE, the considerations
   of the Secure Channel should also adhere to the policy configured at
   each of end of the Secure Channel.  However, the policy controls and
   definitions are out of scope for this document.

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   Where an Attesting Environment serves as an endpoint of a Secure
   Channel used to convey a UCCS, the security assurance required of
   that Attesting Environment by a Relying Party generally calls for the
   Attesting Environment to be implemented using techniques designed to
   provide enhanced protection from an attacker wishing to tamper with
   or forge UCCS originating from that Attesting Environment.  A
   possible approach might be to implement the Attesting Environment in
   a hardened environment such as a TEE [RFC9397] or a TPM [TPM2].

   When UCCS emerge from the Secure Channel and into the receiver, the
   security properties of the secure channel no longer protect the UCCS,
   which now are subject to the same security properties as any other
   unprotected data in the Verifier environment.  If the receiver
   subsequently forwards UCCS, they are treated as though they
   originated within the receiver.

   The Secure Channel context does not govern fully formed CWTs in the
   same way it governs UCCS.  As with Entity Attestation Tokens (EATs,
   see [I-D.ietf-rats-eat]) nested in other EATs (Section 4.2.18.3
   (Nested Tokens) of [I-D.ietf-rats-eat]), the Secure Channel does not
   endorse fully formed CWTs transferred through it.  Effectively, the
   COSE envelope of a CWT (or a nested EAT) shields the CWT Claims Set
   from the endorsement of the secure channel.  (Note that EAT might add
   a nested UCCS Claim, and this statement does not apply to UCCS nested
   into UCCS, only to fully formed CWTs.)

5.  Considerations for Using UCCS in Other RATS Contexts

   This section discusses two additional usage scenarios for UCCS in the
   context of RATS.

5.1.  Delegated Attestation

   Another usage scenario is that of a sub-Attester that has no signing
   keys (for example, to keep the implementation complexity to a
   minimum) and has a Secure Channel, such as local inter-process
   communication, to interact with a lead Attester (see "Composite
   Device", Section 3.3 of [RFC9334]).  The sub-Attester produces a UCCS
   with the required CWT Claims Set and sends the UCCS through the
   Secure Channel to the lead Attester.  The lead Attester then computes
   a cryptographic hash of the UCCS and protects that hash using its
   signing key for Evidence, for example, using a Detached-Submodule-
   Digest or Detached EAT Bundle (Section 5 of [I-D.ietf-rats-eat]).

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5.2.  Privacy Preservation

   A Secure Channel which preserves the privacy of the Attester may
   provide security properties equivalent to COSE, but only inside the
   life-span of the session established.  In general, when a privacy
   preserving Secure Channel is employed for conveying a conceptual
   message, the receiver cannot correlate the message with the senders
   of other received UCCS messages beyond the information the Secure
   Channel authentication provides.

   An Attester must consider whether any UCCS it returns over a privacy
   preserving Secure Channel compromises the privacy in unacceptable
   ways.  As an example, the use of the EAT UEID Claim (Section 4.2.1 of
   [I-D.ietf-rats-eat]) in UCCS over a privacy preserving Secure Channel
   allows a Verifier to correlate UCCS from a single Attesting
   Environment across many Secure Channel sessions.  This may be
   acceptable in some use-cases (e.g., if the Attesting Environment is a
   physical sensor in a factory) and unacceptable in others (e.g., if
   the Attesting Environment is a user device belonging to a child).

6.  IANA Considerations

6.1.  CBOR Tag registration

   In the CBOR Tags registry [IANA.cbor-tags] as defined in Section 9.2
   of RFC 8949 [STD94], IANA is requested to allocate the tag in Table 1
   from the Specification Required space (1+2 size), with the present
   document as the specification reference.

       +========+==========================+======================+
       |    Tag | Data Item                | Semantics            |
       +========+==========================+======================+
       | CPA601 | map (Claims-Set as per   | Unprotected CWT      |
       |        | Appendix A of [RFCthis]) | Claims Set [RFCthis] |
       +--------+--------------------------+----------------------+

                         Table 1: Values for Tags

   // RFC-Editor: This document uses the CPA (code point allocation)
   // convention described in [I-D.bormann-cbor-draft-numbers].  For
   // each usage of the term "CPA", please remove the prefix "CPA" from
   // the indicated value and replace the residue with the value
   // assigned by IANA; perform an analogous substitution for all other
   // occurrences of the prefix "CPA" in the document.  Finally, please
   // remove this note.

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6.2.  Media-Type application/uccs+cbor Registration

   IANA is requested to add the following Media-Type to the "Media
   Types" registry [IANA.media-types].

      +===========+=======================+========================+
      | Name      | Template              | Reference              |
      +===========+=======================+========================+
      | uccs+cbor | application/uccs+cbor | Section 6.2 of RFCthis |
      +-----------+-----------------------+------------------------+

                     Table 2: Media Type Registration

   Type name:  application
   Subtype name:  uccs+cbor
   Required parameters:  n/a
   Optional parameters:  n/a
   Encoding considerations:  binary (CBOR data item)
   Security considerations:  Section 7 of RFCthis
   Interoperability considerations:  none
   Published specification:  RFCthis
   Applications that use this media type:  Applications that transfer
      Unprotected CWT Claims Set(s) (UCCS) over Secure Channels
   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/cbor".  (At
      publication of this document, there is no fragment identification
      syntax defined for "application/cbor".)
   Additional information:  Deprecated alias names for this type:  N/A

                            Magic number(s):  N/A

                            File extension(s):  .uccs

                            Macintosh file type code(s):  N/A
   Person and email address to contact for further information:  RATS WG
      mailing list (rats@ietf.org)
   Intended usage:  COMMON
   Restrictions on usage:  none
   Author/Change controller:  IETF

6.3.  Media-Type application/ujcs+json Registration

   IANA is requested to add the following Media-Type to the "Media
   Types" registry [IANA.media-types].

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      +===========+=======================+========================+
      | Name      | Template              | Reference              |
      +===========+=======================+========================+
      | ujcs+json | application/ujcs+json | Section 6.3 of RFCthis |
      +-----------+-----------------------+------------------------+

                  Table 3: JSON Media Type Registration

   Type name:  application
   Subtype name:  ujcs+json
   Required parameters:  n/a
   Optional parameters:  n/a
   Encoding considerations:  binary (UTF-8)
   Security considerations:  Section 7 of RFCthis
   Interoperability considerations:  none
   Published specification:  RFCthis
   Applications that use this media type:  Applications that transfer
      Unprotected JWT Claims Set(s) (UJCS) over Secure Channels
   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/json".  (At
      publication of this document, there is no fragment identification
      syntax defined for "application/json".)
   Additional information:  Deprecated alias names for this type:  N/A

                            Magic number(s):  N/A

                            File extension(s):  .ujcs

                            Macintosh file type code(s):  N/A
   Person and email address to contact for further information:  RATS WG
      mailing list (rats@ietf.org)
   Intended usage:  COMMON
   Restrictions on usage:  none
   Author/Change controller:  IETF

6.4.  Content-Format registration

   IANA is requested to register a Content-Format number in the "CoAP
   Content-Formats" subregistry, within the "Constrained RESTful
   Environments (CoRE) Parameters" registry [IANA.core-parameters], as
   follows:

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     +=======================+================+========+=============+
     | Content Type          | Content Coding | ID     | Reference   |
     +=======================+================+========+=============+
     | application/uccs+cbor | -              | TBD601 | Section 6.4 |
     |                       |                |        | of RFCthis  |
     +-----------------------+----------------+--------+-------------+

                    Table 4: Content-Format Registration

   // RFC editor: please replace TBD601 by the number actually assigned
   // by IANA (601 is suggested).

7.  Security Considerations

   The security considerations of [STD94] apply.  The security
   considerations of [RFC8392] need to be applied analogously, replacing
   the function of COSE with that of the Secure Channel; in particular
   "it is not only important to protect the CWT in transit but also to
   ensure that the recipient can authenticate the party that assembled
   the claims and created the CWT".

   Section 3 discusses security considerations for Secure Channels, in
   which UCCS might be used.  This document provides the CBOR tag
   definition for UCCS and a discussion on security consideration for
   the use of UCCS in RATS.  Uses of UCCS outside the scope of RATS are
   not covered by this document.  The UCCS specification -- and the use
   of the UCCS CBOR tag, correspondingly -- is not intended for use in a
   scope where a scope-specific security consideration discussion has
   not been conducted, vetted and approved for that use.  In order to be
   able to use the UCCS CBOR tag in another such scope, the secure
   channel and/or the application protocol (e.g., TLS and the protocol
   identified by ALPN) MUST specify the roles of the endpoints in a
   fashion that the security properties of conveying UCCS via a Secure
   Channel between the roles are well-defined.

7.1.  General Considerations

   Implementations of Secure Channels are often separate from the
   application logic that has security requirements on them.  Similar
   security considerations to those described in [STD96] for obtaining
   the required levels of assurance include:

   *  Implementations need to provide sufficient protection for private
      or secret key material used to establish or protect the Secure
      Channel.

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   *  Using a key for more than one algorithm can leak information about
      the key and is not recommended.

   *  An algorithm used to establish or protect the Secure Channel may
      have limits on the number of times that a key can be used without
      leaking information about the key.

   *  Evidence in a UCCS conveyed in a Secure Channel generally cannot
      be used to support trust in the credentials that were used to
      establish that secure channel, as this would create a circular
      dependency.

   The Verifier needs to ensure that the management of key material used
   to establish or protect the Secure Channel is acceptable.  This may
   include factors such as:

   *  Ensuring that any permissions associated with key ownership are
      respected in the establishment of the Secure Channel.

   *  Using cryptographic algorithms appropriately.

   *  Using key material in accordance with any usage restrictions such
      as freshness or algorithm restrictions.

   *  Ensuring that appropriate protections are in place to address
      potential traffic analysis attacks.

7.2.  Algorithm-specific Security Considerations

   Table 5 provides references to some security considerations of
   specific cryptography choices that are discussed in [RFC9053].

            +===================+============================+
            | Algorithm         | Reference                  |
            +===================+============================+
            | AES-CBC-MAC       | Section 3.2.1 of [RFC9053] |
            +-------------------+----------------------------+
            | AES-GCM           | Section 4.1.1 of [RFC9053] |
            +-------------------+----------------------------+
            | AES-CCM           | Section 4.2.1 of [RFC9053] |
            +-------------------+----------------------------+
            | ChaCha20/Poly1305 | Section 4.3.1 of [RFC9053] |
            +-------------------+----------------------------+

                   Table 5: Algorithm-specific Security
                              Considerations

8.  References

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8.1.  Normative References

   [BCP14]    Best Current Practice 14,
              <https://www.rfc-editor.org/info/bcp14>.
              At the time of writing, this BCP comprises the following:

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

              Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [BCP225]   Best Current Practice 225,
              <https://www.rfc-editor.org/info/bcp225>.
              At the time of writing, this BCP comprises the following:

              Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/info/rfc8725>.

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <https://www.iana.org/assignments/cbor-tags>.

   [IANA.cwt] IANA, "CBOR Web Token (CWT) Claims",
              <https://www.iana.org/assignments/cwt>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7519>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/rfc/rfc8392>.

   [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, <https://www.rfc-editor.org/rfc/rfc8610>.

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   [RFC9165]  Bormann, C., "Additional Control Operators for the Concise
              Data Definition Language (CDDL)", RFC 9165,
              DOI 10.17487/RFC9165, December 2021,
              <https://www.rfc-editor.org/rfc/rfc9165>.

   [STD94]    Internet Standard 94,
              <https://www.rfc-editor.org/info/std94>.
              At the time of writing, this STD comprises the following:

              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

8.2.  Informative References

   [I-D.ietf-rats-eat]
              Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
              Wallace, "The Entity Attestation Token (EAT)", Work in
              Progress, Internet-Draft, draft-ietf-rats-eat-31, 6
              September 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rats-eat-31>.

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <https://www.iana.org/assignments/core-parameters>.

   [IANA.media-types]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [NIST-SP800-90Ar1]
              Barker, E. and J. Kelsey, "Recommendation for Random
              Number Generation Using Deterministic Random Bit
              Generators", National Institute of Standards and
              Technology, DOI 10.6028/nist.sp.800-90ar1, June 2015,
              <https://doi.org/10.6028/nist.sp.800-90ar1>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/rfc/rfc4949>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

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   [RFC8747]  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
              2020, <https://www.rfc-editor.org/rfc/rfc8747>.

   [RFC9053]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
              August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.

   [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>.

   [RFC9397]  Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", RFC 9397, DOI 10.17487/RFC9397, July 2023,
              <https://www.rfc-editor.org/rfc/rfc9397>.

   [STD96]    Internet Standard 96,
              <https://www.rfc-editor.org/info/std96>.
              At the time of writing, this STD comprises the following:

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Countersignatures", STD 96, RFC 9338,
              DOI 10.17487/RFC9338, December 2022,
              <https://www.rfc-editor.org/info/rfc9338>.

   [TPM2]     "Trusted Platform Module Library Specification, Family
              “2.0”, Level 00, Revision 01.59 ed., Trusted Computing
              Group", 2019.

Appendix A.  CDDL

   The Concise Data Definition Language (CDDL), as defined in [RFC8610]
   and [RFC9165], provides an easy and unambiguous way to express
   structures for protocol messages and data formats that use CBOR or
   JSON.

   [RFC8392] does not define CDDL for CWT Claims Sets.

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   // RFC-Editor: This document uses the CPA (code point allocation)
   // convention described in [I-D.bormann-cbor-draft-numbers].  Please
   // replace the number 601 in the code blocks below by the value that
   // has been assigned for CPA601 and remove this note.

   The CDDL model in Figure 1 shows how to use CDDL for defining the CWT
   Claims Set defined in [RFC8392].  These CDDL rules have been built
   such that they also can describe [RFC7519] Claims sets by disabling
   feature "cbor" and enabling feature "json".

   UCCS-Untagged = Claims-Set
   UCCS-Tagged = #6.601(UCCS-Untagged)

   Claims-Set = {
    * $$Claims-Set-Claims
    * Claim-Label .feature "extended-claims-label" => any
   }
   Claim-Label = CBOR-ONLY<int> / text
   string-or-uri = text

   $$Claims-Set-Claims //= ( iss-claim-label => string-or-uri )
   $$Claims-Set-Claims //= ( sub-claim-label => string-or-uri )
   $$Claims-Set-Claims //= ( aud-claim-label => string-or-uri )
   $$Claims-Set-Claims //= ( exp-claim-label => ~time )
   $$Claims-Set-Claims //= ( nbf-claim-label => ~time )
   $$Claims-Set-Claims //= ( iat-claim-label => ~time )
   $$Claims-Set-Claims //= ( cti-claim-label => bytes )

   iss-claim-label = JC<"iss", 1>
   sub-claim-label = JC<"sub", 2>
   aud-claim-label = JC<"aud", 3>
   exp-claim-label = JC<"exp", 4>
   nbf-claim-label = JC<"nbf", 5>
   iat-claim-label = JC<"iat", 6>
   cti-claim-label = CBOR-ONLY<7>  ; jti in JWT: different name and text

   JSON-ONLY<J> = J .feature "json"
   CBOR-ONLY<C> = C .feature "cbor"
   JC<J,C> = JSON-ONLY<J> / CBOR-ONLY<C>

                  Figure 1: CDDL definition for Claims-Set

   Specifications that define additional Claims should also supply
   additions to the $$Claims-Set-Claims socket, e.g.:

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   ; [RFC8747]
   $$Claims-Set-Claims //= ( 8: CWT-cnf ) ; cnf
   CWT-cnf = {
     (1: CWT-COSE-Key) //
     (2: CWT-Encrypted_COSE_Key) //
     (3: CWT-kid)
   }

   CWT-COSE-Key = COSE_Key
   CWT-Encrypted_COSE_Key = COSE_Encrypt / COSE_Encrypt0
   CWT-kid = bytes

   ;;; Insert the required CDDL from RFC 9052 to complete these
   ;;; definitions.  This can be done manually or automated by a
   ;;; tool that implements an import directive such as:
   ;# import rfc9052

   The above definitions, concepts and security considerations also
   define a JSON-encoded Claims-Set as encapsulated in a JWT.  Such an
   unsigned Claims-Set can be referred to as a "Unprotected JWT Claims
   Set", a "UJCS".  The CDDL definition of Claims-Set in Figure 1 can be
   used for a "UJCS":

   UJCS = Claims-Set

Appendix B.  Example

   This appendix is informative.

   The example CWT Claims Set from Appendix A.1 of [RFC8392] can be
   turned into a UCCS by enclosing it with a tag number CPA601:

    601(
      {
        / iss / 1: "coap://as.example.com",
        / sub / 2: "erikw",
        / aud / 3: "coap://light.example.com",
        / exp / 4: 1444064944,
        / nbf / 5: 1443944944,
        / iat / 6: 1443944944,
        / cti / 7: h'0b71'
      }
    )

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Appendix C.  EAT

   The following CDDL adds UCCS-format and UJCS-format tokens to EAT
   using its predefined extension points (see Section 4.2.18 (submods)
   of [I-D.ietf-rats-eat]).

   $EAT-CBOR-Tagged-Token /= UCCS-Tagged
   $EAT-CBOR-Untagged-Token /= UCCS-Untagged

   $JSON-Selector /= [type: "UJCS", nested-token: UJCS]

Acknowledgements

   Laurence Lundblade suggested some improvements to the CDDL.  Carl
   Wallace provided a very useful review.

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany
   Email: henk.birkholz@ietf.contact

   Jeremy O'Donoghue
   Qualcomm Technologies Inc.
   279 Farnborough Road
   Farnborough
   GU14 7LS
   United Kingdom
   Email: jodonogh@qti.qualcomm.com

   Nancy Cam-Winget
   Cisco Systems
   3550 Cisco Way
   San Jose, CA 95134
   United States of America
   Email: ncamwing@cisco.com

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

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   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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