A CBOR Tag for Unprotected CWT Claims Sets
draft-ietf-rats-uccs-09

RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                           J. O'Donoghue
Expires: 5 September 2024                     Qualcomm Technologies Inc.
                                                           N. Cam-Winget
                                                           Cisco Systems
                                                              C. Bormann
                                                  Universität Bremen TZI
                                                            4 March 2024

               A CBOR Tag for Unprotected CWT Claims Sets
                        draft-ietf-rats-uccs-09

Abstract

   When transported over secure channels, CBOR Web Token (CWT, RFC 8392)
   Claims Sets may not need the protection afforded by wrapping them
   into COSE, as is required for a true CWT.  This specification defines
   a CBOR tag for such unprotected CWT Claims Sets (UCCS) and discusses
   conditions for its proper use.

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.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 5 September 2024.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Deployment and Usage of UCCS  . . . . . . . . . . . . . . . .   4
   3.  Characteristics of a Secure Channel . . . . . . . . . . . . .   5
   4.  UCCS in RATS Conceptual Message Conveyance  . . . . . . . . .   5
   5.  Considerations for Using UCCS in Other RATS Contexts  . . . .   7
     5.1.  Delegated Attestation . . . . . . . . . . . . . . . . . .   7
     5.2.  Privacy Preservation  . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     7.1.  General Considerations  . . . . . . . . . . . . . . . . .   9
     7.2.  AES-CBC_MAC . . . . . . . . . . . . . . . . . . . . . . .   9
     7.3.  AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.4.  AES-CCM . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.5.  ChaCha20 and Poly1305 . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  CDDL . . . . . . . . . . . . . . . . . . . . . . . .  13
   Appendix B.  Example  . . . . . . . . . . . . . . . . . . . . . .  15
   Appendix C.  JSON Support . . . . . . . . . . . . . . . . . . . .  15
   Appendix D.  EAT  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in
   a CBOR Object Signing and Encryption (COSE, [RFC9052]) envelope.
   COSE provides -- amongst 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 pre-existing 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.

   This specification does not change [RFC8392]: A true 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].

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   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.

   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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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

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   Channel characteristics in general and further describe RATS usage
   scenarios and corresponding requirements for UCCS deployment.

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 [RFC8725] using the algorithm "none".  RFC 8725 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 [RFC8725] apply in an analogous way to the use of UCCS as
   elaborated on in this document.

   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.

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.

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   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 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.

   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.

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   The Secure Channel context does not govern fully formed CWTs in the
   same way it governs UCCS.  As with EATs 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]).

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.

   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).

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6.  IANA Considerations

   In the CBOR Tags registry [IANA.cbor-tags] as defined in Section 9.2
   of [RFC8949], 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.

7.  Security Considerations

   The security considerations of [RFC8949] apply.  The security
   considerations of [RFC8392] need to be applied analogously, replacing
   the function of COSE with that of the Secure Channel.

   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.

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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 [RFC9052] 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.

   *  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.

   The remaining subsections of this section highlight some aspects of
   specific cryptography choices that are detailed further in [RFC9053].

7.2.  AES-CBC_MAC

   *  A given key should only be used for messages of fixed or known
      length.

   *  Different keys should be used for authentication and encryption
      operations.

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   *  A mechanism to ensure that IV cannot be modified is required.

   Section 3.2.1 of [RFC9053] contains a detailed explanation of these
   considerations.

7.3.  AES-GCM

   *  The key and nonce pair is unique for every encrypted message.

   *  The maximum number of messages to be encrypted for a given key is
      not exceeded.

   Section 4.1.1 of [RFC9053] contains a detailed explanation of these
   considerations.

7.4.  AES-CCM

   *  The key and nonce pair is unique for every encrypted message.

   *  The maximum number of messages to be encrypted for a given block
      cipher is not exceeded.

   *  The number of messages both successfully and unsuccessfully
      decrypted is used to determine when rekeying is required.

   Section 4.2.1 of [RFC9053] contains a detailed explanation of these
   considerations.

7.5.  ChaCha20 and Poly1305

   *  The nonce is unique for every encrypted message.

   *  The number of messages both successfully and unsuccessfully
      decrypted is used to determine when rekeying is required.

   Section 4.3.1 of [RFC9053] contains a detailed explanation of these
   considerations.

8.  References

8.1.  Normative References

   [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>.

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   [RFC2119]  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/rfc/rfc2119>.

   [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>.

   [RFC8174]  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/rfc/rfc8174>.

   [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>.

   [RFC8725]  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/rfc/rfc8725>.

   [RFC8949]  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/rfc/rfc8949>.

   [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>.

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-25, 15
              January 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rats-eat-25>.

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   [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>.

   [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>.

   [RFC9052]  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/rfc/rfc9052>.

   [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>.

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

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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.

   // 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.

   This specification proposes using the definitions in Figure 1 for the
   CWT Claims Set defined in [RFC8392].  Note that these definitions
   have been built such that they also can describe [RFC7519] Claims
   sets by disabling feature "cbor" and enabling feature "json", but
   this flexibility is not the subject of the present specification.

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   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

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'
      }
    )

Appendix C.  JSON Support

   This appendix is informative.

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

   UJCS = Claims-Set

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Appendix D.  EAT

   This appendix is informative.

   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@sit.fraunhofer.de

   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

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   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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