RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                           J. O'Donoghue
Expires: 14 January 16 July 2022                         Qualcomm Technologies Inc.
                                                           N. Cam-Winget
                                                           Cisco Systems
                                                              C. Bormann
                                                  Universität Bremen TZI
                                                            13 July 2021
                                                         12 January 2022

               A CBOR Tag for Unprotected CWT Claims Sets


   CBOR Web Token (CWT, RFC 8392) Claims Sets sometimes do 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.

   // The present version (-01) has a few editorial improvements over
   // -00 and attempts to address points from Thomas Fossati's
   // 2021-03-16 review, for further discussion at IETF 111.

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 (rats) Working Group mailing list (mailto: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

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 14 January 16 July 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Example Use Cases . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Characteristics of a Secure Channel . . . . . . . . . . . . .   4
     3.1.  UCCS and Remote ATtestation procedureS (RATS) . . . . . .   5
     3.2.  Privacy Preserving Channels . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.1.  General Considerations  . . . . . . . . . . . . . . . . .   7
     5.2.  AES-CBC_MAC . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  AES-CCM . . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     5.5.  ChaCha20 and Poly1305 . . . . . . . . . . . . . . . . . .   8   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9  10
   Appendix A.  CDDL . . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Example  . . . . . . . . . . . . . . . . . . . . . .  10  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  14

1.  Introduction

   A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in
   a CBOR Object Signing and Encryption (COSE, [RFC8152]) envelope.
   COSE provides -- amongst other things -- the end-to-end data origin
   authentication and integrity protection employed by RFC 8392 and
   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

   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) and the
   conveyance of Evidence from an Attester to a Verifier.

   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 secured 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

   The terms Attester, Attesting Environment and Verifier are used as in

   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:  A protected communication channel between two peers
      that can ensure the same qualities associated for UCCS conveyance
      as CWT conveyance without any additional protection.

   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",
   "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.  Example Use Cases

   Use cases involving the conveyance of Claims, in particular, remote
   attestation procedures (RATS, see [I-D.ietf-rats-architecture])
   require a standardized data definition and encoding format that can
   be transferred and transported using different communication
   channels.  As these are Claims, [RFC8392] is 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
   [I-D.ietf-teep-architecture]) or 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 further describe the RATS usage scenario 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.  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 Remote ATtestation
   procedureS (RATS) Secure Channels, the following subsection 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.

3.1.  UCCS and Remote ATtestation procedureS (RATS)

   For the purposes of this section, the Verifier is the receiver of the
   UCCS and the Attester is the provider 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.

   As a minimum requirement in the scope of RATS Claims, the Verifier
   MUST authenticate the Attester as part of the establishment of the
   Secure Channel.  Furthermore, the channel MUST provide integrity of
   the communication from the Attester to the Verifier.  If
   confidentiality is also required, the receiving side needs to be
   authenticated as well; this can be achieved if the Verifier and the
   Attester mutually authenticate when establishing the Secure Channel.

   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.

   A Secure Channel established or maintained using weak cryptography
   may not provide the assurance required by a relying party of the
   authenticity and integrity of the UCCS.

   Ultimately, it is up to the Verifier's policy to determine whether to
   accept a UCCS from the Attester 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 the Attester and the Verifier.  However, the policy controls
   and definitions are out of scope for this document.

   Where the security assurance required of an attesting environment by
   a relying party requires it, the attesting environment may be
   implemented using techniques designed to provide enhanced protection
   from an attacker wishing to tamper with or forge UCCS.  A possible
   approach might be to implement the attesting environment in a
   hardened environment such as a TEE [I-D.ietf-teep-architecture] or a
   TPM [TPM2].

   When UCCS emerge from the Secure Channel and into the Verifier, the
   security properties of the Secure Channel no longer apply and UCCS
   have the same properties as any other unprotected data in the
   Verifier environment.  If the Verifier subsequently forwards UCCS,
   they are treated as though they originated within the Verifier.

   As with EATs nested in other EATs (Section 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 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)

3.2.  Privacy Preserving Channels

   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, a Verifier cannot
   correlate UCCS received in different sessions from the same attesting
   environment based on the cryptographic mechanisms used when a privacy
   preserving Secure Channel is employed.

   In the case of a Remote Attestation, the 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 [I-D.ietf-rats-eat] Claim 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 device belonging to a

4.  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.

       |    Tag | Data Item | Semantics                            |
       | TBD601 | map       | Unprotected CWT Claims Set [RFCthis] |

                          Table 1: Values for Tags

5.  Security Considerations

   The security considerations of [RFC8949] apply.  The security
   considerations of [RFC8392] need to be applied analogously, replacing
   the role of COSE with that of the Secured 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 Remote ATtestation procedureS (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.

5.1.  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
   [I-D.ietf-cose-rfc8152bis-struct] 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

   *  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.

   The Verifier needs to ensure that the management of key material used
   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.

   *  Cryptographic algorithms are used appropriately.

   *  Key material is used 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.


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

   *  Different keys should be used for authentication and encryption

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

   Section 3.2.1 of [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
   explanation of these considerations.

5.3.  AES-GCM

   *  The key and nonce pair are 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 [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
   explanation of these considerations.

5.4.  AES-CCM

   *  The key and nonce pair are 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 [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
   explanation of these considerations.

5.5.  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 [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
   explanation of these considerations.

6.  References

6.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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

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

   [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/info/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/info/rfc8392>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,

6.2.  Informative References

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,

              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,

              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture", Work
              in Progress, Internet-Draft, draft-ietf-rats-architecture-
              12, 23 April
              14, 9 December 2021, <https://www.ietf.org/archive/id/draft-
              ietf-rats-architecture-12.txt>. <https://www.ietf.org/archive/id/

              Mandyam, G.,
              Lundblade, L., Ballesteros, M., Mandyam, G., and J. O'Donoghue, "The Entity
              Attestation Token (EAT)", Work in Progress, Internet-Draft, draft-ietf-rats-eat-10, 7 June Internet-
              Draft, draft-ietf-rats-eat-11, 24 October 2021, <https://www.ietf.org/archive/id/draft-ietf-rats-

              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-teep-architecture-14, 22 February
              ietf-teep-architecture-15, 12 July 2021,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8693]  Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J.,
              and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693,
              DOI 10.17487/RFC8693, January 2020,

   [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/info/rfc8747>.

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

Appendix A.  CDDL

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

   This specification proposes using the definitions in Figure 1 for the
   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.

   Claims-Set = {
    * $$Claims-Set-Claims
    * Claim-Label .feature "extended-claims-label" => any
   Claim-Label = 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"

                  Figure 1: CDDL definition for Claims-Set

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

   ; [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

   ; [RFC8693]
   $$Claims-Set-Claims //= ( 9: CWT-scope ) ; scope
   ; TO DO: understand what this means:
   ; scope The scope of an access token as defined in [RFC6749].
   ; scope 9 byte string or text string [IESG] [RFC8693, Section 4.2]
   CWT-scope = bytes / text

   ; [RFC-ietf-ace-oauth-authz-45, Section 5.10]
   $$Claims-Set-Claims //= ( 38: CWT-ace-profile ) ; ace_profile
   CWT-ace-profile = $CWT-ACE-Profiles /
     int .feature "ace_profile-extend"
   ; fill in from IANA registry
   ;   https://www.iana.org/assignments/ace/ace.xhtml#ace-profiles :
   $CWT-ACE-Profiles /= 1 ; coap_dtls

   $$Claims-Set-Claims //= ( 39: CWT-cnonce ) ; cnonce
   CWT-cnonce = bytes

   $$Claims-Set-Claims //= ( 40: CWT-exi ) ; exi
   CWT-exi = uint ; in seconds (5.10.3)

   ;;; insert CDDL from 9052-to-be to complete these CDDL definitions.

Appendix B.  Example

   The example CWT Claims Set from Appendix A.1 of [RFC8392] can be
   turned into an UCCS by enclosing it with a tag number TBD601:

        / 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'


   Laurence Lundblade suggested some improvements to the CDDL.

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt

   Email: henk.birkholz@sit.fraunhofer.de

   Jeremy O'Donoghue
   Qualcomm Technologies Inc.
   279 Farnborough Road
   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

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