Messaging Layer Security credentials using Selective Disclosure CBOR Web Tokens
draft-mahy-mls-sd-cwt-credential-00

Messaging Layer Security                                         R. Mahy
Internet-Draft                            Rohan Mahy Consulting Services
Intended status: Informational                              2 March 2025
Expires: 3 September 2025

Messaging Layer Security credentials using Selective Disclosure CBOR Web
                                 Tokens
                  draft-mahy-mls-sd-cwt-credential-00

Abstract

   The Messaging Layer Security (MLS) protocol contains Credentials used
   to authenticate an MLS client with a signature key pair.  Selective
   Disclosure CBOR Web Tokens (SD-CWT) and Selective Disclosure JSON Web
   Tokens (SD-JWT) define token formats where the holder can selectively
   reveal claims about itself with strong integrity protection and
   cryptographic binding to the holder's key.  This document defines MLS
   credentials for both these token types.

About This Document

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

   The latest revision of this draft can be found at
   https://rohanmahy.github.io/mls-sd-cwt-credential/draft-mahy-mls-sd-
   cwt-credential.html.  Status information for this document may be
   found at https://datatracker.ietf.org/doc/draft-mahy-mls-sd-cwt-
   credential/.

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

   Source for this draft and an issue tracker can be found at
   https://github.com/rohanmahy/mls-sd-cwt-credential.

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-
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  New MLS Credential types  . . . . . . . . . . . . . . . . . .   3
     3.1.  MLS SD-CWT Credential . . . . . . . . . . . . . . . . . .   4
     3.2.  MLS SD-JWT Credential . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  SD-CWT Credential . . . . . . . . . . . . . . . . . . . .   6
     6.2.  SD-JWT Credential . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   This document defines new MLS [RFC9420] credential types for SD-CWT
   [I-D.ietf-spice-sd-cwt] and SD-JWT
   [I-D.ietf-oauth-selective-disclosure-jwt] tokens respectively.  The
   SD-CWT Credential contains a Selective Disclosure Key Binding Token
   (SD-KBT).  The SD-JWT Credential contains SD-JWT with Key Binding
   (SD-JWT+KB), which could be represented in the traditional data
   format, or in a more compact binary encoding.

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   The "holder" of one of these tokens could be the MLS client including
   the token in its Credential in its LeafNode (in a group or in a
   KeyPackage) or in an ExternalSender structure.

2.  Conventions and Definitions

   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.

   The term Credential is used as defined in Section 5.3 of [RFC9420].
   The terms MLS Distribution Service (DS) and MLS Authentication
   Service (AS) are used as defined in [I-D.ietf-mls-architecture].  The
   terms MLS client, MLS group, LeafNode, KeyPackage, PublicMessage,
   PrivateMessage, ratchet tree, and GroupInfo are likewise common MLS
   terms defined in [RFC9420].

3.  New MLS Credential types

   This document extends the list of defined CredentialTypes in MLS to
   include sd_cwt and sd_jwt types.  Additional syntax and semantics are
   defined in the following subsections.

   struct {
       CredentialType credential_type;
       select (Credential.credential_type) {
           case basic:
               opaque identity<V>;
           case x509:
               Certificate certificates<V>;
           ...
           case sd_cwt:
               opaque sd_kbt<V>;
           case sd_jwt:
               SdJwt sd_jwt;
       };
   } Credential;

   The MLS architecture [I-D.ietf-mls-architecture] describes the
   Authentication Services as having the following three services (i.e.
   requirements):

   1.  Issue credentials to clients that attest to bindings between
       identities and signature key pairs

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   2.  Enable a client to verify that a credential presented by another
       client is valid with respect to a reference identifier

   3.  Enable a group member to verify that a credential represents the
       same client as another credential

   The consequence of this is that the consumer of the SD-CWT or SD-JWT
   needs to be able to determine both the MLS client and the application
   identity referred to in a token in a Credential.

3.1.  MLS SD-CWT Credential

   An MLS SD-CWT Credential contains a single SD-KBT, containing an SD-
   CWT in the KBT protected header.  The SD-CWT contains zero of more
   disclosures (in the sd_claims header field).

   Any party that can view the credential can read the disclosed claims.
   For example if LeafNodes are visible to the MLS DS, because MLS
   handshake messages are conveyed in PublicMessage, the disclosed
   claims would also be visible to the DS.

   The SD-CWT inside the credential MAY include zero or more encrypted
   disclosures (in the sd_encrypted_claims header field).  Each
   encrypted disclosure is separately AEAD encrypted with a per-
   disclosure unique ephemeral key and salt.  The per-disclosure
   encryption key allows the holder/MLS client to disclose an element to
   a specific subset of members, or (in the common case when the DS is
   privy to the ratchet tree) only to members of the group.  A proof of
   concept to decrypt encrypted disclosures only for members of the
   group is described in [I-D.mahy-mls-member-secrets].

   The audience in the SD-KBT is either a representation of the MLS
   group, or a higher-level application structure associated with an MLS
   group or tightly-coupled collection of groups (for example, a chat
   room which maintains one MLS group for the main discussion and
   another for moderators to discuss the moderation of the room) such
   that being in one group without the collection would be nonsensical.

   The subject in the SD-CWT represents a specific MLS client (for
   example a COSE key thumbprint, or a client ID URI).  It should not
   use an identifier which represents multiple signature key pairs of
   the same type, or represents the same "user" on multiple devices.

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3.2.  MLS SD-JWT Credential

   The SD-JWT Credential can be represented in the classic SD-JWT+KB
   data format defined in Section 4 of
   [I-D.ietf-oauth-selective-disclosure-jwt] (shown below), or in a more
   compact binary representation.  MLS SD-JWT Credentials MUST include
   the Key Binding.

      *TODO*: Discuss if the LeafNode signature over the Credential is
      sufficient

   The classic format uses only characters from the unpadded base64url
   character set (Section 5 of [RFC4648]) plus the period (.) character
   to separate the three parts of the Issuer-signed JWT, and the tilde
   (~) character to separate disclosures from the other components.

<Issuer-signed JWT>~<Disclosure 1>~<Disclosure 2>~...~<Disclosure N>~<KB-JWT>

                Figure 1: SD-JWT+KB in classic format

   This document also defines a "compacted" format where each of the
   components of the Issuer-signed JWT, every Disclosure, and the KB-JWT
   are base64url decoded and stored in individual fields in the SdJwt
   struct.

   struct {
       Bool compacted;
       select (compacted) {
           case true:
               opaque protected<V>;
               opaque payload<V>;
               opaque signature<V>;
               SdJwtDisclosure disclosures<V>;
               opaque sd_jwt_key_binding<V>;
           case false:
               opaque sd_jwt_kb<V>;
       };
   } SdJwt;

   enum {
     false(0),
     true(1)
   } Bool;

      The compacted variant allows implementations to tradeoff reduced
      size for the extra processing cost of base64url encoding and
      decoding the Credential.

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4.  Security Considerations

   The privacy considerations in SD-CWT, SD-JWT, and MLS apply.  TODO
   more security.

5.  Privacy Considerations

   The privacy considerations in SD-CWT and SD-JWT apply.  The privacy
   considerations of MLS are largely discussed in
   [I-D.ietf-mls-architecture].  TODO more privacy.

6.  IANA Considerations

   This document requests IANA to add the following entries to the MLS
   Credential Types registry.  Please replace RFCXXXX with the RFC of
   this document.

6.1.  SD-CWT Credential

   *  Value: 0x0005 (suggested)

   *  Name: sd_cwt

   *  Recommended: Y

   *  Reference: RFCXXXX

6.2.  SD-JWT Credential

   *  Value: 0x0006 (suggested)

   *  Name: sd_jwt

   *  Recommended: Y

   *  Reference: RFCXXXX

7.  References

7.1.  Normative References

   [I-D.ietf-mls-architecture]
              Beurdouche, B., Rescorla, E., Omara, E., Inguva, S., and
              A. Duric, "The Messaging Layer Security (MLS)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-mls-architecture-15, 3 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mls-
              architecture-15>.

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   [I-D.ietf-oauth-selective-disclosure-jwt]
              Fett, D., Yasuda, K., and B. Campbell, "Selective
              Disclosure for JWTs (SD-JWT)", Work in Progress, Internet-
              Draft, draft-ietf-oauth-selective-disclosure-jwt-17, 1
              March 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-oauth-selective-disclosure-jwt-17>.

   [I-D.ietf-spice-sd-cwt]
              Prorock, M., Steele, O., Birkholz, H., and R. Mahy, "SPICE
              SD-CWT", Work in Progress, Internet-Draft, draft-ietf-
              spice-sd-cwt-02, 4 December 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spice-
              sd-cwt-02>.

   [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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [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>.

   [RFC9420]  Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
              Omara, E., and K. Cohn-Gordon, "The Messaging Layer
              Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
              July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.

7.2.  Informative References

   [I-D.mahy-mls-member-secrets]
              "*** BROKEN REFERENCE ***".

Acknowledgments

   Thanks to Richard Barnes for his comment.

Author's Address

   Rohan Mahy
   Rohan Mahy Consulting Services
   Email: rohan.ietf@gmail.com

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