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UserInfo Verifiable Credentials as MLS Credentials

Document Type Active Internet-Draft (individual)
Authors Richard Barnes , Suhas Nandakumar
Last updated 2023-03-13
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Intended RFC status (None)
Stream Stream state (No stream defined)
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Messaging Layer Security                                       R. Barnes
Internet-Draft                                             S. Nandakumar
Intended status: Informational                                     Cisco
Expires: 14 September 2023                                 13 March 2023

           UserInfo Verifiable Credentials as MLS Credentials


   This specification extends Message Layer Security (MLS) credentials
   framework with a new credential type, "UserInfoVC", based on the
   OpenID Connect UserInfo Verifiable Credential type
   "UserInfoCredential".  A UserInfo Verifiable Credential encapsulates
   the UserInfo claims from the OpenID provider as a Verifiable
   Credential that can be presented to a third-party Verifier.  These
   credentials can be easily provisioned to MLS clients using the OpenID
   Connect login flows, augmented with type "UserInfoCredential".  The
   credential itself is an object associating identity attributes to the
   signature public key that the client will use in MLS, signed by the
   OpenID Provider.  In situations where the OpenID Provider is distinct
   from the MLS Delivery Service, these credentials provide end-to-end
   secure identity assurance.

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Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
     2.1.  Terminology
   3.  Concept
   4.  UserInfoVC
     4.1.  Credential Validation
     4.2.  Mapping between JWK Key Types and MLS Ciphersuites
   5.  Security Considerations
   6.  Privacy Considerations
   7.  IANA Considerations
     7.1.  MLS Credential Type
   8.  Normative References
   Authors' Addresses

1.  Introduction

   MLS provides end-to-end authenticated key exchange [@!I-D.ietf-mls-
   protocol].  As described in the MLS architecture, MLS requires an
   Authentication Service (AS) as well as a Delivery Service (DS) [@!I-
   D.ietf-mls-architecture].  The full security goals of MLS are only
   realized if the AS and DS are non-colluding.  In other worlds,
   applications can deploy MLS to get end-to-end encryption (acting as
   MLS Delivery Service), but they need to partner with a non-colluding
   Authentication Service in order to achieve full end-to-end security.

   OpenID Connect is widely used to integrate identity providers with
   applications, but its current core protocol doesn't provide the
   binding to cryptographic keys required for end-to-end security.  When
   OpenID Connect is coupled with the "Verifiable Credentials"
   framework, however, it can be used to provision clients with signed
   "UserInfo VC" objects that contain the critical elements of a
   credential to be used in MLS:

   *  Identity attributes for the user of a client

   *  A public key whose private key is held by a client

   *  A signature over the above by a trusted identity provider

   The required updates to OpenID Connect are specfied in
   [OpenIDUserInfoVC].  That document defines a profile of the OpenID
   for Verifiable Credential Issuance protocol for issuing "UserInfo
   Verifiable Credentials".  These credentials bind a signature key pair
   to the user attributes typically exposed through the OpenID Connect
   UserInfo endpoint.

   In this document, we describe a "UserInfoVC" credential type for MLS
   that encapsulates a signed UserInfo object as Verifiable Credential,
   so that it can be used for authenticating an MLS client.  We also
   describe the validation process that MLS clients use to verify
   UserInfoVC objects that they receive via MLS.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

2.1.  Terminology

   This specification uses terms from the MLS Protocol specification.
   In particular, we refer to the MLS Credential object, which
   represents an association between a client's identity and the
   signature key that the client will use to messages in the MLS key
   exchange protocol.

3.  Concept

      |    | (1) Generate signature key pair
      |    V
   +----------+                                   +----------+
   |          |<~~~(2) OpenID Connect Login~~~~~~>|          |
   |          |                                   |          |
   |          |                                   |          |
   |          |-------(3) Credential Request----->|  OpenID  |
   | Client 1 |     (type=UserInfoCredential,     | Provider |
   |          |      token & proof)               |   (OP)   |
   |          |                                   |          |
   |          |<------(4) Credential Response-----|          |
   |          |         (credential)              |          |
   +----------+                                   +----------+
         :                                             ^
         : (5) UserInfoVC in MLS KeyPackage            |
         :                                             |
         v                                             |
   +----------+                                        |
   |          |                                        |
   |          | (6) Fetch JWK set, Verify JWT          |
   |          |        Signature                       |
   | Client 2 |<----------------------------------------
   |          |----+
   |          |    | (7) Validate vc claim using
   |          |<---+     OP's JWK

               OpenID Connect UserInfo VC MLS Credential Flow

   The basic steps showing OIDC Verifiable Credential based MLS
   credential flow are shown above.

   Client 1 acts as an Holder (in the VC model) and as an MLS client.
   Client 2 is an MLS client and acts as Verifier (in the VC model) and
   implements certain OpenID Connect operations that enable it to verify
   signed UserInfo VC objects.

   1.  Client 1 generates a signature key pair using an algorithm that
       is supported by both MLS and UserInfo VC.

   2.  Client 1 performs an OpenID Connect login interaction with the
       scope "userinfo_credential" to obtain UserInfo VCs.

   3.  Client 1 sends a Credential Request specifying that it desires a
       UserInfo VC, together with a proof that it controls the private
       key of a signature key pair and the access token.

   4.  The OpenID Provider verifies the proof and create a Credential
       Response containing the UserInfo VC attesting the claims that
       would have been provided by the UserInfo endpoint and public key
       corresponding to the private key used to compute the proof in the
       Credential Request.

   5.  Client 1 generates a UserInfoVC MLS Credential object with the
       signed UserInfo VC JWT.  Client 1 embeds the UserInfoVC in an MLS
       KeyPackage object and signs the KeyPackage object with the
       corresponding private key.

   6.  Client 1 sends the KeyPackage to Client 2, e.g., by posting it to
       a directory from which Client 2 fetches it when it wants to add
       Client 1 to a group.

   7.  Client 2 verifies the signature on the KeyPackage and extracts
       the UserInfoVC credential.  Client 2 uses OpenID Connect
       Discovery to fetch the OpenID Provider's JWK set.

   8.  Client 2 verifies the signed UserInfo VC using the the
       appropriate key from the OpenID Provider's JWK set.

   If all checks pass, Client 2 has a high degree of assurance of the
   identity of Client 1.  At this point Client 1's KeyPackage (including
   the VerifiableCredential) will be included in the MLS group's ratchet
   tree and distributed to the other members of the group.  The other
   members of the group can verify the VerifiableCredential in the same
   way as Client 2.

4.  UserInfoVC

   A new credential type UserInfoVC is defined as shown below.  This
   credential type is indicated with CredentialType userinfo_vc (see
   Section 7).

   struct {
       opaque vc<0..2^32-1>;
   } UserInfoVC;

   The vc field contains the signed JWT-formatted UserInfo VC object (as
   defined in [OpenIDUserInfoVC]), encoded using UTF-8.  The payload of
   object MUST provide iss and vc claims.  The iss claim is used to look
   up the OpenID Provider's metadata.  The vc claim contains
   authenticated user attributes and a public key binding.
   Specifically, the field contains a did:jwk
   URI describing the subject's public key as a JWK.

4.1.  Credential Validation

   An MLS client validates a UserInfoVC credential in the context of an
   MLS LeafNode with the following steps:

   *  Verify that the jwt field parses successfully into a JWT
      [!@RFC7519], whose payload parses into UserInfo object as defined
      in Section 5.3.2 of [!@OpenID].

   *  Verify that an iss claim is present in the UserInfo VC payload and
      that "iss" value represents and issuer that is trusted according
      to the client's local policy.

   *  Verify the JWT signature:

      -  Fetch the issuer metadata using OIDC Discovery

      -  Use the jwks_uri field in the metadata to fetch the JWK set.

      -  Verify that the JWT signature verifies under one of the keys in
         the JWK set.

   *  Verify the key binding:

      -  Verify that a vc claim is present in the UserInfo VC payload.

      -  Verify that the value of the claim is a JSON object that
         contains a credentialSubject field, as defined in Section 4 of

      -  Verify id field exists and it MUST be a a Decentralized
         Identifier with DID method jwk (W3c.did-core).

      -  Verify that the jwk field parses as a JWK.

      -  Verify that the signature_key in the LeafNode matches the key
         in the id field.

   If all of the above checks pass, the client can use the signature key
   in the JWK for verifying MLS signatures using the signature scheme
   corresponding to the kty and crv parameters in the JWK.  The identity
   attributes in the JWT should be associated with the MLS client that
   presented the credential.

4.2.  Mapping between JWK Key Types and MLS Ciphersuites

   Below table maps JWK key types (kty) and elliptic curves (crv) to the
   equivalent MLS signature scheme.

             | kty |   crv   | TLS/MLS signature scheme     |
             |  EC |  P-256  | ECDSA with P-256 and SHA-256 |
             |  EC |  P-384  | ECDSA with P-384 and SHA-384 |
             |  EC |  P-521  | ECDSA with P-521 and SHA-512 |
             |  EC | Ed25519 | Ed25519                      |
             |  EC |  Ed448  | Ed448                        |

                                 Table 1

5.  Security Considerations

   The validation procedures specified verify that a JWT came from a
   given issuer.  It doesn't veirfy that the issuer is authorative for
   the claimed attributes.  The client needs to verify that the issuer
   is trusted to assert the claimed attributes.

6.  Privacy Considerations

   UserInfo can contain sensitive info such as human names, phone
   numbers, and using these credentials in MLS will expose this
   information to other group members, and potentially others if used in
   a prepublished KeyPackage.

7.  IANA Considerations

7.1.  MLS Credential Type

   IANA is requested to register add the following new entry to the MLS
   Credential Type registry.

            | Value  | Name        | Recommended | Reference |
            | 0x0003 | userinfo-vc | Y           | RFC XXXX  |

                                 Table 2

8.  Normative References

              Ansari, M., Barnes, R., Kasselman, P., and K. Yasuda,
              "OpenID Connect UserInfo Verifiable Credentials 1.0", 15
              December 2022, <


   TODO acknowledge.

Authors' Addresses

   Richard Barnes

   Suhas Nandakumar