Additional MLS Credentials

Introduction MLS provides end-to-end authenticated key exchange . Each client participating in an MLS group is authenticated with a credential. The MLS credential structure is extensible: New MLS credential formats can be defined which support new mechanisms for authenticating clients. In this document, we define two new types of credential:

Credentials based on OpenID Connect UserInfo Verifiable Credentials
Multi-credentials that present several credentials at once

UserInfo Verifiable Credentials (VCs) are a mechanism by which an OpenID Provider can bind user attributes to a signature key pair. OpenID Connect is already widely deployed as a mechanism for connecting authentication services to applications, and the OpenID Foundation is in the process of standardizing the extensions required for OpenID Providers to issue UserInfo VCs. Multi-credentials address use cases where there might not be a single credential that captures all of a client's authenticated attributes. For example, an enterprise messaging client may wish to provide attributes both from its messaging service, to prove that its user has a given handle in that service, and from its corporate owner, to prove that its user is an employee of the corporation. Multi-credentials can also be used in migration scenarios, where some clients in a group might wish to rely on a newer type of credential, but other clients haven't yet been upgraded.

Terminology 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 RFC 2119 [RFC2119]. 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 pair that the client will use to sign messages in the MLS key exchange protocol.

UserInfo Verifiable Credentials As described in the MLS architecture, MLS requires an Authentication Service (AS) as well as a Delivery Service (DS) . The full security goals of MLS are only realized if the AS and DS are non-colluding. In other words, 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 use in MLS. 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 . 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. A "UserInfoVC" credential encapsulates a UserInfo Verifiable Credential object, 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.

UserInfo VC Life-Cycle

The protocol interactions to issue and verify a UserInfo VC | | | | | | | | | | | |-------(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 in . Client 1 is an MLS client that acts as a Holder in the VC model. Client 2 is also an MLS client, but acts in the Verifier role in the VC model. Both clients implement certain OpenID Connect operations to obtain and verify UserInfo VC objects.

Client 1 generates a signature key pair using an algorithm that is supported by both MLS and UserInfo VC.
Client 1 performs an OpenID Connect login interaction with the scope "userinfo_credential" to obtain UserInfo VCs.
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.
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.
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.
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.
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.
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.

UserInfoVC A new credential type UserInfoVC is defined as shown below. This credential type is indicated with the CredentialType value userinfo_vc (see ). ; } UserInfoVC; ]]> The jwt field contains the signed JWT-formatted UserInfo VC object (as defined in ), 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 vc.credentialSubject.id contains a did:jwk URI describing the subject's public key as a JWK.

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 [@!OpenID.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 openid-userinfo-vc.
- 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.

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

Multi-Credentials New credential types MultiCredential and WeakMultiCredential are defined as shown below. These credential types are indicated with CredentialType values multi and weak-multi (see ). ; } CredentialBinding struct { CredentialBinding bindings; } MultiCredential; struct { CredentialBinding bindings; } WeakMultiCredential; ]]> The two types of credentials are processed in exactly the same way. The only difference is in how they are treated when evaluating support by other clients, as discussed below.

Credential Bindings A multi-credential consists of a collection of "credential bindings". Each credential binding is a signed statement by the holder of the credential that the signature key in the LeafNode belongs to the holder of that credential. Specifically, the signature is computed using the MLS SignWithLabel function, with label "CredentialBindingTBS" and with a content that covers the contents of the CredentialBinding, plus the signature_key field from the LeafNode in which this credential will be embedded. The cipher_suite for a credential is NOT REQUIRED to match the cipher suite for the MLS group in which it is used, but MUST meet the support requirements with regard to support by group members discussed below.

Verifying a Multi-Credential A credential binding is supported by a client if the client supports the credential type and cipher suite of the binding. A credential binding is valid in the context of a given LeafNode if both of the following are true:

The credential is valid according to the MLS Authentication Service.
The credential_key corresponds to the specified credential, in the same way that the signature_key would have to correspond to the credential if the credential were presented in a LeafNode.
The signature field is valid with respect to the signature_key value in the leaf node.

A client that receives a credential of type multi in a LeafNode MUST verify that all of the following are true:

All members of the group support credential type multi.
For each credential binding in the multi-credential:
- Every member of the group supports the cipher suite and credential type values for the binding.
- The binding is valid in the context of the LeafNode.

A client that receives a credential of type weak-multi in a LeafNode MUST verify that all of the following are true:

All members of the group support credential type multi.
Each member of the group supports at least one binding in the multi-credential. (Different members may support different subsets.)
Every binding that this client supports is valid in the context of the LeafNode.

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

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.

IANA Considerations

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

Value	Name	Recommended	Reference
0x0003	userinfo-vc	Y	RFC XXXX
0x0004	multi	Y	RFC XXXX
0x0005	weak-multi	Y	RFC XXXX