Internet DRAFT - draft-barnes-mls-addl-creds
draft-barnes-mls-addl-creds
Messaging Layer Security R. Barnes
Internet-Draft S. Nandakumar
Intended status: Informational Cisco
Expires: 5 September 2024 4 March 2024
Additional MLS Credentials
draft-barnes-mls-addl-creds-01
Abstract
This specification defines two new kinds of credentials for use
within the Message Layer Security (MLS) credential framework:
UserInfo Verifiable Credentials and multi-credentials. UserInfo
Verifiable Credentials allow clients to present credentials that
associate OpenID Connect attributes to a signature key pair held by
the client. Multi-credentials allow clients to present authenticated
attributes from multiple sources, or to present credentials in
different formats to support groups with heterogeneous credential
support.
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-barnes-mls-addl-creds/.
Discussion of this document takes place on the Messaging Layer
Security Working Group mailing list (mailto:mls@ietf.org), which is
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Source for this draft and an issue tracker can be found at
https://github.com/bifurcation/mls-userinfo-vc.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. UserInfo Verifiable Credentials . . . . . . . . . . . . . . . 3
3.1. UserInfo VC Life-Cycle . . . . . . . . . . . . . . . . . 4
3.2. UserInfoVC . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Credential Validation . . . . . . . . . . . . . . . . . . 7
3.4. Mapping between JWK Key Types and MLS Ciphersuites . . . 8
4. Multi-Credentials . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Credential Bindings . . . . . . . . . . . . . . . . . . . 9
4.2. Verifying a Multi-Credential . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7.1. MLS Credential Types . . . . . . . . . . . . . . . . . . 11
8. Normative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
MLS provides end-to-end authenticated key exchange
[I-D.ietf-mls-protocol]. 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.
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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.
2. 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.
3. UserInfo Verifiable Credentials
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 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.
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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
[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.
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.
3.1. UserInfo VC Life-Cycle
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+----+
| | (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
Figure 1: The protocol interactions to issue and verify a UserInfo VC
The basic steps showing OIDC Verifiable Credential based MLS
credential flow are shown in Figure 1.
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.
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.
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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.
3.2. UserInfoVC
A new credential type UserInfoVC is defined as shown below. This
credential type is indicated with the CredentialType value
userinfo_vc (see Section 7).
struct {
opaque jwt<0..2^32-1>;
} UserInfoVC;
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The jwt 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 vc.credentialSubject.id contains a did:jwk
URI describing the subject's public key as a JWK.
3.3. 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.
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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.
3.4. 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
4. 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 Section 7).
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struct {
CipherSuite cipher_suite;
Credential credential;
SignaturePublicKey credential_key;
/* SignWithLabel(., "CredentialBindingTBS", CredentialBindingTBS) */
opaque signature<V>;
} CredentialBinding
struct {
CredentialBinding bindings<V>;
} MultiCredential;
struct {
CredentialBinding bindings<V>;
} 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.
4.1. 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.
struct {
CipherSuite cipher_suite;
Credential credential;
SignaturePublicKey credential_key;
SignaturePublicKey signature_key;
} CredentialBindingTBS;
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.
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4.2. 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.
5. 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.
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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 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 |
+--------+-------------+-------------+-----------+
Table 2
8. 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-12, 3 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
architecture-12>.
[I-D.ietf-mls-protocol]
Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", Work in Progress, Internet-
Draft, draft-ietf-mls-protocol-20, 27 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
protocol-20>.
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[OpenIDUserInfoVC]
Ansari, M., Barnes, R., Kasselman, P., and K. Yasuda,
"OpenID Connect UserInfo Verifiable Credentials 1.0", 15
December 2022, <https://openid.net/specs/openid-connect-
userinfo-vc-1_0.html>.
Authors' Addresses
Richard Barnes
Cisco
Email: rlb@ipv.sx
Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
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