Raw-Public-Key and Pre-Shared-Key as OAuth client credentials

This document describes Transport Layer Security (TLS) authentication using Raw-Public-Key and Pre-Shared-Key as the mechanism for OAuth client authentication. Examples of endpoint requiring client authentication are token and introspection. The OAuth 2.0 Authorization Framework defines a shared secret method of client authentication but also allows for the definition and use of additional client authentication mechanisms when interacting with the authorization server's token endpoint. This document describes two additional mechanisms of client authentication utilizing Raw-Public-Key and Pre-Shared-Key TLS , which provide better security characteristics than shared secrets. To get most bennefits and improved security with these new client credential types it is recomended to use the 'one credential per Client Software Instance' paradigm. This can be achived by letting the client dynamicly register as described in .

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

The following section defines, as an extension of OAuth 2.0, Section 2.3, using Pre-Shared-Key with TLS to authenticate the client. This method is registered as 'tls_client_psk' in "OAuth Token Endpoint Authentication Methods" registry. If this method is to be used, the client and the Authorization Server MUST share a secret key, and they MUST agree on an identifier for this key. The (D)TLS handshake MUST be done according to , with the client indicating support for one or more Pre-Shared-Key cipher suites and authorization server selecting a Pre-Shared-Key cipher suite. In order to enable the authorization server to select the correct pre-shared-key the client MUST send the key identifier in the psk-identity field of the ClientKeyExchange message. How the authorization server maps the identifier to a pre-shared-key, and to a specific client is out of scope for this specification. Note that the key identifier MUST be 2^16 bytes or shorter, in order to fit into the psk-identity field.

The following section defines, as an extension of OAuth 2.0, Section 2.3, the use of Raw-Public-Key with (D)TLS to authenticate the client. This method is registered as 'tls_client_rpk' in "OAuth Token Endpoint Authentication Methods" registry. The (D)TLS handshake MUST be done according to , with the client indicating support for Raw-Public-Key certificates and the authorization server asking client send its Raw Public Key certificate. Since the client cannot send an explicit client or key identifier in the handshake, the authorization server MUST derive a client identifier from RPK that the client uses. Note to implementers: Authorization servers can use the following method to map a Raw Public Key to a client identifier: The client identifier is generated from the Raw Public Key using the procedure specified in section 3 of. The digest is calculated on the Raw Public Key only (not on the SubjectPublicKeyInfo used in the handshake). An example is shown in .

For dynamic registration of a RPK this specification registers the new parameter 'rpk' to the Client Registration Metadata Registry. When used this parameter MUST contain a JSON Web Key representing the public key of the client. When 'rpk' is present in the registration request 'token_endpoint_auth_method' MUST include 'tls_client_rpk'. For dynamic registration of a PSK this specification registers the new parameter 'psk' to the Client Registration Metadata Registry. When used this parameter MUST contain a JSON Web Key representing the key of the client. When registering the client can include the key in the registrations request or the authorisation can generate the key and return it. If the 'psk' attribute is present in a request 'token_endpoint_auth_method' MUST include 'tls_client_psk'. To request the authorisation server to generate the key the client includes 'tls_client_psk' in 'token_endpoint_auth_method' but does not send 'psk' attribute. The 'jwks' and 'jwks_uri' is not used to avoid conflict and confusion with application layer keys.

This document is highly inspired by written by B. Campbell, J. Bradley, N. Sakimura and T. Lodderstedt.

This specification requests registration of the following value in the IANA "OAuth Dynamic Client Registration Metadata" registry established by .

Client Metadata Name: rpk Client Metadata Description: JWK for client Raw-Public-Key, can be included in request. Change Controller: IESG Specification Document(s): [[ this specification ]] Client Metadata Name: psk Client Metadata Description: JWK for client Pre-Shared-Key, can be included both in request and response. Change Controller: IESG Specification Document(s): [[ this specification ]]

This specification requests registration of the following value in the IANA "OAuth Token Endpoint Authentication Methods" registry established by .

Token Endpoint Authentication Method Name: tls_client_rpk Change Controller: IESG Specification Document(s): [[ this specification ]] Token Endpoint Authentication Method Name: tls_client_psk Change Controller: IESG Specification Document(s): [[ this specification ]]

TBD