For Proof of Possession OAuth2 tokens, in addition to the access token, what must be presented to the authorization server?

For Proof of Possession (PoP) using mutual TLS (mTLS), as defined in RFC 8705, the client must authenticate itself to the authorization server using its X.509 certificate. During the TLS handshake, the client presents its certificate, and the authorization server can then bind the issued access token to that specific client certificate. This ensures that only the client in possession of the corresponding private key can use the token, preventing it from being used as a simple bearer token if stolen.

A. Nonce: A nonce is a cryptographic value used to prevent replay attacks, but it is not the primary credential for PoP in this context.

B. Client JSON Web Key (JWK): While a JWK can be used in other PoP mechanisms (e.g., DPoP), for mTLS, the standard X.509 client certificate is the required credential.

C. State: The state parameter is used during the authorization code flow to mitigate Cross-Site Request Forgery (CSRF) and is not related to PoP token presentation.

1. Ping Identity AM Documentation, "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens". This section explains that for mTLS, the client authenticates with a certificate during the TLS handshake when requesting a token.

2. Internet Engineering Task Force (IETF), RFC 8705, "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens", Section 2.1 "Mutual-TLS for Client Authentication". This section specifies that the client authenticates by presenting its X.509 certificate.