RFC 8414 - OAuth 2.0 Authorization Server Metadata endpoint.

Serves the discovery document at /.well-known/oauth-authorization-server (RFC 8414 §3) so that clients can discover the issuer, the endpoint URLs, and the capabilities the authorization server advertises.

The document is assembled by Attesto.Discovery.metadata/2; this controller contributes transport concerns only and adds no policy of its own. Every protocol member - the issuer, the token endpoint (token_endpoint), the JWKS location (jwks_uri), the PKCE challenge methods (code_challenge_methods_supported, fixed to S256 per RFC 7636 §4.2), and the DPoP algorithms (dpop_signing_alg_values_supported, RFC

9449. • is derived by the core builder from the protocol configuration.

The capability members reflect exactly what the server supports: grant_types_supported is read from AttestoPhoenix.Config.grant_types_supported/1 (every grant the token endpoint dispatches by default — authorization_code, refresh_token, client_credentials, and OAuth token exchange — narrowed when the host configures :grant_types_supported, and the token endpoint enforces the same set; enabled optional grants are added only when that setting is nil, while an explicit list must contain their exact URNs); token_endpoint_auth_methods_supported lists the client-authentication methods it accepts. Its default is client_secret_basic, client_secret_post, private_key_jwt, and none for PKCE-using public clients; an explicit config list may add mTLS or wallet-attestation methods. The PAR endpoint is advertised separately as pushed_authorization_request_endpoint.

The host-specific members - the supported scopes (scopes_supported), the authorization endpoint, and the dynamic registration endpoint (registration_endpoint, RFC 7591, advertised only when registration is enabled) - are read from AttestoPhoenix.Config and passed through, never hardcoded here.

The response carries no secrets and is identical for every caller, so it is served unauthenticated. RFC 8414 §3.1 permits caching of the metadata response, so a public, cacheable Cache-Control header is set.

Wiring

The router pipeline must place the AttestoPhoenix.Config under conn.private[:attesto_phoenix_config] (the same key the other endpoints read) and the derived Attesto.Config under conn.private[:attesto_protocol_config]. Both are required; a missing value raises rather than serving a partial document, because a partial discovery document would misdirect clients to endpoints that may not exist.