Transaction Token Authorization Grant Profile for OAuth Identity and Authorization Chaining

This specification defines a profile of the OAuth Identity and Authorization Chaining Across Domains [I-D.ietf-oauth-identity-chaining] mechanism that uses a Transaction Token (Txn-Token) [I-D.ietf-oauth-transaction-tokens] as the subject token in a Token Exchange [RFC8693] request to obtain a JWT Authorization Grant for crossing a trust boundary. A Txn-Token is scoped to a single trust domain and represents the full authorization context of an in-progress transaction, regardless of whether that transaction was initiated by a human user calling an external API, by an internal system event, or by an automated workload. This profile specifies how a service operating within that trust domain can present its Txn-Token to obtain a JWT Authorization Grant that carries the necessary context across a trust boundary, enabling an access token to be issued for a partner service, without exposing internal trust-domain credentials or token formats beyond the trust boundary.