RFC 7521 Deep Dive: Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants

Introduction In OAuth 2.0, client authentication typically uses a client_id and client_secret pair (or PKCE). For obtaining access tokens, common authorization grants include the "Authorization Code Grant" involving user authentication, and the "Client Credentials Grant" used for batch processing. However, in actual enterprise environments or complex system integrations, the following requirements may arise: "We already have a robust authentication infrastructure using SAML or JWT within the company. Can we leverage this for OAuth 2.0 client authentication?" "The user is offline, and the server (client) wants to act on the user's behalf. Can we get an access token using a pre-approved 'assertion' without handing over a password?" "I don't want to send 'shared secrets' like client secrets over the network. I want to authenticate with a token signed using public-key cryptography." RFC 7521 (Assertion Framework for OAuth 2.0 Client Authentication and Authorization Grants) was created to s