OIDC Authorization Code Flow Explained With Sequence Diagrams

The OpenID Connect (OIDC) authorization code flow is a secure authentication sequence that enhances the OAuth 2.0 authorization framework by adding identity verification, primarily using ID tokens. This flow involves multiple steps: discovery to locate necessary endpoints, an authorization request that redirects users for login, and a server-to-server token exchange that uses Proof Key for Code Exchange (PKCE) to securely swap a short-lived authorization code for tokens. PKCE is now a mandatory component to prevent intercepted codes from being misused, as it binds the code to the client that requested it. The flow avoids exposing tokens in the browser URL, enhancing security by using parameters like state and nonce to prevent cross-site request forgery (CSRF) and replay attacks. The ID token, a signed JSON Web Token (JWT), confirms user identity, while the access token authorizes API calls, with each requiring distinct validation steps. This robust flow is recommended over the deprecated implicit flow, ensuring confidentiality and integrity of sensitive information across web and native applications.

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