auth.mom / OAuth 2.0 and OpenID Connect: A Practical Guide

OAuth 2.0 and OpenID Connect: A Practical Guide

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OAuth 2.0: Authorization, Not Authentication

OAuth 2.0 is commonly described as an authentication protocol, but that is technically incorrect and the distinction matters. OAuth 2.0 is an authorization framework — it allows an application to obtain limited access to a user's resources without knowing the user's credentials. The user authenticates directly with an authorization server (e.g., Google); the application receives an access token that it can use to call APIs on the user's behalf.

OpenID Connect (OIDC) is the authentication layer built on top of OAuth 2.0. It adds an ID token — a JSON Web Token containing claims about the authenticated user — and a standard way to retrieve user profile information. When you "Sign in with Google," OIDC is the protocol making it work.

The Authorization Code Flow

The most secure and widely recommended flow for web applications:

1
User clicks "Sign in with [Provider]"
2
App redirects to the authorization server with client ID, requested scopes, redirect URI, state, and PKCE code challenge
3
User authenticates with the provider and consents to the requested scopes
4
Authorization server redirects back with a short-lived authorization code
5
App server exchanges the code (plus client secret + PKCE verifier) for access token, refresh token, and ID token
6
App validates the ID token signature and reads the user's claims
⚠️ PKCE should always be used, even for confidential clients, to prevent authorization code interception attacks.

ID Token Claims

sub
Subject — unique user ID
email
User's email address
email_verified
Boolean — verified by provider
iss
Issuer URL
aud
Audience — your client ID
exp
Expiration Unix timestamp
iat
Issued-at Unix timestamp
name
Display name

These claims are signed by the authorization server — your application can verify the signature and trust the contents without a round-trip to the server.

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