JWT + OAuth2 + OIDC + PKCE Complete small Guide

The flow will be: Authentication foundation Session vs JWT JWT deep dive JWT security Access/Refresh tokens OAuth2 relationship with JWT End-to-end production flow PKCE Storage strategies summary 1. Authentication Fundamentals Every secure application needs answers to two questions: Authentication "Who are you?" Example: User enters: username password MFA System verifies identity. Result: User is Bhargav Authorization "What are you allowed to do?" Example: User: Bhargav Permissions: READ_ORDERS CREATE_ORDER DELETE_ORDER Authentication happens first. Authorization happens after. Authentication | v Authorization 2. Traditional Session-Based Authentication (Stateful) Before JWT, applications commonly used sessions. Flow User logs in: Browser | | username/password | v Server Server creates: Session ID = abc123 Stores: Database / Memory abc123 | | User: Bhargav Role: ADMIN Browser receives: Cookie: SESSION_ID=abc123 Every Request Browser sends: GET /orders Cookie: SESSION_ID=abc123 Server: Receive Session ID | v Search session storage | v Find user | v Allow request Problems with Sessions 1. Server maintains state The server must remember: Session ID | v User Information 2. Scaling problem Imagine multiple servers: Load Balancer / \ Server A Server B User logs in: Server A Session stored here Next request: Server B No session found Solutions: Sticky sessions Shared session database 3. JWT Authentication (Stateless) JWT solves this by putting information inside the token. JWT: JSON Web Token It is a compact, signed representation of claims between two parties. Example: eyJhbGciOiJIUzI1Ni... JWT vs Session Session Server stores user state: Server Session ID | v User Data JWT Token contains information: JWT Header + Payload + Signature Server does not need to store session information. 4. JWT Structure A JWT has three parts: HEADER.PAYLOAD.SIGNATURE Example: xxxxx.yyyyy.zzzzz Part 1: Header Contains metadata. Example: { "alg" : "RS256" , "typ" : "JWT" } Meaning: JWT uses RS