JWT Basics — Quick Start Guide

What Is a JWT?

A JSON Web Token (JWT) is a compact, URL-safe string used to transmit authenticated claims between parties. It is the foundation of modern API authentication and OAuth 2.0 flows.

Token Structure

Every JWT has three parts separated by dots:

eyJhbGci....  .  eyJzdWI....  .  SflKxwRJSM....
   HEADER          PAYLOAD         SIGNATURE

Essential Claims

• sub — Subject (user ID)
• exp — Expiration timestamp
• iss — Issuer
• aud — Audience
• iat — Issued at

Try It Yourself

Paste any token into our JWT Decoder to see these parts decoded instantly.

Continue to JWT Authentication Explained for the full picture.

Understanding JWT Basics — Quick Start Guide in Production

Developers search for JWT Basics — Quick Start Guide when building API authentication with JSON Web Tokens. JWTs are used by OAuth 2.0, OpenID Connect, Auth0, Firebase, AWS Cognito, and Keycloak. Always validate exp, iss, and aud server-side — decoding alone proves nothing about authenticity.

JWT Structure Recap

Every JWT has three dot-separated segments: header (algorithm), payload (claims), signature (proof). Use JWT Decoder to inspect and JWT Validator to verify before trusting any claim value in production code.

Common Pitfalls

• Algorithm confusion (none attack) — whitelist allowed algorithms
• Secrets in the payload — payload is only Base64-encoded, not encrypted
• Ignoring clock skew on exp and nbf
• Weak HMAC secrets — use 256-bit random keys
• Skipping signature verification — always call verify(), not decode()
• Storing tokens in localStorage — XSS can steal them

Further Reading

Browse related resources: JWT Decoder, JWT Validator, JWT Basics, JWT Authentication, JWT Errors, Algorithms, Glossary, and Learning Path.