Understanding JWT Tokens: Structure, Claims, and Security

JSON Web Tokens (JWT) have become the standard for secure information transmission between parties. Understanding their structure and proper usage is crucial for modern web development.

JWT Structure

A JWT consists of three parts separated by dots:

1. Header: Algorithm and token type
2. Payload: Claims (data)
3. Signature: Verification hash

Common JWT Claims

Registered Claims

• iss (Issuer): Who created the token
• sub (Subject): Who the token is about
• aud (Audience): Who the token is for
• exp (Expiration): When the token expires
• iat (Issued At): When the token was created

Private Claims

Custom claims like user roles, permissions, or any application-specific data.

JWT Security Best Practices

1. Always verify signatures: Never trust unverified tokens
2. Check expiration: Reject expired tokens
3. Use HTTPS: Protect tokens in transit
4. Keep secrets safe: Never expose signing keys
5. Set reasonable expiration: Balance security and usability

When to Use JWT

• Stateless authentication
• Single sign-on (SSO)
• Information exchange
• Authorization

Our JWT Decoder tool helps you inspect tokens during development and debugging.