JWT Decoder: Understanding JSON Web Tokens

JSON Web Tokens (JWT) are the standard for API authentication and authorization. Understanding how to decode and validate JWTs is essential for debugging authentication issues and securing your applications.

What is This?

A JWT is a compact, URL-safe token composed of three Base64-encoded parts separated by dots: Header.Payload.Signature. The header contains token type and algorithm, the payload contains claims (user data), and the signature verifies authenticity.

How to Decode JWT Tokens

1. Copy the JWT token from your application (usually from Authorization header)
2. Paste the token into the JWT decoder
3. View the decoded header (algorithm, token type)
4. View the decoded payload (claims: user ID, expiration, etc.)
5. Verify the signature (optionally provide secret key)
6. Check expiration time and validate claims

Benefits

• Debug Authentication: Understand why tokens are invalid or expired
• Inspect Claims: See user data, roles, and permissions encoded in token
• Verify Expiration: Check if tokens are still valid
• Security Analysis: Identify weak signing algorithms (avoid HS256 with weak secrets)
• API Troubleshooting: Debug 401 Unauthorized errors by examining tokens

Common Use Cases

Authentication Debugging

When users report login issues, decode their JWT to check expiration (exp claim), issuer (iss), and audience (aud). Verify the token hasn't expired and contains expected claims. Common issue: clock skew between servers causes premature expiration.

API Integration

When integrating with third-party APIs using OAuth2/OIDC, decode access tokens and ID tokens to understand what data is available. ID tokens contain user profile (email, name), access tokens contain scopes (read, write).

Role-Based Access Control

JWTs often contain roles or permissions in custom claims. Decode tokens to verify users have correct roles: {roles: ["admin", "user"]}. Backend should always validate roles, never trust client-side claims.

Single Sign-On (SSO)

SSO systems like Okta, Auth0, and Azure AD use JWTs. Decode tokens to troubleshoot SSO integration issues, verify user attributes, and understand how identity flows between applications.

Tips & Tricks

• Never trust JWT claims blindly: Always verify signature on the server
• Use strong secrets: HS256 requires strong secrets (32+ random characters)
• Prefer RS256: RSA public/private key signing is more secure than HMAC
• Short expiration times: Use 15-60 minute expiration with refresh tokens
• Validate all claims: Check exp (expiration), nbf (not before), iss (issuer), aud (audience)
• Store tokens securely: Use httpOnly cookies or memory, never localStorage for sensitive tokens

Conclusion

JWT tokens are ubiquitous in modern web authentication, from OAuth2 to API keys. Understanding JWT structure and being able to decode tokens is essential for debugging authentication issues and building secure applications. Our JWT decoder instantly reveals header, payload, and signature verification status. Remember: JWTs are signed but not encrypted - sensitive data in JWTs is visible to anyone with the token.