JWT Decoder
Decode and inspect JSON Web Tokens instantly
JSON Web Token (JWT)
About JWT Decoder
JSON Web Tokens (JWT) are an open standard (RFC 7519) for securely transmitting information between parties as a JSON object.
Features:
- Instant client-side decoding - your tokens never leave your browser
- View header and payload contents in formatted JSON
- Copy decoded data with one click
- Real-time validation and error messages
Note: This tool only decodes the JWT. It does not validate the signature. JWTs can be decoded by anyone, but only verified with the secret key.
Understanding JWT Tokens
What are JWT Tokens?
JSON Web Tokens (JWT) are a compact, URL-safe means of representing claims to be transferred between two parties. They are digitally signed, which ensures the integrity of the information and allows the receiver to verify the sender's authenticity. JWTs are commonly used for authentication and information exchange in modern web applications.
JWT Structure
A JWT consists of three parts separated by dots (.), each encoded in Base64:
1. Header
Contains the token type (JWT) and the signing algorithm (e.g., HMAC SHA256 or RSA).
{ "alg": "HS256", "typ": "JWT" }2. Payload
Contains the claims (statements about an entity and additional data). Common claims include sub (subject), iat (issued at), exp (expiration time).
{ "sub": "user123", "name": "John Doe", "iat": 1516239022 }3. Signature
Created by encoding the header and payload, then signing with a secret key. This ensures the token hasn't been tampered with.
HMACSHA256(base64UrlEncode(header) + "." + base64UrlEncode(payload), secret)Common Use Cases
- Authentication: Once a user logs in, subsequent requests include the JWT, allowing access to protected resources without re-authenticating.
- Information Exchange: Securely transmit information between parties, as the signature verifies the sender's identity and data integrity.
- Single Sign-On (SSO): Share authentication across multiple applications and domains.
- API Authorization: Grant access to specific API endpoints based on token claims.
- Stateless Sessions: Store session information client-side, reducing server memory usage.
Security Considerations
- JWTs are not encrypted by default - Anyone can decode and read the header and payload. Never store sensitive data like passwords in JWTs.
- Always use HTTPS - Transmit JWTs only over secure connections to prevent interception.
- Set appropriate expiration times - Use short-lived tokens (exp claim) to minimize risk if a token is compromised.
- Validate signatures - Always verify the token's signature on the server before trusting its contents.
- Use strong secret keys - For HMAC algorithms, use long, random secret keys. For RSA, use appropriate key lengths (2048+ bits).
- Implement token refresh mechanisms - Use refresh tokens for long-term sessions instead of extending JWT expiration indefinitely.
Best Practices
Keep payloads small: Minimize data in tokens as they're sent with every request.
Use standard claims: Leverage registered claims like iss (issuer), sub (subject), aud (audience) for interoperability.
Implement token revocation: Maintain a blacklist or use short expiration times with refresh tokens.
Store tokens securely: Use httpOnly cookies or secure browser storage, avoiding localStorage for sensitive applications.
Monitor and log: Track token usage patterns and detect anomalies that might indicate security breaches.
Use appropriate algorithms: Prefer RS256 (RSA) for public/private key pairs in production environments over HS256 when possible.