JWT Decoder

JWT Decoder Live

What is a JWT?

A JSON Web Token (JWT) is a compact, URL-safe token format used to securely transmit information between parties. A JWT consists of three Base64URL-encoded parts separated by dots: a Header (algorithm and token type), a Payload (claims/data), and a Signature (used to verify integrity). JWTs are widely used in authentication and authorization flows — when you log in to a web application, the server often returns a JWT that your browser sends with every subsequent request.

JWT Structure

• Header: Contains metadata — typically the signing algorithm (alg) such as HS256, RS256, or ES256, and the token type (typ), which is always JWT.
• Payload: Contains claims — statements about the user or session. Registered claims include sub (subject), iss (issuer), aud (audience), exp (expiry), iat (issued at), and nbf (not before). Custom claims can be any key-value pair.
• Signature: Created by signing the encoded header and payload with a secret or private key. It ensures the token hasn't been tampered with. For HMAC algorithms (HS256/384/512), the same secret is used to sign and verify. For RSA/ECDSA (RS256, ES256), a private key signs and the public key verifies.

Why Not Verify in the Browser?

Signature verification requires the secret key (for HMAC) or the private key (for asymmetric algorithms). Exposing these keys in browser JavaScript would completely defeat the purpose of signing — any attacker could read the key from the browser's developer tools. Verification must always happen on a trusted server. This tool safely decodes (not verifies) the JWT, which is useful for debugging, inspecting claims, and checking expiry without needing the key.

Common JWT Algorithms

• HS256/HS384/HS512: HMAC with SHA-256/384/512. Symmetric — same key signs and verifies. Fast and simple, but the secret must be shared between parties.
• RS256/RS384/RS512: RSA with SHA-256/384/512. Asymmetric — private key signs, public key verifies. Common in enterprise SSO and OAuth 2.0.
• ES256/ES384/ES512: ECDSA with SHA-256/384/512. Asymmetric like RSA but with much smaller keys. Preferred for mobile and IoT.
• PS256/PS384/PS512: RSA-PSS — similar to RSA but with probabilistic padding for extra security.
• none: No signature. Only acceptable for testing — never use in production.