JWT Decoder

JSON Web Tokens (JWTs) are compact, self-contained credentials used by modern web applications to authenticate users and authorize API requests. A JWT looks like a long random string, but it actually contains three parts encoded in Base64: a header describing the token type, a payload containing user information or claims, and a signature proving the token hasn't been tampered with. This tool decodes JWTs to reveal what information they carry, making it essential for debugging authentication issues and understanding token contents.

Structure of a JWT

A JWT consists of three Base64-encoded sections separated by dots: header.payload.signature. The header typically specifies the hashing algorithm (HS256, RS256, etc.). The payload contains "claims"—data the issuer asserts, such as user ID, email, roles, or expiration time. The signature proves a server created the token and it hasn't been modified. This tool automatically splits the JWT and decodes each part into readable JSON.

Debugging authentication

When your application fails to authenticate a user, you often need to inspect the JWT to see what went wrong. Did the token expire? Does the payload contain the expected user data? This decoder answers those questions instantly. It shows the issued-at timestamp (iat), expiration timestamp (exp), and calculates how much time remains before the token expires.

Important security note

This tool decodes the JWT structure but does not verify the signature. Any client can decode the visible parts of a JWT—the security comes from the signature, which only the server can verify using a secret key or public key. Never trust JWT claims on the client side without server-side verification. Use this tool for debugging and inspection only, not for security decisions.

Common claims

Typical JWT payloads include sub (subject/user ID), iat (issued at), exp (expiration), aud (audience), and custom claims defined by your application. The tool displays all claims in formatted JSON for easy reading.