JWT Decoder

What is a JWT Decoder?

A JWT Decoder reads a JSON Web Token and displays its three parts in human-readable format. Every JWT contains a Header (algorithm & type), a Payload (claims — user data, expiry, issuer), and a Signature. This tool decodes all three instantly, shows expiry status with a live countdown timer, highlights standard claims with labels, detects security issues like alg:none, and supports HS256/HS384/HS512 signature verification using the browser Web Crypto API.

JWT Structure — How It Works

A JWT looks like xxxxx.yyyyy.zzzzz — three Base64URL-encoded strings joined by dots. The header and payload are readable by anyone; the signature is what proves authenticity. Only the party holding the secret or public key can verify the signature. This means JWTs should never contain sensitive data like passwords.