JWT Decoder

A JWT decoder splits a JSON Web Token into its three parts and reads the two that are readable. Paste a token and you get the header, the payload with every claim explained, and a live countdown to expiry.

The important thing to understand about a JWT, and the reason this page runs entirely in your browser: the payload is not encrypted. It is base64, an encoding, not a cipher. Anyone holding the token can read everything inside it with no key at all – which is what this tool is doing. The signature does not hide the contents; it only proves they have not been altered.

That has a direct consequence. A JWT is a live credential, usually a working session. If you paste one into a tool that sends it to a server, you have handed someone a valid login for as long as the token lasts. This tool has no server to send it to.

The three parts

Header declares the signing algorithm and, often, a kid naming which key was used.

Payload carries the claims – who issued the token, who it is about, what it is allowed to do, and when it stops being valid.

Signature is the part that cannot be read. It is a cryptographic hash of the first two parts, made with a key only the issuer holds.

The tool colours all three so you can see where each one starts and ends, which is usually the first confusing thing about a token.

Claims, in plain English

Registered claims are three-letter abbreviations that are easy to misread. Each one is labelled: iss is the issuer, sub is the subject the token is about, aud is the audience it was minted for, jti is a unique ID used to stop a token being replayed.

The three time claims – exp, nbf and iat – are stored as Unix seconds, which is unreadable at a glance and easy to mistake for milliseconds. Each is shown as a local datetime alongside the raw number.

Four things it checks that most decoders do not

Expiry, counted down live. Not just the timestamp, but how long is actually left, updating each second and turning amber under five minutes.

alg set to none. This means the token is unsigned. Anyone can change the payload and it will still parse. A server that accepts such a token is vulnerable, and this is a real class of authentication bypass rather than a theoretical one.

nbf in the future. The token is not valid yet. From an application’s point of view this looks exactly like a broken token, and it is easy to spend an hour on it without knowing the claim exists.

iat in the future. The issuing server’s clock is ahead. This causes intermittent rejections that are very hard to reproduce.

It also flags a payload containing anything password-shaped, since a JWT is the wrong place to put a secret that anyone holding the token can read.

Decoding is not validating

Decoding shows what a token says. It does not prove the token is genuine. A token that decodes perfectly may still have been forged, so the panel reads “signature not verified” until you verify it.

You can verify it here, with one deliberate limit.

Tokens signed with RS256, PS256, ES256 and their siblings are verified with the issuer’s public key, which is meant to be public – published at a JWKS endpoint, printed in documentation, safe to hand out. Paste it and the check runs in your browser through the Web Crypto API. Nothing is uploaded, and the result tells you whether this exact token was signed by the matching private key and has not been altered since.

Tokens signed with HS256 and its siblings cannot be verified here, and that is on purpose. HMAC uses one shared secret to both sign and verify, so checking an HS256 signature means handing over the secret that protects your entire authentication system. Plenty of tools will ask you for it. We will not. Verify those on your server, with a library, using a key your server already holds.

If you paste a private key by mistake, the tool spots it and refuses before attempting anything.

Two things verification still does not tell you: whether the token has been revoked, and whether your server would accept it – your server has its own rules about issuer, audience and clock tolerance.