What's inside a JWT
A JSON Web Token is three base64url strings joined by dots:
header.payload.signature- The header says how the token was signed, for example
{"alg": "HS256", "typ": "JWT"}. - The payload holds the claims: who the token is about (
sub), who issued it (iss), who it's for (aud) and when it expires (exp), plus anything the issuer adds, such as roles. - The signature is computed from the first two parts and a key. If anyone changes the header or payload, the signature no longer matches.
Decoding only reverses the base64url step, which is why you can read any token without a key. That also means a JWT payload is not private: don't put passwords, card numbers or other secrets in it.
Checking expiry
The times in a JWT are Unix timestamps in seconds. The decoder turns exp, iat and nbf into readable dates, in UTC and in your local time zone, and shows at the top whether the token is still valid. A common bug is writing milliseconds instead of seconds (a 13-digit number); the decoder warns you when it sees that.
Verifying the signature
Paste the key under Verify signature:
- HS256 / HS384 / HS512: the shared secret, exactly as the server uses it.
- RS*, PS* and ES*: the public key as PEM (
-----BEGIN PUBLIC KEY-----) or as JSON (a JWK, or a JWKS with akeysarray; the key whosekidmatches the header is used).
Verification uses your browser's built-in Web Crypto API. A green result means the token was signed with that key and hasn't been changed since. Your server still has to check exp, aud and iss itself.
Warnings the decoder shows
alg: "none": the token isn't signed at all. Servers must never accept it.- No
expclaim: the token is valid forever unless the server tracks it. - Very long lifetimes (over 30 days): a leaked token stays usable for a long time.
- Times written in milliseconds or as strings.
Frequently asked questions
Is it safe to paste a real token here?
The token is decoded and verified inside your browser and is never sent to our server or saved. Still, treat production tokens like passwords: anyone who has a valid token can use it until it expires.
Why can I read the payload without the secret?
A standard JWT is signed, not encrypted. The header and payload are only base64url-encoded, so anyone can read them. The signature stops people from changing the contents, not from reading them. Never put passwords or other secrets in a JWT payload.
Which algorithms can be verified?
HS256, HS384 and HS512 with a shared secret; RS256, RS384, RS512, PS256, PS384 and PS512 with an RSA public key; and ES256, ES384 and ES512 with an EC public key. Keys can be pasted as PEM (BEGIN PUBLIC KEY) or as a JWK or JWKS.
What do exp, iat and nbf mean?
They are times in seconds since 1 January 1970 (UTC). exp is when the token expires, iat is when it was issued and nbf is the time before which it must not be accepted. The decoder shows each one as a date and as a relative time such as "in 45 minutes".