JWT decoder and signer

Decode & verify — paste a token (a leading Bearer is ignored). The header, payload, and signature are coloured, the JSON is pretty-printed, and every claim is explained, with exp, nbf, iat, and auth_time shown as dates and as “in 3 hours” / “2 days ago”. Problems are flagged: alg: none, expired or not-yet-valid tokens, missing exp, very long lifetimes, crit headers, and jku/x5u key URLs.

To verify, enter the shared secret for HS256/384/512 (as text, Base64, or hex), or a public key for RS, PS, ES, or EdDSA tokens as a PEM (BEGIN PUBLIC KEY), a JWK, or a whole JWKS — the key whose kid matches the token is used.

Build & sign — choose the algorithm, edit the header and payload JSON, add iat, nbf, exp, or jti with one click, and sign with a secret or a private key (PEM BEGIN PRIVATE KEY or JWK). Open in decoder checks the result.

Keys — generate RSA (2048–4096 bits), EC (P-256, P-384, P-521), Ed25519, or HMAC test keys, exported as PEM and JWK with an RFC 7638 thumbprint as the kid. Use them for testing only.

Tokens and keys stay in the page; the token is kept in browser storage only if you tick remember. PKCS#1 (BEGIN RSA PRIVATE KEY) and SEC1 (BEGIN EC PRIVATE KEY) PEMs are not supported by WebCrypto — convert them with openssl pkcs8 -topk8 -nocrypt.