Token Bench

Paste a JWT to read its header, payload and claims. Paste a key — an HMAC secret, a JWK, or a whole JWKS — to find out whether the signature actually holds. HS, RS, PS and ES algorithms, checked with the browser's own Web Crypto, and the right key resolved out of a JWKS by the token's kid. Then the part every other JWT tool leaves to you: when the signature does not verify, it says why. Not by guessing — it takes each hypothesis, changes that one thing, re-runs the check, and only reports a cause it watched start verifying. It will tell you that the token was re-encoded in transit and the signature covers the exact characters it no longer has; that your secret works base64- or hex-decoded, which is a different key from the literal string and a decision your library makes rather than the token; that the token is signed HS512 while you are checking HS256; that the ECDSA signature is ASN.1 DER, the form OpenSSL, Go and most Java signers emit and RFC 7518 forbids, when it wants a fixed-width r and s; or that the signature is perfectly good and the JWKS simply was not searched to the key that made it, which is what a key rotation looks like from the outside. It also names algorithm confusion out loud: a token claiming HMAC while you hold a public key is a forgery anyone who can read your JWKS could have written, and if HMAC-ing with that key reproduces the signature it says so. When no hypothesis holds it says that too, because refusing to invent a reason is the whole point. Nothing is uploaded.