JWT Decoder & Generator

Paste a JSON Web Token to decode its header and payload, verify its signature (HS256/384/512, RS256/384/512, or ES256/384), and check standard claims like exp and aud. Runs entirely client-side — your token never leaves your browser.

JWT HMAC Secret (for HS256/HS384/HS512) Public Key — PEM or JWKS JSON (for RS256/RS384/RS512, ES256/ES384) Expected audience (optional) Expected issuer (optional) Clock skew tolerance (seconds)

Header Payload

Generate a New Token (HMAC)

Header (defaults will be used if left blank) Payload Secret

How to use it

Paste a token and click Decode to see its header, payload, and a claims table with human-readable expiry info. To verify the signature: for HS* algorithms enter the shared secret; for RS*/ES* paste the signer's public key as PEM, or a JWKS JSON blob (the key matching the token's kid is picked automatically, falling back to the first compatible key). Set an expected audience/issuer to flag mismatches, and a clock-skew tolerance if your systems' clocks may drift.

Frequently Asked Questions

Which algorithms are supported?

Verification and generation: HS256, HS384, HS512 (shared secret). Verification only: RS256, RS384, RS512, ES256, ES384 (public key). EdDSA (Ed25519) isn't supported — browser support for it is still inconsistent, and we'd rather leave it out than ship a check that silently fails in some browsers.

Can I verify against a JWKS URL?

Not by URL — fetching a remote JWKS would mean this page makes a network request, which breaks the "nothing leaves your browser" guarantee every other tool on Cryonel follows. Instead, paste the JWKS JSON directly (copy it from your identity provider's /.well-known/jwks.json endpoint) and it's matched locally.

What does the "alg: none" warning mean?

A token with "alg":"none" has no signature at all — anyone can create one with arbitrary claims. It's a well-known JWT vulnerability class; a backend that accepts these can be trivially impersonated. Cryonel decodes them but never treats them as verified.

Why does the signature matter?

The signature lets you confirm the token was issued by the server and hasn't been tampered with. The header and payload are readable by anyone (they aren't encrypted), so never put sensitive data in the payload.

Is it safe to enter my secret or key on this site?

Nothing is sent to a server — all computation happens in your browser via the Web Crypto API. That said, avoid entering production secrets into third-party tools.