JWT Decoder

JWT Decoder reads a JSON Web Token and checks its signature — entirely in your browser.

Decoding

Paste a token into Encoded token. It is coloured by part: header.payload.signature.

• You can paste a whole Authorization: Bearer eyJ… header, with quotes or line breaks — it’s cleaned up for you.
• The bar at the top says whether the token is Expired, Not valid yet, or Not expired, using exp and nbf.
• Claims lists every payload field. Times (exp, iat, nbf, auth_time) are shown as dates in your time zone, with “in 3 hours” / “2 days ago”. Switch to JSON to see the raw payload.

Decoding is not verification. Anyone can read a JWT. Only a valid signature proves who issued it.

Verifying the signature

Paste the key into Verify signature. The token’s alg decides which key you need:

alg	Key to paste
HS256 HS384 HS512	The shared secret. Tick Secret is base64 if your secret is stored base64-encoded.
RS256/384/512, PS256/384/512	RSA public key
ES256 ES384 ES512	EC public key (P-256 / P-384 / P-521)
EdDSA	Ed25519 public key

Public keys can be:

• PEM — -----BEGIN PUBLIC KEY-----…
• JWK — a single key object {"kty":"RSA","n":"…","e":"AQAB"}
• JWKS — the whole {"keys":[…]} document from your provider’s /.well-known/jwks.json. The key is picked by the token’s kid.

Private keys are refused on purpose — you never need one to verify.

Other formats: for a certificate, run openssl x509 -pubkey -noout -in cert.pem; for a BEGIN RSA PUBLIC KEY (PKCS#1) key, run openssl rsa -RSAPublicKey_in -in key.pem -pubout.

Warnings

CurlyJSON flags common mistakes:

• alg: "none" — an unsigned token. Servers must reject it.
• No exp — the token never expires.
• exp in milliseconds instead of seconds.
• Time claims stored as strings.
• HMAC secrets shorter than the hash size (for example under 32 bytes for HS256).

Privacy

Tokens are processed with your browser’s Web Crypto API. Unlike the other tools, the JWT Decoder doesn’t remember your input — tokens are credentials, so nothing is saved, even in your browser. Encrypted tokens (JWE, five parts) can’t be decoded without the private key.