JWT Decoder

Read a token, check it, or sign a test one

A JWT is three Base64url parts: header, payload, signature. The header and payload are JSON, and they are not encrypted. Anyone with the token can read them. The signature shows the payload was not edited, if you have the HMAC secret or the public key. This page decodes, verifies and generates in the browser. It flags alg none, a missing or due exp, a time written in milliseconds, and an HMAC secret shorter than 32 bytes. The token is not uploaded. A checksum on the hash page is not a JWT signature. exp is a Unix time in seconds; other zones are on the Unix timestamp page.

How to use it

1. The encoded token is on the left. Paste it there. On the right, Header and Payload are two tabs. With Update as you type on, both boxes fill in. The times under Payload use this computer’s local clock. There is no grace period: if exp is now or earlier, it is marked expired.
2. Show Example loads an HS256 token whose payload is {"sub":"user-1"} and sets HMAC secret to secret. Secret is Base64 stays off. Verify says the signature matches. Security audit says the secret is 6 bytes and that there is no exp. The sample is short on purpose.
3. Verify checks the token box, not unsaved edits in Header or Payload. HS256, HS384 and HS512 use HMAC secret. RS, PS and ES use the key pair under Algorithm: Public key to check, Private key to sign. Generate key fills both. Generate builds a new token from those two boxes. The Algorithm menu wins over the alg written in the header.
4. Expiration starts on Keep claims, so Generate leaves iat and exp alone. Choose 1 hour and Generate writes both as seconds from now. Template User fills sub 1234567890, name John Doe and email [email protected]. Template Access fills sub user-1 and scope read.
5. Copy under the token copies the token. Copy on Header or Payload copies that box. Verify with JWKS reads JWKS address in this browser and downloads keys only. The token is not sent. Clear All empties the token, both JSON boxes, the secret, the key and the address.

What is checked

HS256, HS384 and HS512 use the secret. A secret under 32 bytes still signs, and the audit warns. Tick Secret is Base64 only when the secret is Base64, not the raw text. RS256, RS384, RS512, PS256, PS384, PS512, ES256, ES384 and ES512 use a PEM or a JWK. A public key verifies. A PKCS#8 private key generates, and an RSA private key can also verify because the public half is inside it. Paste SPKI (BEGIN PUBLIC KEY), PKCS#8 (BEGIN PRIVATE KEY), or PKCS#1 RSA. A certificate is rejected. An EC key that says BEGIN EC PRIVATE KEY is rejected; export PKCS#8. alg none is never treated as valid. Verify follows the alg in the header. An HMAC secret is not used to check an RSA token. exp, nbf and iat are seconds. A 13-digit value is flagged, not converted. JWKS must be a set of keys, or one JWK. Several keys and no kid is an error. An array in Header or Payload blocks Generate.

Where people use it

• Reading sub and exp on a token you already hold.
• Signing a local HS256 token for a server whose secret you set yourself.
• Checking an RS256 token with the public PEM, or with a JWKS address that allows browsers.
• Seeing a short test secret before it is copied into a config.

Questions