About the JWT Decoder
Introduction
A JSON Web Token (JWT, RFC 7519) is the compact, URL-safe token that most APIs, single sign-on systems and OAuth 2.0 / OpenID Connect providers use to say who you are and what you may do. This tool decodes a token into its header and claims, tells you whether it is expired, verifies its signature with a secret or public key, and lets you edit the claims and sign a new token – handy for debugging authentication, testing how an API handles expired or tampered tokens, and learning how JWTs work.
How to use it
Paste a token into Encoded token. A leading Bearer or a whole Authorization: header line is removed automatically.
Read the header and payload on the right. Time claims (exp, nbf, iat, auth_time) are shown as UTC dates with "in 5 min" / "3 h ago", and the box under the token says whether it is valid right now.
To check the signature, enter the secret (HS256/384/512) or paste the public key (RS*, PS*, ES*, EdDSA) as PEM, X.509 certificate, JWK or a whole JWKS – the right key is picked by the token's kid.
Edit the header or payload, or use the Set buttons for iat, exp and nbf. The token is re-encoded immediately; with the secret or private key it is re-signed too, otherwise the old signature is struck through because it no longer matches.
Short links: develzone.org/jwt, /jws and /bearer. Pasting a token into the search box on any page opens it here.
Anatomy of a token
A signed JWT has three Base64url parts separated by dots, shown in three colours:
Header – the algorithm (alg), usually the type (typ: "JWT") and often a key ID (kid).
Payload – the claims: registered ones such as issuer (iss), subject (sub), audience (aud), expiry (exp), plus anything the issuer adds – roles, scopes, e-mail.
Signature – computed over header.payload with a secret or private key. Change one character of the first two parts and it no longer matches.
The payload is only encoded, not encrypted – anyone holding the token can read it. Never put passwords or secrets into claims. Tokens with five parts are encrypted (JWE); their payload cannot be read without the recipient's key.
Supported algorithms
HS256, HS384, HS512 – HMAC with one shared secret. Use at least as many random bytes as the hash size (32 for HS256).
RS256, RS384, RS512 and PS256, PS384, PS512 – RSA signatures (PKCS#1 v1.5 and PSS).
ES256, ES384, ES512 – ECDSA on the P-256, P-384 and P-521 curves.
EdDSA (Ed25519) – needs a current browser.
Keys are accepted as SPKI / PKCS#8 PEM, X.509 certificates, JWK and JWKS. Old PKCS#1 (BEGIN RSA PRIVATE KEY) and SEC1 PEMs are not – the tool tells you the openssl command to convert them.
Good to know
A decoded token proves nothing until its signature is verified with a key you trust, and exp, nbf, iss and aud are checked. Servers must reject alg: "none".
Times are in seconds since 1970 (UTC). Convert any of them with the Unix timestamp converter.
Just need one part decoded? The Base64url decoder does that.
Privacy
Decoding, verifying and signing all happen in your browser with the Web Crypto API. Tokens, secrets and keys are never sent to a server and not stored; a token passed in a link is removed from the address bar as soon as the page has read it. Still, treat production tokens like passwords – they are valid credentials until they expire.