Local-Only JWT Decoder

A JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed. JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA or ECDSA.

Why Local Decoding is Critical

Developers often use online tools to decode JWTs during debugging. However, many of these third-party websites send your token to their backend servers. If you paste a production token containing sensitive user claims or active session data, you are essentially leaking access credentials to a third party.

Our Local-Only JWT Decoder solves this. By relying strictly on client-side JavaScript (using the browser's native Base64 decoding APIs), we guarantee that your token is processed entirely on your local machine. No network requests are made. No data is transmitted.

Structure of a JWT

In its compact form, JSON Web Tokens consist of three parts separated by dots (.), which are:

• Header: Typically consists of two parts: the type of the token, which is JWT, and the signing algorithm being used, such as HMAC SHA256 or RSA.
• Payload: Contains the claims. Claims are statements about an entity (typically, the user) and additional data. There are three types of claims: registered, public, and private claims.
• Signature: To create the signature part, you have to take the encoded header, the encoded payload, a secret, the algorithm specified in the header, and sign that.

: While you can decode the Header and Payload of a JWT without the secret key, you cannot verify if the token has been tampered with or if it is authentically signed without the secret. Our tool deliberately omits signature verification to remain 100% offline and stateless, protecting you from accidentally transmitting your private verification keys over the internet.