A JSON Web Token (JWT, pronounced "jot") is an open standard (RFC 7519) for securely transmitting information between parties as a compact, URL-safe string. JWTs are widely used for authentication and authorization in modern web applications, single sign-on (SSO) systems, and API security. Unlike traditional session-based authentication that stores state on the server, JWTs are self-contained tokens that carry all the necessary user information within the token itself, making them ideal for stateless, scalable architectures and microservices.
Every JWT consists of three Base64URL-encoded parts separated by dots. The header specifies the token type (JWT) and the signing algorithm (such as HS256 or RS256). The payload contains the claims -- pieces of information about the user or session, including standard claims like sub (subject), iat (issued at), exp (expiration time), and any custom claims your application needs. The signature is created by combining the encoded header, encoded payload, and a secret key using the specified algorithm, ensuring that the token has not been tampered with. While the header and payload are only encoded (not encrypted), the signature guarantees their integrity.
Developers use JWT decoders to inspect token contents during API development and debugging, verify that claims such as expiration times and roles are set correctly, and troubleshoot authentication issues. This tool is also valuable for security audits -- checking that tokens do not contain sensitive data in the payload, since JWTs are only signed, not encrypted by default. Our browser-based decoder parses your tokens entirely client-side, so sensitive authentication tokens are never sent to any server. Simply paste a JWT to instantly view its decoded header, payload, and signature components.