JWT Encoder

A JSON Web Token (JWT, pronounced "jot") is a compact, URL-safe way of representing claims between two parties — typically an authentication server and an application. When you log into a website or an API, a JWT is often what gets created on the server and sent back to your browser, and every subsequent request you make to that service carries the JWT (usually in an Authorization header) to prove who you are without the server needing to maintain a session record for you. A JWT consists of three Base64URL-encoded parts separated by dots: header.payload.signature. The header describes which algorithm was used to sign it (for example, {"alg":"HS256","typ":"JWT"}). The payload contains the claims — the actual data you want to transmit, such as a user ID, an expiration time, or any custom fields your application uses. The signature is a cryptographic hash of the header and payload, created with the algorithm and a secret key that only the signing server knows, which lets anyone who knows the secret verify that the token has not been tampered with since it was created.\n\nAn important distinction exists between encoding and signing a JWT (what this tool does) and decoding and verifying one (what a separate JWT Decoder tool does). Encoding means constructing the token — creating the header, serializing the payload as JSON, Base64URL-encoding both parts, signing them, and assembling the final three-part string. A decoder reads an existing token — it splits the three parts, decodes the header and payload to show you what is inside, but it cannot verify the signature because verification requires the secret key, which only your server has (or should have — never paste a real production secret into any online tool).\n\nThis encoder comes with a critical security warning: NEVER paste a real production secret key into this or any online tool. Even though this tool runs entirely in your browser and never transmits data, secrets entered into a web page can be read by browser extensions, developer tools, or malware on the local machine. This tool is designed for development, testing, and learning — generate JWTs with test secrets to understand the format, debug payload structure, or prototype authentication flows.