Describe the process of implementing JWT (JSON Web Token) authentication, detailing how tokens are created, signed, and verified.

JWT (JSON Web Token) authentication is a stateless authentication mechanism that uses digitally signed JSON objects to securely transmit information between parties. The process involves three key steps: creation, signing, and verification. 1. Creation: The first step is to create the JWT. A JWT consists of three parts: a header, a payload, and a signature. a. Header: The header typically specifies the token type (JWT) and the signing algorithm being used (e.g., HS256, RS256). It's a JSON object that is Base64 URL encoded. b. Payload: The payload contains the claims, which are statements about the user or entity. Claims can be registered claims (e.g., `iss` (issuer), `sub` (subject), `aud` (audience), `exp` (expiration time)), public claims (defined by the application), and private claims (custom claims). It's also a JSON object that is Base64 URL encoded. A common claim is the user ID. c. Signature: The signature is created by taking the Base64 URL encoded header, the Base64 URL encoded payload, a secret key (for symmetric algorithms like HS256) or a private key (for asymmetric algorithms like RS256), the specified algorithm, and signing them. 2. Signing: The signing process ensures that the token cannot be tampered with. The algorithm specified in the header is used to sign the token. For example, using the HS256 algorithm: `signature = HMACSHA256(base64UrlEncode(header) + '.' + base64UrlEncode(payload), secret);` The `secret` is a secret key known only to the server. For asymmetric algorithms like RS256, the server uses its private key to sign the token. 3. Verification: When the client sends the JWT to the server, the server needs to verify its authenticity. This involves: a. Checking the signature: The server uses the same algorithm and secret key (or the public key for asymmetric algorithms) to re-calculate the signature based on the header and payload. If the calculated signature matches the signature in the JWT, it confirms that the token hasn't been tampered with. b. Validating the claims: The server validates the claims in the payload, such as the expiration time (`exp`), the issuer (`iss`), and the audience (`aud`), to ensure that the token is still valid and authorized for the current request. If any of these checks fail, the token is considered invalid, and the request is rejected. After successful verification, the server can trust the information in the JWT and grant the client access to the requested resources. This entire process enables a stateless authentication mechanism, where the server doesn't need to maintain session information for each user.