Understanding JWT Tokens: Decoding, Verification, and Security Best Practices

# Understanding JWT Tokens: Decoding, Verification, and Security Best Practices JSON Web Tokens (JWT) are an open standard (RFC 7519) for transmitting secure claims between two parties. They are widely used for stateless session management in modern single-page applications (SPAs) and mobile apps. ## Structure of a JWT Token A JWT string consists of three base64url-encoded sections separated by dots (`.`): 1. **Header**: Specifies the signing algorithm (e.g. HS256, RS256) and token type (`"typ": "JWT"`). 2. **Payload**: Contains claims such as user ID (`sub`), expiration time (`exp`), issued-at timestamp (`iat`), and custom user roles. 3. **Signature**: Cryptographic signature generated using the secret key to verify token integrity. ## Security Considerations - **Token Expiry (`exp`)**: Tokens should always include an expiration claim to limit damage in case of token leakage. - **Never Store Secrets in Payload**: JWT payloads are Base64Url decoded—not encrypted. Anyone who intercepts a JWT can read its contents. Never place passwords or sensitive API keys inside claims.