JWT Explained: How Token Signatures Verify Identity Without Server Memory

JSON Web Tokens (JWTs) allow APIs to authenticate users without the server storing session data, as the client carries a self-contained proof of identity. A JWT consists of a base64-encoded header and payload — readable by anyone — plus a cryptographic signature generated using a secret key known only to the server. Any tampering with the payload invalidates the signature, causing the server to reject the token. A key trade-off is that JWTs cannot be instantly revoked; the standard workaround is using short-lived access tokens paired with revocable refresh tokens. Since the payload is not encrypted, sensitive information should never be stored in it, and security depends on HTTPS and safe client-side storage practices.