HS256

HS256 is an algorithm that creates and verifies cryptographic signatures using HMAC with the SHA-256 hash function. It is commonly referred to as "HMAC-SHA256" and is used to provide data integrity and authentication for messages, most notably within JSON Web Tokens (JWT).

In the context of JWT, HS256 is a symmetric signing algorithm. The token consists of a base64url-encoded header, a base64url-encoded payload, and a signature. The signature is produced by applying HMAC-SHA256 to the concatenation of the header and payload: signature = HMAC-SHA256(key, base64url(header) + "." + base64url(payload)). The resulting signature is then base64url-encoded and appended to the token. Verification uses the same secret key to recompute the signature and compare it to the one in the token.

Security considerations include the need to keep the shared secret confidential. The same key is used to sign and verify tokens, so a leaked key enables forgery of tokens for all relying services. Recommended practices include using a sufficiently long, random secret (often at least 256 bits), rotating keys regularly, and using key identifiers to distinguish keys. It is important to note that HS256 does not encrypt the payload; JWTs signed with HS256 provide integrity and authenticity but not confidentiality. If encryption of the payload is required, additional measures such as JWE or transport-layer encryption should be used. For scenarios requiring asymmetric verification, alternatives like RS256 or ES256 may be preferred.