HMAC-SHA256

HMAC (Hash-based Message Authentication Code) is the canonical way to combine a secret key and an arbitrary message into an authentication tag. The construction is intentionally simple: `HMAC(k, m) = H((k ⊕ opad) || H((k ⊕ ipad) || m))`. The double-hash with two padded keys neutralizes length-extension attacks on the underlying hash, which is why you never want to roll your own `H(key || message)`: that pattern *is* exploitable on SHA-2. HMAC-SHA256 is the default for new API signing schemes (AWS V4, Stripe webhooks, GitHub webhooks, JWT HS256). If you're picking between HMAC-SHA256 and Poly1305 / GMAC / BLAKE3-keyed, HMAC-SHA256 is the safest interoperable choice; the others are faster but harder to glue into existing ecosystems.