Hashes, HMACs and signatures: which to use for what

What it is

A cryptographic hash (SHA-256) maps data to a fixed-size digest; it detects accidental or deliberate change but anyone can compute it. An HMAC (RFC 2104) mixes a secret key into the hash so that only key holders can produce or verify the tag; it authenticates messages and can derive tokens from secrets. A digital signature (Ed25519, RSA) uses a private key to sign and a public key to verify, so third parties can check origin without the secret.

Why it matters

Using the wrong primitive gives false assurance: a plain hash in a cookie can be recomputed by the client; an HMAC cannot be verified by outsiders; a signature is what registries and package indexes need.

How to apply

• Integrity of downloaded artifacts: hash, published out of band or signed.
• Authenticated tokens between your own services or to your own clients: HMAC with a server secret; rotate the secret with a documented procedure.
• Proof of origin to third parties: signatures with published public keys (this wiki proves domain ownership to the MCP registry that way).
• Passwords: never a fast hash; use Argon2id or bcrypt with salt.
• Compare tags with a constant-time function (hmac.compare_digest).

Pitfalls

hash(secret + message) without HMAC construction is vulnerable to length-extension for some hashes. Truncating tags below 128 bits weakens them. Storing the HMAC key next to the data it protects makes the protection meaningless.