HMAC

A dangerous sentence appears often in device security designs: “We use SHA-256, so it is secure.”

That is usually incomplete. SHA-256 is a hash function. It turns data into a fixed-size digest, but it does not know who computed it and cannot stop an attacker from modifying data and recomputing the digest.

Hashes, MACs, and signatures all output values that look like checksums, but they solve different problems:

Hash: did the data match this fingerprint, assuming the digest itself is trusted
MAC: was this generated by someone holding the same shared key
Signature: was this generated by a private-key holder, verifiable by public key

Mixing them up directly affects firmware verification, device authentication, message integrity, and audit logs.