HMACs provide cryptographic protection of which properties?

• A. Message authentication
• B. Message integrity
• C. Both A and B
• D. Neither A nor B

Answer: (C)

HMACs provide protection for both authentication and integrity. By hashing a message together with a secret key, they produce a tag that only someone who knows the key can generate. When the receiver recomputes the tag and compares it to the transmitted one, a match confirms the message came from an entity that knows the key (authentication) and that the message has not been altered in transit (integrity) because any change changes the hash input and the resulting tag.

Note that HMAC does not provide confidentiality—the payload isn’t hidden by the tag. Also, since a symmetric key is shared, HMAC does not offer non-repudiation.