HMACs provide cryptographic protection of which attribute?

• A. Authentication
• B. Nonrepudiation
• C. Both A and B
• D. Neither A nor B

Answer: (A)

HMACs are hash-based message authentication codes that use a shared secret key to bind a hash to a message. This provides data origin authentication and integrity: the recipient can verify that the message came from someone who knows the key and that it hasn’t been tampered with in transit. They do not provide nonrepudiation because the key is shared; the sender could be anyone who knows the secret, so there’s no proof of who specifically sent it. Nonrepudiation requires digital signatures with public-key cryptography. So, the attribute protected by HMACs is authentication.