Rina and Naseem share a MAC key. Naseem produces a message with a valid MAC and claims Rina sent it. Can a third party settle the dispute?

Answer

No — either party could have produced that MAC, so it identifies neither

Options - A. No — either party could have produced that MAC, so it identifies neither - B. Yes — only Rina could have produced a valid MAC on her own message - C. Yes, provided the MAC is built on a collision-resistant hash function - D. No — because a MAC cannot detect alterations to the message Why - A. Correct. The key is shared, so both are equally capable of producing it — that is precisely why a MAC gives no non-repudiation. - B. The mirror error. A valid MAC proves someone holding the shared key made it, and that is two people. - C. Hash strength is irrelevant here. A perfect hash does not stop a legitimate key-holder from computing a legitimate MAC. - D. A true conclusion reached by a false route — MACs detect alteration perfectly well. The reason is key sharing, not weak integrity. Non-repudiation needs a secret that exactly one party holds. Any authenticator computed under a shared key is disqualified before you examine its cryptographic strength.

Stallings & Brown 5e ch2 §2.4; ch21 §21.5