What is the function of a message authentication code (MAC)?

• A. Encrypts data for transmission
• B. Generates random keys
• C. Verifies message integrity
• D. Decrypts ciphertext

Answer: (C)

A message authentication code (MAC) plays a critical role in ensuring the integrity and authenticity of a message. Specifically, it is a short piece of information generated from the message content and a secret key, which is used to verify that the message has not been altered in transit. When a sender creates a MAC for a message, they use a cryptographic hash function along with a secret key, producing a unique code. The recipient can then use the same secret key to generate a MAC from the received message and compare it with the MAC sent by the sender. If the two MACs match, it confirms that the message has not been tampered with and is indeed from the expected sender, thus validating both the message's integrity and authenticity. This verification process is vital in communication systems to prevent unauthorized changes to data and to confirm the identity of the sender.

The functions of encrypting data, generating random keys, and decrypting ciphertext refer to different aspects of information security, such as confidentiality and key management, rather than the specific role of MACs in integrity verification.