Define “message authentication code” (MAC).

A message authentication code (MAC) is indeed defined as a short piece of information used to authenticate a message and confirm its integrity and authenticity. It functions by allowing both the sender and the receiver of a message to verify that the message has not been altered in transit and that it indeed comes from a legitimate source.

When a sender creates a MAC, it typically involves a secret key and the message itself. This process generates a unique code that is then sent along with the message. Upon receipt, the receiver can execute the same MAC process using the secret key and the received message. If the code generated matches the one sent, the message is deemed intact and authentic.

The other options diverge from this definition of MAC. A type of key used only for encryption does not encompass the validation roles of a MAC. A detailed report generated after an encryption process does not relate to the concept of message authentication or integrity verification. A user-defined code that changes frequently describes a different security measure that may relate to passwords or dynamic challenge-response systems, rather than the specific, key-based authentication process that a MAC entails. Thus, the definition of MAC as a piece of information that ensures both authenticity and integrity is what makes the first choice accurate.