What is a Message Authentication Code (MAC)?

• A. A public key encryption method
• B. A cryptographic function involving a secret key
• C. A type of symmetric cipher
• D. A hash function

Answer: (B)

A Message Authentication Code (MAC) serves to verify both the integrity and authenticity of a message. It achieves this by using a cryptographic function that relies on a secret key. When a sender generates a MAC for a message, it combines the message with this secret key and processes it through a cryptographic algorithm. The resulting MAC is then sent along with the message itself.

The recipient, who shares the same secret key, can independently compute the MAC for the received message and compare it to the MAC sent with the message. If these MACs match, it assures the recipient that the message has not been altered in transit and confirms the identity of the sender who possesses the secret key, thereby satisfying the requirements for both integrity and authenticity.

Other options do not accurately describe a MAC: public key encryption methods employ different mechanisms involving key pairs, symmetric ciphers are a broader category focusing on the encryption and decryption of data without specifically emphasizing message integrity with a key, and while a hash function is involved in various cryptographic operations, it does not involve a secret key and cannot provide the same security guarantees for authenticity as a MAC.