What does the message authentication code ensure?

A message authentication code (MAC) is a cryptographic checksum on data that provides a way to verify both the integrity and authenticity of a message. It is generated by a specific algorithm using a shared secret key and the original message. When the recipient receives the message along with the MAC, they can perform the same algorithm using the shared secret key and ensure that the MAC matches what they compute. This confirms that the data has not been altered in transit (ensuring integrity) and that it originated from the expected sender, as only someone with access to the secret key could generate a valid MAC for that message (ensuring authenticity). Thus, the primary role of a MAC is to safeguard against unauthorized modifications and to assert the legitimacy of the source of the message.