A message authentication code (MAC) is a short piece of information used to verify the integrity and authenticity of a message. It is generated by applying a cryptographic algorithm to the message and a secret key, ensuring that only someone with the key can produce the same MAC for the same message. This helps detect any unauthorized changes to the message during transmission.
When a recipient receives a message along with its MAC, they can use the same algorithm and key to generate their own MAC. If the two MACs match, it confirms that the message is authentic and has not been altered.