Final answer:
The recipient of a message uses the MAC to verify the integrity and authenticity of a message using a shared secret key. This ensures the message hasn't been altered and confirms the sender's identity, which is essential for secure communications.
Step-by-step explanation:
When the recipient of a message receives it along with a Message Authentication Code (MAC), they use a shared secret key to perform a verification process. The recipient computes the MAC on the received message using the same algorithm and secret key that the sender used. If the MAC that the recipient computes matches the MAC that was sent with the message, it ensures the integrity of the message – confirming that it has not been altered in transit – and also authenticates the identity of the sender, since only someone with access to the secret key could have created that MAC.
The use of MACs is critical in digital security to prevent tampering and forgery of messages. Ensuring data integrity and authentication in this way is a fundamental aspect of secure communications over potentially insecure channels such as the internet. Without a MAC, data could be intercepted and changed, or fake messages could be sent without detection.