11.5 Message authentication codes
If Alice wants to securely transmit a message m to Bob, she must use a so-called Message Authentication Code (MAC) to prevent Eve from tampering with that message. More precisely, a MAC prevents Mallory from doing the following:
Modifying m without Bob noticing it
Presenting Bob a message m′ generated by Mallory, m′≠m, without Bob noticing that m′ was not sent by Alice
Therefore, a MAC helps us to achieve the two security objectives integrity protection and message authentication (see Chapter 2, Secure Channel and the CIA Triad and Chapter 5, Entity Authentication). Note that a MAC cannot prevent the tampering itself, nor can it prevent message replay. The active attacker Mallory can always manipulate the genuine message m, or present Bob with the message m′ and pretend that it was sent by Alice. A MAC only gives Bob the ability to detect that something went wrong during the transmission of the message he...
