2.2. Message Authentication Codes
A message authentication code (MAC) function computes a MAC from a message and a secret key. If the originator and the receiver share knowledge of that secret key, the receiver can calculate the same function of the message and secret key and see if it matches the MAC accompanying the message. See Figure 2-2. If the MAC matches, then you know, within the strength of the MAC function and key, that some program with possession of the secret produced the MAC. Of course, every program that can verify the MAC needs to know this secret. Thus all of them can create valid MACs even if they should only receive and verify these codes.
Figure 2-2. Message authentication codes
A simple MAC function might append the secret ...
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