 Charles, who transmits Alice's packets to Bob, interleaves the packets with corresponding bogus packets (called "chaff") with corresponding serial numbers, arbitrary symbols, and a random number in place of the MAC.Charles does not need to know the key to do that (real MAC are large enough that it is extremely unlikely to generate a valid one by chance, unlike in the example).Bob uses the MAC to find the authentic messages and drops the "chaff" messages.