Key recovery attack on ANSI retail MAC

A new type of attack is introduced which takes advantage of MAC truncation to simplify key recovery attacks based on MAC verifications. One example of the attack is described which, in certain circumstances, enables a more efficient attack than was previously known to be launched against the ANSI retail MAC. The existence of this attack means that truncation for this MAC scheme should be used with greater care than was previously believed, and very short MACs should be avoided altogether