Method and apparatus for encrypting input bit sequence

摘要

An output bit sequences is derived from an initial bit sequence and this output bit sequence is used to encrypt an input bit sequence in a first mode of operation or not to so encrypt the input bit sequence in a second mode of operation. The mode of operation is switched automatically whenever the output bit sequence contains a predetermined trap bit sequence. As a result of this automatic switching between such encryption and no such encryption, unauthorized determination of secret codes is thwarted.