Plateau characteristics

Plateau characteristics are a special type of characteristics whose probability depends on the key and can have only 2 values. For a (usually small) subset of the keys it has a non-zero probability and for all other keys its probability is zero. In this paper we prove that for a large group of ciphers, including the AES, all two-round characteristics are plateau characteristics. For the AES and other ciphers with a similar structure we show that the vast majority of characteristics over 4 or more rounds are plateau characteristics. In the case of the AES, for most keys there are two-round characteristics with fixed-key probability equal to 32/2 while the Maximum Expected Differential Probability (MEDP) of two-round differentials is at most 13.25/2 .