Improved cryptanalysis of the FOX block cipher

Abstract

In this paper, we analyze the pseudorandomness of the high level structure of FOX64, and describe a 2-round pseudorandomness distinguisher and a 3-round strong pseudorandomness distinguisher, and thus prove that 3-round and 4-round are necessary to achieve the pseudorandomness and strong pseudorandomness respectively. We also find a 4-round impossible difference characteristic. By using it, an adversary can attack 5, 6 and 7-round FOX64 with 269, 2133 and 2197 encryptions respectively. which improves the best known attack by a factor of 240.4. This attack can be extended to 5-round FOX128 with 2133 encryptions. © 2010 Springer-Verlag.