Total break of Zorro using linear and differential attacks

Abstract

An AES-like lightweight block cipher, namely Zorro, was proposed in CHES 2013. While it has a 16-byte state, it uses only 4 S-Boxes per round. This weak nonlinearity was widely criticized, insofar as it has been directly exploited in all the attacks on Zorro reported by now, including the weak key, reduced round, and even full round attacks. In this paper, using some properties discovered by Wang <em>et al.</em> we present new differential and linear attacks on Zorro, both of which recover the full secret key with practical complexities. These attacks are based on very efficient distinguishers that have only two active S-Boxes per four rounds. The time complexities of our differential and linear attacks are 2^55.40 and 2^45.44 and the data complexity are 2^55.15 chosen plaintexts and 2^45.44 known plaintexts, respectively. The results clearly show that the block cipher Zorro does not have enough security against differential and linear attacks.