Non-linear approximations in linear cryptanalysis

Abstract

By considering the role of non-linear approximations in lin- ear cryptanalysis we obtain a generalization of Matsui’s linear crypta- nalytic techniques. This approach allows the cryptanalyst greater flexibil- ity in mounting a linear cryptanalytic attack and we demonstrate the effectiveness of our non-linear techniques with some simple attacks on LOKI91. These attacks potentially allow for the recovery of seven addi- tional bits of key information with less than 1/4 of the plaintext that is required using current linear cryptanalytic methods.