International Association
for Cryptologic Research

In this paper we continue the previous line of research on the analysis of the differential properties of the lightweight block ciphers Simon and Speck. We apply a recently proposed technique

for automatic search for differential trails in ARX ciphers and improve the trails in Simon32 and Simon48 previously reported as best. We further extend the search technique for the case of differen-

tials and improve the best previously reported differentials on Simon32, Simon48 and Simon64 by exploiting more effectively the strong differential effect of the cipher. We also present improved trails and differentials on Speck32, Speck48 and Speck64. Using these new results we improve the currently best known attacks on several versions of Simon and Speck. A second major contribution of the paper

is a graph based algorithm (linear time) for the computation of the exact differential probability of the main building block of Simon: an AND operation preceded by two bitwise shift operations. This gives

us a better insight into the differential property of the Simon round function and differential effect in the cipher. Our algorithm is general and works for any rotation constants. The presented techniques

are generic and are therefore applicable to a broader class of ARX designs.