A Brief Comparison of Simon and Simeck

Simeck is a new lightweight block cipher design based on combining the Simon and Speck block cipher. While the design allows a smaller and more efficient hardware implementation, its security margins are not well understood. The lack of design rationals of its predecessors further leaves some uncertainty on the security of Simeck. In this work we give a short analysis of the impact of the design changes by comparing the lower bounds for differential and linear characteristics with Simon. We also give a comparison of the effort of finding those bounds, which surprisingly is significant less for Simeck while covering a larger number of rounds. Furthermore, we provide new differentials for Simeck which can cover more rounds compared to previous results on Simon. Based on this we mount key recovery attacks on 19/26/33 rounds of Simeck32/48/64, which also give insights on the reduced key guessing effort due to the different set of rotation constants.