Differential Attacks on Reduced SIMON Versions with Dynamic Key-guessing Techniques

SIMON is a family of lightweight block ciphers which are designed by the U.S National Security Agency in 2013. It has totally 10 versions corresponding to different block size 2n and key length lk, named as SIMON2n/lk. In this paper, we present a new differential attack by considering the sufficient bit conditions of the previous differential paths. Based on the bit conditions, we successfully propose a new type of dynamic key-guessing technique which greatly reduces the key space guessed. Our attacks work on the reduced SIMON of all 10 suggested versions, which improve the best previous results by 2 to 4 rounds. For verification, we implemented a practical attack on 19-round SIMON32 in a PC, and the experimental data confirm the correctness of the attack, which also fit the theoretical complexity and success rate very well. It is remarked that, our cryptanalysis only provides a more accurate security evaluation, and it does not mean the security problem of the whole SIMON family.