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 $l_k$, named as SIMON$2n/l_k$. In this paper, we present a new 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 18-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.
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