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SMS4 is a 128-bit block cipher used in the WAPI standard for wireless networks in China. In this paper, we analyze the security of the SMS4 block cipher against differential cryptanalysis. Firstly, we prove three theorems and one corollary that reflect relationships of 5- and 6-round SMS4. Next, by these relationships, we clarify the minimum number of active S-boxes in 6-, 7- and 12-round SMS4 respectively...