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SMS4 is a 128-bit block cipher used in the WAPI standard for protecting data packets in WLAN. In this paper, a differential power analysis attack method on every byte of round keys is presented. Through this attack, the round keys of the last four rounds of SMS4 can be obtained, and then the 128- bit encryption key can be found out. The results of simulation experiments indicate that this attack is effective and practical on the SMS4 cipher.