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International Association for Cryptologic Research (IACR)
Abstract
SMS4 is a 32-round unbalanced Feistel block cipher with its block
size and key size being 128 bits. As a fundamental block cipher used
in the WAPI standard, the Chinese national standard for WLAN, it has
been widely implemented in Chinese WLAN industry. In this paper, we
present a modified branch-and-bound algorithm which can be used for
searching multiple linear characteristics for SMS4-like unbalanced
Feistel block ciphers. Furthermore, we find a series of 5-round
iterative linear characteristics of SMS4 when applying the modified
algorithm in SMS4. Then based on each 5-round iterative linear
characteristic mentioned above, an 18-round linear characteristic of
SMS4 can be constructed, thus leading to a list of 18-round linear
characteristics of SMS4. According to the framework of Biryukov $et\
al.$ from Crpto 2004, a key recovery attack can be mounted on
22-round SMS4 by utilizing the above multiple linear
characteristics. As a matter of fact, our result has much lower data
complexity than the previously best known cryptanalytic result on
22-round SMS4, which is also the previously best known result on
SMS4
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