International Association
for Cryptologic Research

In this paper, we focus on the automatic differential cryptanalysis of ARX block ciphers with respect to XOR-difference, and develop Mouha et al.'s framework of finding differential characteristics by adding a new method for constructing long characteristics from short ones. The new method reduces the searching time a lot and makes it possible to search differential characteristics for ARX block ciphers with large word sizes. Also, we take the differential effect into consideration and find that the differential probability increases by a factor of $4\sim2^{10}$ when multiple characteristics are counted in. The efficiency of our method is demonstrated by improved attacks of SPECK and LEA, which cover 1, 1, 4 and 5 more rounds of SPECK48, SPECK64, SPECK96 and SPECK128, respectively, and 2 more rounds of LEA-128.