International Association
for Cryptologic Research

MIBS is a 32-round lightweight block cipher following a Feistel structure with the block length of 64-bit and the key length of 64 or 80 bits. In this paper, the properties of the key scheduling algorithm are investigated and lots of repeated bits among the different round keys are found. Moreover, the optimal guessing order of the unknown key bits is obtained by using the greedy algorithm. At last, combined with the early abort technique, the differential cryptanalyses are improved to 15 rounds both of MIBS-64 and MIBS-80. For MIBS-64, the data complexity is $2^{59}$, and the time complexity is $2^{46.2}$ encryptions. For MIBS-80, the data complexity is $2^{59}$, and the time complexity is $2^{51.7}$ encryptions. The key scheduling algorithm of MIBS is similar to some other lightweight block ciphers, and we hope further similarities will help build better attacks for them as well.