Related-key rectangle cryptanalysis of reduced-round block cipher MIBS

A related-key rectangle attack treats a block cipher as a cascade of two sub-ciphers to construct distinguishers. In this paper, by introducing related-key differential characteristics with high probability for each sub-cipher, we construct a distinguisher for 13-round MIBS80, a Feistel block cipher with key length of 80 bits, and launch a key-recovery attack on 15-round MIBS80 with time complexity of 2⁶⁷ and data complexity of 2⁴⁹. A similar attack is also launched on 13-round MIBS64, a version of the cipher with 64-bit keys. This is the first and a textbook related-key rectangle cryptanalysis on MIBS block cipher.