Title: The Improved 32nd-Order Differential Attack on 8 Rounds of MISTY2 without FL Functions

Issue Number: Vol. 2, No. 3
Year of Publication: Oct - 2013
Page Numbers: 27-34
Authors: Yasutaka Igarashi, Toshinobu Kaneko, Yutaka Eguchi, Takahiro Murai, Ryutaro
Journal Name: International Journal of Cyber-Security and Digital Forensics (IJCSDF)
- Hong Kong


We study the 32nd-order differential attack on 8 rounds of MISTY2 without FL functions. MISTY2 is a 64-bit block cipher with a 128-bit secret key proposed by Matsui of Mitsubishi Electric Corp. in 1996. We found the new 32nd-order differential characteristic of MISTY2 without FL functions, which makes the 32nd-order differential of the upper 23 bits out of a 32-bit input to the 8th-round FO function be zero. Using the characteristics, we show that 8 rounds of MISTY2 without FL functions can be simply attacked with 235 blocks of chosen plain text and 281.4 times of FO operation. Moreover we reduce the number of times of FO operation required for this attack by using a modulo 2 occurrence distribution, which is derived by a partial sum technique proposed by Ferguson et al. We apply this distribution to the intermediate data of encryption function, and show that the number of times of FO operation can be reduced to 257.4. This work is the first 8-round attack on MISTY2 as far as we know, while previously known higher-order differential attacks on MISTY2 are 5-round attack and 7-round attack.