Cryptanalysis of Raindrop and FBC

Abstract

This paper introduces the results of several different security analysis of two new block ciphers: Raindrop and FBC, which are the two candidates of block cipher designs submitted to the Chinese Cryptographic Algorithms Design Competition in 2019. Raindrop applies two-branch Feistel structure, while FBC block cipher is based on the four-way dual Feistel structure design. We give detailed security evaluation on Raindrop and FBC, using differential, linear, impossible difference and boomerang cryptanalysis approaches. For Raindrop, we achieved the results as follows: differential attack on 12-round Raindrop based on the 11-round distinguisher with the computational complexity of $2^{62.41}$; linear attack on 13-round Raindrop based on 12-round distinguisher with the computational complexity of $2^{106.3}$; impossible differential attack on 18-round Raindrop based on 12-round distinguisher with the computational complexity of $2^{102.83}$; and boomerang attack on 15-round Raindrop based on 14-round distinguisher with the computational complexity of $2^{224.6}$. For FBC, results are as follows: differential attack on 12-round FBC based on 11-round distinguisher with the computational complexity of $2^{93.41}$; linear attack on 11-round FBC based on 10-round distinguisher with the computational complexity of $2^{112.54}$; impossible differential attack on 11-round FBC based on 7-round distinguisher with the computational complexity of $2^{94.54}$; and boomerang attack on 13-round FBC based on 12-round distinguisher with the computational complexity of $2^{247.67}$. At present, the best records achieved are 18-round impossible differential attack for Raindrop-128-128 and 13-round boomerang attack for FBC128-256. The statistical distinguishers we built are similar to the proposals but we provide the concrete key recovery attacks in this study.