A related key impossible differential attack against 22 rounds of the lightweight block cipher LBlock

Abstract

LBlock is a new lightweight block cipher proposed by Wu and Zhang (2011) [12] at ACNS 2011. It is based on a modified 32-round Feistel structure. It uses keys of length 80 bits and message blocks of length 64 bits.In this letter, we examine the security arguments given in the original article and we show that we can improve the impossible differential attack given in the original article on 20 rounds by constructing a 22-round related key impossible differential attack that relies on intrinsic weaknesses of the key schedule. This attack has a complexity of 270 cipher operations using 247 plaintexts. This result was already published in Minier and Naya-Plasencia (2011) [9].