LBlock-s is the kernel block cipher of the authentication encryption algorithm LAC submitted to CAESAR competition.The general structure of LBlock-s is almost the same as that of LBlock

but LBlock-s adopts an improved key schedule algorithm with better diffusion property.Using the shifting relation of subkeys derived by the key schedule algorithm

an impossible differential cryptanalysis on 21-round LBlock-s was presented based on a 14-round impossible differential.The time and data complexities are 2

67.61

21-round encryptions and 2

63

chosen plaintexts respectively

and the number of subkey bits needed to be guessed is 72.Using partial-matching method

an impossible differential cryptanalysis on

LBlock-s up to 23-round was also presented with time complexity less than exhaustion of all key bits.This work is useful for the security analysis of LAC algorithm.