Multidimensional Zero-Correlation Linear Cryptanalysis on 23-Round LBlock-s

摘要

LBlock-s is the kernel block cipher of the authentication encryption algorithm LAC submitted to CAESAR competition. The LBlock-s algorithm is almost the same as LBlock except that the former adopts an improved key schedule algorithm with better diffusion property. Using the shifting relation of certain subkeys derived by the new key schedule algorithm, we present a multidimensional zero-correlation linear cryptanalysis on 23-round LBlock-s. The time complexity of the attack is about 2~(75.4) 23-round encryptions, where 2~(62.3) known plaintexts are used and 60 subkey bits are guessed, which is three bits less than that of LBlock. Our research showed that the improved key schedule algorithm did not enhance their ability to protect against zero-correlation linear cryptanalysis, and it is better to use the irregular bit-shifting to disturb the shifting relation between subkeys.