Cryptanalysis of the LSH hash functions

Abstract

In this paper, we study the security of the LSH hash functions. We find that the wide-pipe MD-structural LSH hash functions apply the 17th PGV scheme which is backward attackable. This property equips us with trivial attacks including pseudo-preimage, free-start collision, and Type II boomerang. These attacks can never be available to previous MD-structural hash functions like Skein. We stress that such trivial attacks can only be regarded as distinguishers rather than real threat to the LSH in nowadays' practical applications. But we should still be cautious about the possible malicious use of LSH in specific situations in the future. We also launch 14-round boomerang attacks on LSH-512 and LSH-256 hash functions with complexities 2308 and 2242, respectively. We verify the correctness of our boomerang attacks by giving practical 11-round boomerang quartets. To the best of our knowledge, these are the first practically verifiable boomerang results on the LSH hash functions. These boomerang results indicate that the round functions of LSH are well designed. Copyright © 2016 John Wiley & Sons, Ltd.