Near-Collisions on the Reduced-Round Compression Functions of Skein and BLAKE

The SHA-3 competition organized by NIST [1] aims to find a new hash standard as a replacement of SHA-2. Till now, 14 submissions have been selected as the second round candidates, including Skein and BLAKE, both of which have components based on modular addition, rotation and bitwise XOR (ARX). In this paper, we propose improved near-collision attacks on the reduced-round compression functions of Skein and a variant of BLAKE. The attacks are based on linear differentials of the modular additions. The computational complexity of near-collision attacks on a 4-round compression function of BLAKE-32, 4-round and 5-round compression functions of BLAKE-64 are 2, 2 and 2 respectively, and the attacks on a 24-round compression functions of Skein-256, Skein-512 and Skein-1024 have a complexity of 2, 2 and 2 respectively.