Bicliques for Preimages: Attacks on Skein-512 and the SHA-2 family

We introduce a new concept in splice-and-cut attacks on hash functions, which bridges the gap between preimage attacks and a powerful method of differential cryptanalysis. The new concept is called biclique, for its system of equations resembling a complete bipartite graph. In view of the current SHA-3 competition, we apply our method to the finalist Skein and demonstrate the first attack on a 22-round version of Skein-512 in the most relevant hash function setting. Then we present the best attacks on the SHA-2 family of hash functions, breaking 45 out of the 64 rounds of SHA-256, 50 rounds of the 80 rounds of SHA-512, and many more rounds in the less relevant compression function setting.

• Type: Journal
• Year: 2011
• Where: IACR
• Authors: Dmitry Khovratovich, Christian Rechberger, Alexandra Savelieva
• Keywords: Bipartite Graph, Hash Function, Hash Functions, IACR 2011