International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Attacking Reduced Round SHA-256

Somitra Kumar Sanadhya
Palash Sarkar
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Abstract: The SHA-256 hash function has started getting attention recently by the cryptanalysis community due to the various weaknesses found in its predecessors such as MD4, MD5, SHA-0 and SHA-1. We make two contributions in this work. First we describe message modification techniques and use them to obtain an algorithm to generate message pairs which collide for the actual SHA-256 reduced to 18 steps. Our second contribution is to present differential paths for 19, 20, 21, 22 and 23 steps of SHA-256. We construct parity check equations in a novel way to find these characteristics. Further, the 19-step differential path presented here is constructed by using only 15 local collisions, as against the previously known 19-step near collision differential path which consists of interleaving of 23 local collisions. Our 19-step differential path can also be seen as a single local collision at the message word level. We use a linearized local collision in this work. These results do not cause any threat to the security of the SHA-256 hash function.
  title={Attacking Reduced Round SHA-256},
  booktitle={IACR Eprint archive},
  keywords={Cryptanalysis, SHA-256 Hash function, Reduced round attacks.},
  note={A shorter version of this paper will appear in the proceedings of ACNS 2008. 14014 received 31 Mar 2008, last revised 15 May 2008},
  author={Somitra Kumar Sanadhya and Palash Sarkar},