Collisions for Step-Reduced SHA-256

In this article we find collisions for step-reduced SHA-256. We develop a differential that holds with high probability if the message satisfies certain conditions. We solve the equations that arise from the conditions. Due to the carefully chosen differential and word differences, the message expansion of SHA-256 has little effect on spreading the differences in the words. This helps us to find full collision for 21-step reduced SHA-256, semi-free start collision, i.e. collision for a different initial value, for 23-step reduced SHA-256, and semi-free start near collision (with only 15 bit difference out of 256 bits) for 25-step reduced SHA-256.

• Type: Conference
• Year: 2008
• Where: FSE
• Authors: Ivica Nikolic, Alex Biryukov
• Keywords: 21-step Reduced Sha-256, Collision, Cryptology, FSE 2008, Step-reduced Sha-256