Improved Cryptanalysis of Reduced RIPEMD-160.

In this article, we propose an improved cryptanalysis of the double-branch hash function standard RIPEMD-160. Using a carefully designed non-linear path search tool, we study the potential differential paths that can be constructed from a difference in a single message word and show that some of these message words can lead to very good differential path candidates. Leveraging the recent freedom degree utilization technique from Landelle and Peyrin to merge two branch instances, we eventually manage to obtain a semi-free-start collision attack for 42 steps of the RIPEMD-160 compression function, while the previously best know result reached 36 steps. In addition, we also describe a 36-step semi-free-start collision attack which starts from the first step.