Y. Li et al., "New Records in Collision Attacks on RIPEMD-160 and SHA-256"¹

RIPEMD-160 and SHA-256 are two hash functions used to generate the bitcoin address. In particular, RIPEMD-160 is an ISO/IEC standard and SHA-256 has been widely used in the world. Due to their complex designs, the progress to find (semi-free-start) collisions for the two hash functions is slow. Recently at EUROCRYPT 2023, Liu et al. presented the first collision attack on 36 steps of RIPEMD-160 and the first MILP-based method to find collision-generating signed differential characteristics. We continue this line of research and implement the MILP-based method with a SAT/SMT-based method. Furthermore, we observe that the collision attack on RIPEMD-160 can be improved to 40 steps with different message differences. We have practically found a colliding message pair for 40-step RIPEMD-160 in 16 hours with 115 threads. Moreover, we also report the first semi-free-start (SFS) colliding message pair for 39-step SHA-256, which can be found in about 3 hours with 120 threads. These results update the best (SFS) collision attacks on RIPEMD-160 and SHA-256. Especially, we have made some progress on SHA-256 since the last update on (SFS) collision attacks on it at EUROCRYPT 2013, where the first practical SFS collision attack on 38-step SHA-256 was found.

#ResearchPapers #IACR #PracticalCollisions #RIPEMD-160 #SHA256 #SAT #SMT
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¹ https://eprint.iacr.org/2023/285

F. Liu et al., "Analysis of RIPEMD-160: New Collision Attacks and Finding Characteristics with MILP"¹

The hash function RIPEMD-160 is an ISO/IEC standard and is being used to generate the bitcoin address together with SHA-256. Despite the fact that many hash functions in the MD-SHA hash family have been broken, RIPEMD-160 remains secure and the best collision attack could only reach up to 34 out of 80 rounds, which was published at CRYPTO 2019. In this paper, we propose a new collision attack on RIPEMD-160 that can reach up to 36 rounds with time complexity 2^ 64.5. This new attack is facilitated by a new strategy to choose the message differences and new techniques to simultaneously handle the differential conditions on both branches. Moreover, different from all the previous work on RIPEMD-160, we utilize a MILP-based method to search for differential characteristics, where we construct a model to accurately describe the signed difference transitions through its round function. As far as we know, this is the first model targeting the signed difference transitions for the MD-SHA hash family. Indeed, we are more motivated to design this model by the fact that many automatic tools to search for such differential characteristics are not publicly available and implementing them from scratch is too time-consuming and difficult. Hence, we expect that this can be an alternative easy tool for future research, which only requires to write down some simple linear inequalities.

#ResearchPapers #IACR #Cryptanalysis #RIPEMD-160 #CollisionAttack #ModularDifference #SignedDifference #MILP
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¹ https://eprint.iacr.org/2023/277