Collision Attacks on Round-Reduced Gimli-Hash/Ascon-Xof/Ascon-Hash
The NIST-approved lightweight cryptography competition is an ongoing project to look for some algorithms as lightweight cryptographic standards. Recently, NIST chooses 32 algorithms from the 57 submissions as Round 2 candidates. Gimli and Ascon are both the Round 2 candidates. In this paper, we analyze the security of their hash mode against collision attacks. Concretely, we mount collision attacks on three hash functions: Gimli-Hash, Ascon-Xof and Ascon-Hash. These three hash functions are all based on sponge constructions. We give two attack strategies for searching collisions in sponge-based hash functions. Following one strategy, we give two non-practical collision attacks: a 6-round collision attack on Gimli-Hash with time complexity 2 and a 2-round collision attack on Ascon-Hash with time complexity 2. Following the other strategy, we give a practical attack on 2-round Ascon-Xof with a 64-bit output. The time complexity is 2. We search for the differential characteristics using the MILP technique and the target differential algorithm.