Quantum security of Trojan message attacks on Merkle–Damgård hash construction

In this paper, we promote Trojan message attacks against Merkle–Damgård hash functions and their concatenation combiner in quantum settings for the first time. Two main quantum scenarios are considered, involving the scenarios where a substantial amount of cheap quantum random access memory (qRAM) is available and where qRAM is limited and expensive to access. We first discuss the construction of diamond structures and analyze the corresponding time complexity in both of these quantum scenarios. Secondly, we propose quantum versions of the generic Trojan message attacks on Merkle–Damgård hash functions as well as their improved versions by combining with diamond structures and expandable messages, and then determine their cost. Finally, we propose Trojan message attack against Merkle–Damgård hash concatenation combiner in quantum setting. The results show that Trojan message attacks can be improved significantly with quantum computers under both scenarios, so the security of hash constructions in classical setting requires careful re-evaluation before being deployed to the post-quantum cryptography schemes.