New Differential-Based Distinguishers for Ascon via Constraint Programming

As the winner of the NIST lightweight cryptography project, Ascon has undergone extensive self-evaluation and third-party cryptanalysis. In this paper, we use constraint programming (CP) as a tool to analyze the Ascon permutation and propose several differential-based distinguishers. We first propose a search methodology for finding truncated differentials for Ascon with CP, the core of which is modeling with the undisturbed bits of the S-box. By using this method, we find the five- and six-round truncated differentials with a probability of 2⁻⁴⁴ and 2⁻¹⁶², respectively. Considering the application of permutation in the context, we also provide the five- and six-round truncated differential distinguishers under the weak-key setting. Then, inspired by our five-round truncated differentials, we propose a six-round boomerang characteristic, and based on this, we obtain the five- and six-round sandwich distinguishers with a complexity of 2⁷⁰ and 2¹³⁴, respectively. Using the CP tool again and specifying that the “3-3” differential pattern is satisfied in the middle rounds, we propose a six-round differential characteristic with a probability of 2⁻²⁸⁰, which increases the probability by 2²⁵ compared to the best known six-round differential characteristic.