Boosting Differential-Linear Cryptanalysis of ChaCha7 with MILP

In this paper, we present an improved differential-linear cryptanalysis of the ChaCha stream cipher. Our main contributions are new distinguishers that were able to build thanks following improvements: a) considered a larger search space, including 2-bit differences (besides 1-bit differences) for difference at beginning differential part trail; b) better choice mask between and linear parts; c) carefully crafted MILP tool finds trails with higher correlation part. We eventually obtain distinguisher reduced 7 rounds requires 2166.89 computations, improving previous record (ASIACRYPT 2022) by factor 247. Also, 7.5 2251.4 being first time against rounds. Using our tool, also found 5-round distinguisher. When combined probabilistic neutral bits (PNB) framework, key-recovery attack on computational complexity 2206.8, 214.2 upon recent result published EUROCRYPT 2022.