Improved Differential-Linear Cryptanalysis of Reduced Rounds of ChaCha

ChaCha is a stream cipher introduced by Daniel Bernstein as a variant of Salsa20. Since the release of ChaCha, it has received the attention of many researchers as it has been widely deployed. In this study, we derive a new linear approximation for ChaCha with a higher probability bias. In addition, we found a combination of input/output differences corresponding to a new linear relationship. Furthermore, we proved that the proposed bias can be used to attack 7-round ChaCha with a reduced computational complexity from \(2^{221.95}\) to \(2^{120.9}\) .