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Revamped Differential-Linear Cryptanalysis on Reduced Round ChaCha
Date Issued
01-01-2022
Author(s)
Dey, Sabyasachi
Garai, Hirendra Kumar
Indian Institute of Technology, Madras
Sharma, Nitin Kumar
Abstract
In this paper, we provide several improvements over the existing differential-linear attacks on ChaCha. ChaCha is a stream cipher which has 20 rounds. At CRYPTO 2020, Beierle et al. observed a differential in the 3.5-th round if the right pairs are chosen. They produced an improved attack using this, but showed that to achieve a right pair, we need 2 5 iterations on average. In this direction, we provide a technique to find the right pairs with the help of listing. Also, we provide a strategical improvement in PNB construction, modification of complexity calculation and an alternative attack method using two input-output pairs. Using these, we improve the time complexity, reducing it to 2 221.95 from 2 230.86 reported by Beierle et al. for 256 bit version of ChaCha. Also, after a decade, we improve existing complexity (Shi et al. ICISC 2012) for a 6-round of 128 bit version of ChaCha by more than 11 million times and produce the first-ever attack on 6.5-round ChaCha128 with time complexity 2 123.04.
Volume
13277 LNCS