Cryptology ePrint Archive: Report 2022/536

Revamped Differential-Linear Cryptanalysis on Reduced Round ChaCha

Sabyasachi Dey and Hirendra Kumar Garai and Santanu Sarkar and Nitin Kumar Sharma

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 ChaCha$128$ with time complexity $2^{123.04}.$

Category / Keywords: secret-key cryptography / Stream cipher, ARX, ChaCha, Probabilistic Neutral Bits (PNBs), Differential attack

Original Publication (in the same form): IACR-EUROCRYPT-2022

Date: received 3 May 2022

Contact author: sarkar santanu bir1 at gmail com

Available format(s): PDF | BibTeX Citation

Version: 20220510:080301 (All versions of this report)

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