### PNB-focused Differential Cryptanalysis of ChaCha Stream Cipher

Shotaro Miyashita, Ryoma Ito, and Atsuko Miyaji

##### Abstract

This study focuses on differential cryptanalysis of the ChaCha stream cipher. In the conventional approach, an adversary first searches for an input/output differential pair with the highest differential bias and then analyzes the probabilistic neutral bits (PNB) based on the obtained input/output differential pair. However, although the time and data complexities for the attack can be estimated by the differential bias and PNB obtained by this approach, the combination of the differential bias and PNB is not always optimal. In addition, the existing studies have not performed a comprehensive analysis of the PNB; thus, they have not provided an upper bound on the number of rounds required for a differential attack that uses a single-bit truncated differential to be successful. To address these limitations, we propose a PNB-focused differential attack on reduced-round ChaCha by first comprehensively analyzing the PNB for all possible single-bit truncated output differences and then searching for the input/output differential pair with the highest differential bias based on the obtained PNB. The best existing attack on ChaCha, proposed by Beierle et al. at CRYPTO 2020, works on up to 7 rounds, whereas the most extended attack we observed works on up to 7.25 rounds using the proposed PNB-focused approach. The time complexity, data complexity, and success probability of the proposed attack are $$2^{255.62}$$, $$2^{48.36}$$, and 0.5, respectively. Although the proposed attack is less efficient than a brute force attack, it is the first dedicated attack on the target and provides both a baseline and useful components (i.e., differential bias and PNB) for improved attacks.

Available format(s)
Category
Secret-key cryptography
Publication info
Published elsewhere. ACISP 2022
Keywords
Contact author(s)
miyashita @ cy2sec comm eng osaka-u ac jp
itorym @ nict go jp
miyaji @ comm eng osaka-u ac jp
History
2022-05-06: last of 2 revisions
See all versions
Short URL
https://ia.cr/2021/1537

CC BY

BibTeX

@misc{cryptoeprint:2021/1537,
author = {Shotaro Miyashita and Ryoma Ito and Atsuko Miyaji},
title = {PNB-focused Differential Cryptanalysis of ChaCha Stream Cipher},
howpublished = {Cryptology ePrint Archive, Paper 2021/1537},
year = {2021},
note = {\url{https://eprint.iacr.org/2021/1537}},
url = {https://eprint.iacr.org/2021/1537}
}

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