Paper 2025/428

On Improved Cryptanalytic Results against ChaCha for Reduced Rounds ≥ 7

Abstract

In this paper, we analyze the subtle issues of complexity estimates related to state-of-the-art cryptanalytic efforts on ChaCha. In this regard, we demonstrate that the currently best-known cryptanalytic result on $7$-round ChaCha with time $2^{189.7}$ and data $2^{102.63}$ [Xu et al., ToSC 2024] can be estimated as $2^{178.12}$ for time and $2^{101.09}$ for data complexity. We improve the best-known result for the $$ round by obtaining an improved set of Probabilistic Neutral Bits and considering our revised estimation. Our result with time complexity $$ and data complexity $$ improves the result of Xu et al., where they could achieve time and data complexity $$ and $$, respectively. For both the $$ and $$ rounds, we can show an improvement of the order of $$ in the time complexity. For $$-round, we improve the result of Dey [IEEE-IT 2024], which reports the time and data complexity of $$ and $$, respectively. By applying the formula of the same paper and incorporating additional PNBs, we obtain improved time and data complexity of $$ and $$, respectively. Thus, this paper describes the currently best-known cryptanalytic results against reduced round ChaCha. Our results do not affect the security claims of the complete algorithm with 20 rounds. Also, we provide a rebuttal of the Work by Wang et al. \cite{wangeprint} and analyze their claim about the error in the ``Divide-and-Conquer'' Approach.