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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 7.25 round by obtaining an improved set of Probabilistic Neutral Bits and considering our revised estimation. Our result with time complexity \(2^{212.43}\) and data complexity \(2^{100.56}\) improves the result of Xu et al., where they could achieve time and data complexity \(2^{223.9}\) and \(2^{100.80}\) , respectively. For both the 7 and 7.25 rounds, we can show an improvement of the order of \(2^{11}\) in the time complexity. For 7.5-round, we improve the result of Dey [IEEE-IT 2024], which reports the time and data complexity of \(2^{255.24}\) and \(2^{32.64}\) , respectively. By applying the formula of the same paper and incorporating additional PNBs, we obtain improved time and data complexity of \({2^{253.23}}\) and \(2^{34.47}\) , 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.

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On Improved Cryptanalytic Results Against ChaCha for Reduced Rounds \(\ge 7\)

  • 会议论文

  • 发表日期：

    2025年

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  • Nitin Kumar Sharma,
  • Sabyasachi Dey,
  • Santanu Sarkar,
  • Subhamoy Maitra

摘要

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 7.25 round by obtaining an improved set of Probabilistic Neutral Bits and considering our revised estimation. Our result with time complexity \(2^{212.43}\) and data complexity \(2^{100.56}\) improves the result of Xu et al., where they could achieve time and data complexity \(2^{223.9}\) and \(2^{100.80}\) , respectively. For both the 7 and 7.25 rounds, we can show an improvement of the order of \(2^{11}\) in the time complexity. For 7.5-round, we improve the result of Dey [IEEE-IT 2024], which reports the time and data complexity of \(2^{255.24}\) and \(2^{32.64}\) , respectively. By applying the formula of the same paper and incorporating additional PNBs, we obtain improved time and data complexity of \({2^{253.23}}\) and \(2^{34.47}\) , 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.