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In this paper, we investigate the neutral bits used in ARX ciphers from the perspective of the boomerang connectivity table. Two propositions are employed to obtain the probability of neutral bits for modular additions, and their proofs are given so that the probabilities can be calculated theoretically for the first time. Then, we apply our method to the neutral bits for neural-differential cryptanalysis of Speck, verify the probabilities, and find some neutral bits not used in previous works. This method is also utilized to reevaluate the probability of neutral bits for differential cryptanalysis of ChaCha. What’s more, a new notion named truncated boomerang connectivity table (TBCT) is also proposed to formalize the calculation of probabilities in the case of truncated differentials.

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A Note on Neutral Bits for ARX Ciphers from the Perspective of BCT

  • 会议论文

  • 发表日期：

    2025年

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  • Jiahao Zhao,
  • Qianqian Yang,
  • Ling Song,
  • Lei Hu

摘要

In this paper, we investigate the neutral bits used in ARX ciphers from the perspective of the boomerang connectivity table. Two propositions are employed to obtain the probability of neutral bits for modular additions, and their proofs are given so that the probabilities can be calculated theoretically for the first time. Then, we apply our method to the neutral bits for neural-differential cryptanalysis of Speck, verify the probabilities, and find some neutral bits not used in previous works. This method is also utilized to reevaluate the probability of neutral bits for differential cryptanalysis of ChaCha. What’s more, a new notion named truncated boomerang connectivity table (TBCT) is also proposed to formalize the calculation of probabilities in the case of truncated differentials.