International Association for Cryptologic Research

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Differential-Linear Cryptanalysis of Reduced Round ChaCha

Authors:
Zhichao Xu , Information Engineering University, Zheng Zhou, China
Hong Xu , Information Engineering University, Zheng Zhou, China
Lin Tan , Information Engineering University, Zheng Zhou, China
Wenfeng Qi , Information Engineering University, Zheng Zhou, China
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DOI: 10.46586/tosc.v2024.i2.166-189
URL: https://tosc.iacr.org/index.php/ToSC/article/view/11626
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Abstract: ChaCha is a well-known stream cipher that has been used in many network protocols and software. In this paper, we study the security of reduced round ChaCha. First, by considering the differential-linear hull effect, we improve the correlation of a four-round differential-linear distinguisher proposed at FSE 2023 by providing other intermediate linear masks. Then, based on the four-round differential-linear distinguisher and the PNB method, by using the assignment 100 ··· 00 for consecutive PNBs, higher backward correlation is obtained and improved key recovery attacks of 7-round and 7.25-round ChaCha are obtained with time complexity 2189.7 and 2223.9, which improve the previously best-known attacks by 217.1 and 214.44, respectively. Finally, we consider the equivalence of the security between (R + 0.25)-round and (R + 0.5)⊕-round ChaCha, and show that (R + 0.25)-round and (R + 0.5)⊕-round ChaCha provide the same security against chosen(known) plaintext attacks. As a result, improved differential-linear cryptanalysis of 7.5⊕-round ChaCha can also be obtained similarly to that of 7.25-round ChaCha, which improves the previously best-known attack by 219.
BibTeX
@article{tosc-2024-34380,
  title={Differential-Linear Cryptanalysis of Reduced Round ChaCha},
  journal={IACR Transactions on Symmetric Cryptology},
  publisher={Ruhr-Universität Bochum},
  volume={024 No. 2},
  pages={166-189},
  url={https://tosc.iacr.org/index.php/ToSC/article/view/11626},
  doi={10.46586/tosc.v2024.i2.166-189},
  author={Zhichao Xu and Hong Xu and Lin Tan and Wenfeng Qi},
  year=2024
}