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Boosting Differential-Linear Cryptanalysis of ChaCha7 with MILP

Authors:
Emanuele Bellini , Cryptography Research Centre, Technology Innovation Institute, Abu Dhabi, UAE
David Gerault , Cryptography Research Centre, Technology Innovation Institute, Abu Dhabi, UAE
Juan Grados , Cryptography Research Centre, Technology Innovation Institute, Abu Dhabi, UAE
Rusydi H. Makarim , Independent Researcher
Thomas Peyrin , Nanyang Technological University, Singapore, Singapore
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DOI: 10.46586/tosc.v2023.i2.189-223
URL: https://tosc.iacr.org/index.php/ToSC/article/view/10983
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Abstract: In this paper, we present an improved differential-linear cryptanalysis of the ChaCha stream cipher. Our main contributions are new differential-linear distinguishers that we were able to build thanks to the following improvements: a) we considered a larger search space, including 2-bit differences (besides 1-bit differences) for the difference at the beginning of the differential part of the differential-linear trail; b) a better choice of mask between the differential and linear parts; c) a carefully crafted MILP tool that finds linear trails with higher correlation for the linear part. We eventually obtain a new distinguisher for ChaCha reduced to 7 rounds that requires 2166.89 computations, improving the previous record (ASIACRYPT 2022) by a factor of 247. Also, we obtain a distinguisher for ChaCha reduced to 7.5 rounds that requires 2251.4 computations, being the first time of a distinguisher against ChaCha reduced to 7.5 rounds. Using our MILP tool, we also found a 5-round differential-linear distinguisher. When combined with the probabilistic neutral bits (PNB) framework, we obtain a key-recovery attack on ChaCha reduced to 7 rounds with a computational complexity of 2206.8, improving by a factor 214.2 upon the recent result published at EUROCRYPT 2022.
BibTeX
@article{tosc-2023-33313,
  title={Boosting Differential-Linear Cryptanalysis of ChaCha7 with MILP},
  journal={IACR Transactions on Symmetric Cryptology},
  publisher={Ruhr-Universit├Ąt Bochum},
  volume={2023, Issue 2},
  pages={189-223},
  url={https://tosc.iacr.org/index.php/ToSC/article/view/10983},
  doi={10.46586/tosc.v2023.i2.189-223},
  author={Emanuele Bellini and David Gerault and Juan Grados and Rusydi H. Makarim and Thomas Peyrin},
  year=2023
}