Moving a Step of ChaCha in Syncopated Rhythm

Shichang Wang; Meicheng Liu; Shiqi Hou; Dongdai Lin

Paper 2023/1087

Moving a Step of ChaCha in Syncopated Rhythm

Shichang Wang, Institute of Information Engineering, Chinese Academy of Sciences, University of Chinese Academy of Sciences

Meicheng Liu, Institute of Information Engineering, Chinese Academy of Sciences, University of Chinese Academy of Sciences

Shiqi Hou, Institute of Information Engineering, Chinese Academy of Sciences, University of Chinese Academy of Sciences

Dongdai Lin, Institute of Information Engineering, Chinese Academy of Sciences, University of Chinese Academy of Sciences

Abstract

The stream cipher ChaCha is one of the most widely used ciphers in the real world, such as in TLS, SSH and so on. In this paper, we study the security of ChaCha via differential cryptanalysis based on probabilistic neutrality bits (PNBs). We introduce the \textit{syncopation} technique for the PNB-based approximation in the backward direction, which significantly amplifies its correlation by utilizing the property of ARX structure. In virtue of this technique, we present a new and efficient method for finding a good set of PNBs. A refined framework of key-recovery attack is then formalized for round-reduced ChaCha. The new techniques allow us to break 7.5 rounds of ChaCha without the last XOR and rotation, as well as to bring faster attacks on 6 rounds and 7 rounds of ChaCha.

Note: In the full version of the paper, we refined the description of formal definitions for the proposed concepts and changed the URL of codes for the verification experiments. Moreover, the details of proofs, applications and experiments of practical attacks are provided in the Supplementary Material of the full version.

Metadata

Available format(s): PDF
Category: Attacks and cryptanalysis
Publication info: A major revision of an IACR publication in CRYPTO 2023
Keywords: Stream Ciphers ChaCha Differential Cryptanalysis PNB Syncopation
Contact author(s): wangshichang @ iie ac cn
liumeicheng @ iie ac cn
houshiqi @ iie ac cn
ddlin @ iie ac cn
History: 2023-07-16: approved; 2023-07-13: received; See all versions
Short URL: https://ia.cr/2023/1087
License: CC BY

BibTeX

@misc{cryptoeprint:2023/1087,
      author = {Shichang Wang and Meicheng Liu and Shiqi Hou and Dongdai Lin},
      title = {Moving a Step of ChaCha in Syncopated Rhythm},
      howpublished = {Cryptology ePrint Archive, Paper 2023/1087},
      year = {2023},
      note = {\url{https://eprint.iacr.org/2023/1087}},
      url = {https://eprint.iacr.org/2023/1087}
}