Moving a Step of ChaCha in Syncopated Rhythm (original) (raw)

Paper 2023/1087

Moving a Step of ChaCha in Syncopated Rhythm

, 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.

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}, url = {https://eprint.iacr.org/2023/1087} }