Paper 2025/1581

Cryptanalysis of ChiLow with Cube-Like Attacks

Abstract

Proposed in EUROCRYPT~2025, \chilow is a family of tweakable block ciphers and a related PRF built on the novel nonlinear $\chichi$ function, designed to enable efficient and secure embedded code encryption. The only key-recovery results of \chilow are from designers which can reach at most 4 out of 8 rounds, which is not enough for a low-latency cipher like \chilow: more cryptanalysis efforts are expected. Considering the low-degree $\chichi$ function, we present three kinds of cube-like attacks on \chilow-32 under both single-tweak and multi-tweak settings, including \begin{itemize} \item[-] a \textit{conditional cube attack} in the multi-tweak setting, which enables full key recovery for 5-round and 6-round instances with time complexities $2^{32}$ and $2^{120}$, data complexities $2^{23.58}$ and $2^{40}$, and negligible memory requirements, respectively. \item[-] a \textit{borderline cube attack} in the multi-tweak setting, which recovers the full key of 5-round \chilow-32 with time, data, and memory complexities of $2^{32}$, $2^{18.58}$, and $2^{33.56}$, respectively. For 6-round \chilow-32, it achieves full key recovery with time, data, and memory complexities of $2^{34}$, $2^{33.58}$, and $2^{54.28}$, respectively. Both attacks are practical. \item [-] an \textit{integral attack} on 7-round \chilow-32 in the single-tweak setting. By combining a 4-round borderline cube with three additional rounds, we reduce the round-key search space from $2^{96}$ to $2^{73}$. Moreover, we present a method to recover the master key based on round-key information, allowing us to recover the master key for 7-round \chilow-32 with a time complexity of $2^{127.78}$. \end{itemize} All of our attacks respect security claims made by the designers. Though our analysis does not compromise the security of the full 8-round \chilow, we hope that our results offer valuable insights into its security properties.