Breaching Forró’s Security With Differential-Linear Foray

Abstract

Forró is a stream cipher proposed by Coutinho et al. in the Journal of Cryptology 2023 as a refinement of ChaCha design by providing better diffusion. The authors showed that existing attacks against ChaCha, which can attack ChaCha until seven rounds, can attack Forró till 5 rounds only. In this study, we provide the first cryptanalysis of Forró after the proposal of the cipher. We provide an attack against 5.5 rounds of Forró with a complexity (2225.04) significantly lower than the brute force search (2256), thus showing that the bound of 5 rounds can be breached. We also improve the attack against 5-round Forró proposed by the designers themselves by a margin of 213.86. These revelations raise concerns regarding Forró’s resilience against cryptographic attacks and underscore the necessity for ongoing scrutiny and refinement in cipher development.

Keywords

• Differential attack
• ARX cipher
• probabilistic neutral bits
• Forró