ChaCha置换的旋转分析
Rotational analysis of ChaCha permutation
摘要：
我们表明，ChaCha20流密码的基本置换不表现为随机置换多达17轮旋转密码分析。特别地，我们通过ChaCha四分之一轮推导出了旋转概率的上界，并展示了如何将该界推广到全轮，然后推广到全置换。所获得的界限表明，发现我们称之为平行旋转碰撞的概率例如对于17轮ChaCha置换小于$2^{-488}$，而对于相同输入大小的随机置换，该概率为$2^{-511}$。我们注意到，我们的攻击不是对ChaCha20流密码的攻击，而是从旋转密码分析的角度对其内部置换的理论分析。
英文摘要：
We show that the underlying permutation of ChaCha20 stream cipher does not behave as a random permutation for up to 17 rounds with respect to rotational cryptanalysis. In particular, we derive a lower and an upper bound for the rotational probability through ChaCha quarter round, we show how to extend the bound to a full round and then to the full permutation. The obtained bounds show that the probability to find what we call a parallel rotational collision is, for example, less than $2^{-488}$ for 17 rounds of ChaCha permutation, while for a random permutation of the same input size, this probability is $2^{-511}$. We remark that our distinguisher is not an attack to ChaCha20 stream cipher, but rather a theoretical analysis of its internal permutation from the point of view of rotational cryptanalysis.
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