## Cryptology ePrint Archive: Report 2020/1049

Rotational analysis of ChaCha permutation

Stefano Barbero and Emanuele Bellini and Rusydi Makarim

Abstract: 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.

Category / Keywords: secret-key cryptography / ChaCha20, Stream Cipher, Rotational cryptanalysis, Permutation, Distinguisher

Date: received 31 Aug 2020

Contact author: eemanuele bellini at gmail com

Available format(s): PDF | BibTeX Citation

Short URL: ia.cr/2020/1049

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