## Cryptology ePrint Archive: Report 2019/287

Security Evaluation for Snow 2.0-like Stream Ciphers Against Correlation Attacks over Extension Fields

A. N. Alekseychuk and S. M. Koniushok and M. V. Poremskyi

Abstract: We propose a general method for security evaluation of SNOW 2.0-like ciphers against correlation attacks that are built similarly to known attacks on SNOW 2.0. Unlike previously known methods, the method we propose is targeted at security proof and allows obtaining lower bounds for efficiency of attacks from the class under consideration directly using parameters of stream cipher components similarly to techniques for security proofs of block ciphers against linear cryptanalysis. The method proposed is based upon automata-theoretic approach to evaluation the imbalance of discrete functions. In particular, we obtain a matrix representation and upper bounds for imbalance of an arbitrary discrete function being realized by a sequence of finite automata. These results generalize a number of previously known statements on matrix (linear) representations for imbalance of functions having specified forms, and may be applied to security proofs for other stream ciphers against correlation attacks. Application of this method to SNOW 2.0 and Strumok ciphers shows that any of the considered correlation attacks on them over the field of the order 256 has an average time complexity not less than $2^{146.20}$ and $2^{249.40}$ respectively, and requires not less than $2^{142.77}$ and, respectively, $2^{249.38}$ keystream symbols.

Category / Keywords: secret-key cryptography / symmetric cryptography, stream cipher, correlation attack, system of noised linear equations, discrete Fourier transform, proof of security, SNOW 2.0, Strumok.