Algebraic methods of cryptanalysis are applicable to present-day ciphers. These methods are based on generation of an equation system, where elements of a ciphertext and a key are chosen as variables of the system. When implementing a linearization method to solve the equation system, we consider a possibility to find partial elements of a key. Generally, cryptosystems use S-blocks, which are the only element contributing to nonlinearity of a ciphering transformation and the level of its strength against cryptanalytic attacks. In this paper we present, by the example of encryption algorithm Kuznechik, an algebraic analysis applicable to some block ciphers.

This paper represents a developed cryptographic information protection algorithm based on a substitutionpermutation network. We describe the cryptographic transformations used in the developed algorithm. One of the features of the algorithm is the simplicity of its modification with regard to different security levels. The algorithm uses a predeveloped S-box tested against differential and linear cryptanalysis. The S-box is consistent with one of the known standards AES and GOST R 34.12-2015. We provide the findings of an avalanche-effect investigation and statistical properties of ciphertexts. The algorithm actually meets the avalanche-effect criterion even after the first round.