Lysytskyi K. Methods and means of constructing block symmetric ciphers with increased stability and speed

The dissertation is devoted to substantiation of a new methodology for assessing the resistance of block symmetric ciphers to attacks of differential and linear cryptanalysis and its further development in the direction of creating and developing a new concept of designing block symmetric ciphers. A large set of theoretical and experimental studies of the indicators of randomness of modern block symmetric ciphers and random substitutions. A new concept of designing block symmetric ciphers is proposed, the basis of which was the use separately or jointly of three methods aimed at increasing the number of S-blocks that are activated in the first cycle of encryption. For the first time, methods for constructing Shup-1M and Shup-2M ciphers with 256-bit input, with improved stability and speed indicators, focused on use in the post-quantum period of cryptography development, have been proposed. The developed approaches and methods were also used for comparative analysis of ciphers submitted at one time to the Ukrainian competition for the selection of the national standard of block symmetric encryption of Ukraine, and later in the study of the cipher Kalyna-2, which became the standard. Recent developments determine the direction of further improvement of the properties and indicators of evidence-based stability of block symmetric ciphers, focused on the use in the postquantum period of cryptography. As a result of the performed researches, the important scientific and technical problem which has practical value for improvement of technologies of block symmetric encryption is solved and consists in development of methods of improvement of dynamic indicators of arrival of block symmetric ciphers to a condition of random substitution on the basis of increase in number of S-blocks. encryption cycles.