Gryshakov S. Method for designing randomized stream ciphers with non-linear random coding

This thesis is devoted to solving actual scientific problem of development the method for designing randomized stream ciphers (RSC) with nonlinear random coding to provide the security of state information resources. Analytical estimates of the parameters that determine the security of the Mihalević-Imai RSC against known ciphertext attacks and chosen initialization vectors attacks are obtained in the thesis for the first time. It was proved for the first time that a class of the Mihalević-Imai RSC has a significant weakness which consists in reducing the amount of information which is necessary for real-time recovery of the plaintext. Analytical bounds of the transmission rate for the Mihalević-Imai RSC given the limitations on the encryption security and the probability of the correct reception of messages by the legitimate receiver are obtained for the first time. The tech-nique for designing RSC was further developed. In contrast to before known ap-proaches, the proposed method is based on the employment of the nonlinear transformations or keyless hash functions for random coding. Obtained new scien-tific results give the developer more capabilities for designing computationally se-cure RSC by enlarging the class of transformations used in the construction of a randomizer. Main practical result of the thesis is a possibility to design provably se-cure RSC without changing the encryption algorithms to ensure the security of state information resources.