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The entry of mankind into the era of high technologies, the rapid growth of computer technology contributes to the expansion of the range of electronic services. To ensure the security of confidential information, personal data, cryptographic systems of traditional cryptography (symmetric cryptosystems) and public key cryptography (asymmetric cryptosystems) are used. As a rule, the former provides security services, the latter provide key distribution. However, in the conditions of totalitarian surveillance in society by the special services of developed countries, cryptographic tabs are embedded in cryptographic algorithms, which, on the one hand, provide “quick” access for special services to confidential information, and on the other hand, allow intruders to break into the cryptosystem and obtain user data. The article proposes a modification of the well-known GOST 28147-89 algorithm, which ensures the "elimination" of possible crypto-bookmarks and an increase in crypto-resistance in the post-quantum period (the emergence of a full-scale quantum computer that allows hacking modern symmetric and asymmetric cryptosystems based on Grover and Shor algorithms). It is proposed to use the procedures for modifying the block-symmetric encryption algorithm (BSEA) GOST 28147-89 (2009, 2015) in OFB mode, which will make it possible to form a pseudo-random sequence based on dynamic changes in the S-box, and provide the required level of security.

The development of information technologies in cyberspace is associated with the development of security services in the components of security: cybersecurity, information security and information security. To provide security services, as a rule, cryptographic algorithms and functions based on symmetric and asymmetric cryptography systems are used. An integral part of such cryptosystems are pseudo-random number generators that allow you to generate a random sequence over a period. One of the evaluation criteria is the duration of the period and its guarantee, i.e. ensuring the randomness produced by the random number generator, another criterion is cryptographic strength - the ability to resist hacking by intruders. On the strength of pseudo-random number generators, symmetric flow cryptosystems are formed that ensure the strength of the cryptogram. The article analyzes the methods for constructing symmetric stream cryptosystems, proposes cryptographic generators of pseudo-random number sequences (PRNGs), the security of which is based on the complexity-theoretic problem of syndromic decoding. An improved method for generating PRNGs is proposed, which makes it possible to provide a guaranteed period of generated sequences.