THE CONCEPT OF NONLINEAR CRYPTOGRAPHIC PRIMITIVES, THEIR STEGANOGRAPHIC APPLICATIONS, AND RELATED AREAS OF USE

The article presents a unified cryptographic-steganographic concept for symmetric encryption systems. There are a large number of developments in the direction of increasing the stability and entropy of symmetric cryptography encryption, but they do not combine the advantages of cryptography and steganography in a single unified design. The object of the research is a class of modified symmetric cryptographic primitives that support ciphertext variability under fixed input conditions and enable the construction of embedded steganographic communication channels. The aim of this study is to develop a cryptographic concept that enables the design and modification of symmetric encryption systems through the introduction of an additional parameter – a modifier – which affects the encryption output and eliminates the deterministic nature of the transformation. The proposed solution is based on the use of a nonlinear cryptographic primitive, formalized as a three-parameter encryption function. The modifier introduces controlled randomization of the encryption output, allowing the system to avoid ciphertext determinism under fixed plaintext and key conditions. This approach enables internal entropy modulation without altering the core structure of the algorithm and maintains compatibility with current cryptographic standards. Furthermore, it allows for the integration of a steganographic channel operating within the modifier domain, facilitating the creation of multi-layered cryptographic constructs with low detectability. The study presents several strategies for modifier generation applicable to different usage scenarios and demonstrates the feasibility of the concept through a software prototype based on a modified DES algorithm. The steganographic potential of the proposed scheme is explored: by leveraging ciphertext variability and modifier sequencing, covert transmission channels are formed that offer plausible deniability and make it difficult to prove the presence of hidden information. The results can be applied to the modification of existing cryptographic algorithms, the development of new protocols, and the construction of multi-layered communication systems with embedded mechanisms for concealing the very fact of data transmission.