Methods and Algorithms for Pseudo-probabilistic Encryption with Shared Key

As a method for providing security of the messages sent via a public channel in the case of potential coercive attacks there had been proposed algorithms and protocols of deniable encryption. The lasts are divided on the following types: 1) schemes with public key, 2) schemes with shares secret key, and 3) no-key schemes. There are introduced pseudo-probabilistic symmetric ciphers that represent a particular variant of implementing deniable encryption algorithms. It is discussed application of the pseudo-probabilistic encryption for constructing special mechanisms of the information protection including steganographic channels hidden in ciphertexts. There are considered methods for designing stream and block pseudo-probabilistic encryption algorithms that implement simultaneous ciphering fake and secret messages so that the generated ciphertext is computationally indistinguishable from the ciphertext obtained as output of the probabilistic encryption of the fake message. The requirement of the ciphertext indistinguishability from the probabilistic encryption has been used as one of the design criteria. To implement this criterion in the construction scheme of the pseudo-probabilistic ciphers it is included step of bijective mapping pairs of intermediate ciphertext blocks of the fake and secret messages into a single expanded block of the output ciphertext. Implementations of the pseudo-probabilistic block ciphers in which algorithms for recovering the fake and secret messages coincide completely are also considered. There are proposed general approaches to constructing no-key encryption protocols and randomized pseudo-probabilistic block ciphers. Concrete implementations of the cryptoschemes of such types are presented.