Stream Pseudo-probabilistic Ciphers.

The paper considers methods and algorithms for stream pseudo-probabilistic encryption and introduces a novel design of such ciphers. In the known algorithms of such type two independent messages (fake and secret ones) are encrypted simultaneously (with using two different keys, fake and secret) and the produced ciphertext is computationally indistinguishable from the ciphertext produced by process of the probabilistic encryption of the fake message using the fake key. However in the known stream pseudo-probabilistic encryption schemes the algorithms for decrypting the fake and secret messages do not coincide completely. Therefore a potential attacker can use the last fact to distinguish the pseudo-probabilistic encryption from the probabilistic one. To provide resistance to such potential attacks in the paper there are proposed stream pseudo-probabilistic ciphers satisfying criterion of the sameness of the algorithms for decrypting the fake and secret messages. The introduced ciphers are sufficiently fast and represent interest for practical application to provide confidentiality of the communication protocols performed using public channels. The randomized pseudo-probabilistic stream ciphers have been also designed.