Pseudo-probabilistic block ciphers and their randomization

Abstract

There is considered implementation of the plan-ahead share-key deniable encryption algorithms that produce the cryptogram that satisfy criterion of the computational indistinguishability from probabilistic encryption of the fake message. This paper introduces a general design of the pseudo-probabilistic block ciphers. The proposed method includes encryption of the secret message block and the fake message block followed by a transformation procedure mapping the pair of intermediate ciphertext blocks into a single block of the output ciphertext. The transformation procedure is implemented in the following two variants: (1) simultaneous encryption of the intermediate ciphertext blocks and (2) solving the system of two linear congruencies. The second variant provides natural possibility to construct pseudo-probabilistic block ciphers in which recovering fake or secret message is performed using the same single decryption algorithm. To provide higher security there are proposed randomized pseudo-probabilistic ciphers. There are also considered designs with different size of the input data blocks corresponding to fake and secret messages.