THEORETICALLY UNBREAKABLE CIPHERS AS THEY SHOULD BE UNDERSTOOD

Abstract

Perfectly-secret ciphers according to the Claude Shannon's theory, which are considered as unbreakable, and more specifically random keystream ciphers, are discussed. An analysis of the sources mentioned in the reference list showed that all of them come to the point that the perfect ciphers according to Claude Shannon's theory are unbreakable. The article introduces some concepts, such as: the probabilistic model of cipher; the perfect cipher, which is secure against a plaintext recovery ciphertext-only attack; the perfect cipher, which is secure against a key recovery ciphertext-only attack; effective plaintext or key recovery attack; ineffective plaintext or key recovery attack; decipherable model of cipher; undecipherable model cipher. The introduced concepts were used to clarify Shannon’s mathematical model and to prove that a statement about unbreakability of the perfect ciphers according to the Claude Shannon's theory, including random keystream cipher, were wrong. The purpose of the article is to attract the attention of specialists to the problem of developing methods for decrypting Vizhener cipher and using them in solving the problem of determining the cipher key of a random gamming according to a ciphertext, as well as developing methods for estimating the complexity and reliability of deciphering the cipher class in question