Constructing probabilistic model for Vigenere OTP cryptanalysis

Abstract

The principles of polyalphabetic in the production of the ciphertext and the single use of the encryption key, are the source of the strength of the Vigenere OTP code, and they are the obstacle and the safe dam against attacks targeting systems that depend on this cipher, and these two principals have achieved success in neutralizing the known methods of analysis and attack. As a practical result of experiments and statistical calculations for the process of producing the horizontal cipher text, meaning the sequence of cipher text characters corresponding to each of the plain text characters sequence and its counterpart the key characters sequence, the principle of multiplicity of the alphabet is not effective in reducing the effect of the statistical characteristics of both the plain text and the key, but on the contrary, the correlational relationships between the characters of the sequences above horizontally have enhanced these statistics and this leads to reducing the possibilities of searching for the components of the ciphertext, and thus enabled us to build a relational statistical model for the correlation of these components is the basis for analyzing and attacking such ciphers by known methods, such as brute force search, attacking with the genetic algorithm, etc., and when we consider the sequence of ciphertext characters together by calculating the statistical relationships, we will find that the impenetrable dam for these ciphers has been worn out and has become not resilient in the face of attacks. The complexity of searching Vigenere OTP ciphers for a cipher sequence length n is O(n26), offset by O(n (7∼10)) as a result of implementing the proposed model, and when the calculations are expanded to include succession of ciphertext characters together, the complexity approaches O(n5).This paper proposes a novel developed model for One Time Pad cryptanalysis, it’s aim is to build a statistical and probabilistic model for exploiting and employing weaknesses in the Vigenere OTP cipher, and to be an entry for analyzing and for attacking by the conventional and unconventional methods.