STUDY OF THE THEORETICAL CORRESPONDENCE INDICES FOR THE UKRAINIAN ALPHABET WITH SPACES

Abstract

Background. Theoretical compatibility indices play an important role in identifying the weaknesses of cryptographic systems and ensuring their stability. After analyzing the available sources of information, this indicator was not found for the Ukrainian alphabet with a space, which makes it impossible to find accurate results in calculations where this indicator is used. Methods. Theoretical correspondence indices have a wide range of applications, but they are most often used in Friedman's First Method when calculating the key size for the Vigener cipher, which is used in the study of the basic concepts of cryptographic systems. The general algorithm for finding indices of theoretical correspondence for the Ukrainian alphabet with a space based on the analysis of a large sample of texts is considered. Results. For calculations, 700 texts were analyzed, the size of which is 1500 elements. The texts were selected using an API from Wikipedia, a free and open multilingual online encyclopedia. The calculation took place for keys of size from 2 to 7. The index of theoretical correspondence was calculated in two stages. At the first stage, practical indices of correspondence were calculated for the whole range of texts, and then, using the standard deviation of the sample, the interval of the most repeated indicators was calculated. The program code for implementing the index calculation algorithm was presented in the form of project components responsible for various aspects of the process. As a result of the study, a variation of the values of the correspondence index was found depending on the size of the text, which emphasizes the need to consider the length of the text in the analysis and development of cryptographic systems, in particular, the selection of encryption keys. It was concluded that the size of the text can affect the results of the calculation of the compatibility indices, but it does not significantly affect the overall security and effectiveness of encryption. Conclusions. In the course of the work, an analysis of the Ukrainian alphabet with a space and its properties was carried out. An algorithm for calculating the index of theoretical correspondence for this alphabet was also developed and implemented. The index of theoretical correspondence was experimentally determined for various texts in the Ukrainian language using the developed algorithm.