Abstract
This article examines a method for determining the key length in polyalphabetic ciphers using the coincidence index (Index of Coincidence, IC). It is shown that traditional frequency analysis is ineffective in analyzing the Vigenère cipher, but the use of the IC allows for a statistical assessment of the probability of letter matches, thereby revealing the true key length. The paper describes the historical role of the method and includes practical calculation examples, a software implementation in Python, and an analysis of the obtained results. It is shown that the coincidence index method allows for a quick and reliable determination of key length, and when combined with additional methods, it ensures effective cryptanalysis of polyalphabetic ciphers.