A cryptanalysis of the autokey cipher using the index of coincidence

Abstract

Modern cryptography builds upon many of the concepts introduced in classical cryptography. This study contributes to research in these fields through a cryptanalysis of a polyalphabetic substitution cipher known as Autokey using Friedman's index of coincidence measure of text character frequency paired with modern computer programming and data collection techniques. Results indicate that under certain constraints on the encrypting key, the index of coincidence can be applied to text decrypted using a small sample of random keys of various lengths to accurately predict the length of the encryption key.