The Autokey cipher, described by Blaise de Vigenère in 1586, solves the periodic weakness of the standard Vigenère cipher by appending the message text to a short keyword to form the keystream.
This paper explores automated methods for breaking the Autokey cipher without a known crib or primer. We discuss the limitations of traditional Index of Coincidence (IC) and present a fitness-based stochastic search using n-gram statistics to recover both the primer length and the plaintext. 1. Introduction auto-key crack only
Auto-key ciphers (e.g., Autokey Cipher, certain stream cipher resynchronization modes) generate key material by combining a short initial key with the plaintext or ciphertext itself. exploits statistical patterns, key-stream reuse, and known-plaintext assumptions to recover the original key or decrypt messages without it. This report covers only automated, targeted attacks against such systems. The Autokey cipher, described by Blaise de Vigenère