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The Autokey cipher defeats Kasiski examination and the Index of Coincidence by never repeating its key. But the short priming key that starts everything off is still just a handful of letters, and one wrong guess collapses the entire decryption for you.
Learn about the Autokey cipher, the Vigenère variant that fixes the repeating-key weakness by folding the plaintext itself into the key stream. That closes the door on Kasiski examination.
Learn about the Beaufort cipher, a Vigenère variant that flips the subtraction around to become its own inverse. The same formula both encrypts and decrypts.
Learn about the Nihilist cipher, the 19th-century Russian revolutionary cipher that turns a Polybius square into a polyalphabetic system by adding coordinate numbers together instead of shifting letters.
Learn about the one-time pad, the only cipher in this series that is mathematically proven unbreakable, and why that proof rests on three conditions that are almost impossible to satisfy in practice.
For three centuries the Vigenère cipher was called 'le chiffre indéchiffrable.' Learn the two statistical techniques, Kasiski examination and the Index of Coincidence, that finally broke it, and watch them recover a real key letter by letter.
The Vigenère Cipher: A Journey Through History and Modern Applications.
