The Vigenère Cipher — The Indecipherable Cipher, Explained
For nearly three hundred years, the Vigenère cipher was considered unbreakable. The French called it "le chiffre indéchiffrable" — the indecipherable cipher — and for good reason. By using a keyword to apply a different Caesar shift at every position, Vigenère flattens the frequency spikes that had betrayed every earlier cipher, and explodes the keyspace from twenty-six keys to astronomically many.
In this lesson we introduce the Vigenère cipher from first principles. We cover its real history (including its famous misattribution), the distinction between monoalphabetic and polyalphabetic ciphers, how to use the tabula recta, and finally a full worked example encrypting a message with a keyword — step by step, letter by letter.
Experiment with Vigenère interactively on the companion site. You can type your own plaintext and keyword, watch each letter route through the tabula recta, and see why the same plaintext letter can produce different ciphertext letters: https://kyri-cou.github.io/cryptography/
This is lesson 4 in the cryptography course. After this, we formalise Vigenère with modular arithmetic and then — at last — show how Babbage and Kasiski cracked the indecipherable.