Vigenere cipher

What it does

Instead of one shift for the whole message, the Vigenere cipher uses a keyword to pick a different shift for each letter. The keyword repeats for as long as the message runs.

That single change breaks the property that makes a Caesar cipher trivial. The letter E no longer maps to one fixed letter; it maps to a different letter depending on where it falls against the keyword. A simple frequency count no longer gives anything away.

Le chiffre indechiffrable

It held its reputation as unbreakable for roughly three centuries and picked up that nickname along the way. The break, when it came, exploited the one thing the cipher cannot hide: the keyword repeats.

Find the repeat length and the message falls apart into that many separate Caesar ciphers, each of which is trivial. Charles Babbage worked this out around 1846, and Friedrich Kasiski published the method in 1863. Counting repeated sequences in the ciphertext gives you the likely period, and from there it is arithmetic.

The practical lesson survives into modern cryptography: a keystream that repeats is a keystream that leaks. Reusing a one-time pad fails for exactly the same reason.

Related

Beaufort is the same idea with the subtraction reversed, which makes it its own inverse. Autokey removes the repetition entirely by continuing the key with the message itself, and is genuinely much stronger.