- Vigenère uses a repeating keyword to shift plaintext characters dynamically.
- Polyalphabetic mapping hides single-letter frequencies.
- Kasiski examination and Index of Coincidence break Vigenère when key length is revealed.
### The Vigenère Cipher
Once known as *le chiffre indéchiffrable* (the indecipherable cipher), Vigenère uses a repeated secret word as the key. Each letter of the key determines the Caesar shift for the corresponding letter of plaintext.
$$C_i = (P_i + K_{i \bmod L}) \pmod{26}$$
#### Breaking Vigenère
By identifying repeated n-grams in the ciphertext, cryptanalysts estimate the key length $L$. Once $L$ is known, the ciphertext is split into $L$ monoalphabetic Caesar streams, which are solved via standard frequency analysis!