Vigenère Cipher

The Vigenère cipher is a polyalphabetic substitution cipher invented in the 16th century and misattributed to Blaise de Vigenère. It uses a keyword to encrypt text: each letter of the key determines a different Caesar shift for the corresponding plaintext letter. The key repeats cyclically. For example, key "KEY" applies shifts K(10), E(4), Y(24) to successive letters. It was considered unbreakable for centuries (le chiffre indéchiffrable) until Charles Babbage and Friedrich Kasiski broke it in the 1800s.

Each plaintext letter is shifted by the corresponding key letter's position in the alphabet (A=0, B=1, ... Z=25). Encode: ciphertext = (plaintext + key) mod 26. Decode: plaintext = (ciphertext - key + 26) mod 26. Non-alphabetic characters are passed through unchanged. The key repeats: for a key "CAT" encrypting "HELLO WORLD", the shifts are C(2), A(0), T(19), C(2), A(0), [space], T(19), C(2), A(0), T(19).

Charles Babbage (1846) and Friedrich Kasiski (1863) independently discovered that recurring patterns in the ciphertext reveal the key length. The Kasiski test finds repeated sequences in the ciphertext; their spacing is likely a multiple of the key length. Once the key length is known, each position is an independent Caesar cipher that can be broken by frequency analysis of every n-th character.

No — it is easily broken with modern computers. Key length detection and frequency analysis defeat it completely. A one-time pad (Vigenère with a truly random key as long as the message, used only once) is theoretically unbreakable, but the key distribution problem makes it impractical. Modern encryption uses AES (symmetric) and RSA/ECDH (asymmetric), which are computationally infeasible to break with current technology.

The Vigenère square (or tabula recta) is a 26×26 grid where row i contains the alphabet shifted by i positions. To encrypt plaintext letter P with key letter K: find row K, column P — the intersection is the ciphertext letter. To decrypt: find row K, the column containing ciphertext letter C — the column header is the plaintext letter. This visual representation makes the cipher easy to use manually.