Vigenère Cipher
The keyword cipher that resisted cracking for 300 years — both ways.
About this cipher
Type a keyword and a message; each letter shifts by the matching keyword letter, cycling — a rolling Caesar cipher that changes shift every position. Decoding reverses it with the same key. Case and punctuation pass through untouched; only letters are enciphered.
Three centuries of unbreakability, and the fall
Published by Giovan Battista Bellaso in 1553 (Vigenère got the credit by historical accident), the cipher earned the name le chiffre indéchiffrable because it defeats simple frequency analysis — E doesn't map to one letter but to several, smearing the statistics flat. It held until the 1860s, when Kasiski published the break: repeated words in the plaintext produce repeated cipher fragments at distances divisible by the key length, and once you know the key length, the cipher collapses into several independent Caesars — each solvable by ordinary frequency counting. The deep lesson survived into modern cryptography: a Vigenère whose key is as long as the message and never reused is the one-time pad, which is provably unbreakable. For real secrets today, AES encryption is next door; this one is for puzzles, escape rooms and history.
Frequently asked questions
How does the Vigenère cipher work?
The keyword repeats along the message, and each letter shifts by its keyword letter's position in the alphabet (A=0 shift, B=1…). LEMON over ATTACK gives different shifts per letter — a rolling Caesar.
Why was it considered unbreakable for so long?
Because each plaintext letter maps to several different cipher letters, flattening the letter-frequency fingerprint that breaks simple ciphers. It took until the 1860s for Kasiski's key-length attack to appear.
How was it eventually broken?
Repeated plaintext words create repeated ciphertext fragments spaced by multiples of the key length. Find that length, split the text into that many Caesar ciphers, and solve each by frequency analysis.
Is Vigenère safe to use for real secrets?
No — it's been thoroughly broken for 160 years and falls to statistical attack in seconds by computer. Use it for games and learning; use modern encryption (like the AES tools here) for anything real.