Vigenère Cipher
A repeating keyword turns the Caesar cipher into a polyalphabetic one. Each plaintext letter is shifted by the next letter of the key.
Per-character mapping
The key repeats across the message, skipping non-letters. Each tile shows the input letter, the key letter that shifts it, and the result.
Vigenère square
The classic tabula recta. To encrypt, find the row given by the key letter and the column given by the plaintext letter — the intersection is the ciphertext. Hover a row or column to highlight it.
How it works
For plaintext letter p and key letter k
(each numbered 0–25), the ciphertext letter is
c = (p + k) mod 26. Decryption flips the sign:
p = (c − k) mod 26. The key cycles to match the message
length, so a 5-letter key produces 5 interleaved Caesar ciphers.
For three centuries Vigenère was reputed to be unbreakable — le chiffre indéchiffrable. Repeating the key is exactly what eventually betrays it, but that is a story for another page.