Vigenère Cipher Tool

The Vigenère cipher encrypts text with a keyword. Each key letter sets a Caesar shift (A = 0, B = 1, ... Z = 25), and the shifts are applied to the message letters in turn, repeating the keyword as needed. Because one plaintext letter can become different ciphertext letters, it is called polyalphabetic. It is often credited to Blaise de Vigenère but was first described by Giovan Battista Bellaso in 1553, and for centuries it was known as le chiffre indéchiffrable, the indecipherable cipher.

Add each key letter to the matching message letter, modulo 26. Encrypting ATTACKATDAWN with the key LEMON gives LXFOPVEFRNHR: A + L = L, T + E = X, T + M = F, A + O = O, C + N = P, then the key repeats with K + L = V, and so on. The Encrypt / Decrypt tab shows this letter by letter in its key stream view.

Standard repeats the key: C = P + K. Autokey uses the key once and then continues with the message itself, so the key never repeats; the receiver needs the same starting keyword. Beaufort computes C = K - P, which is its own inverse, so encrypting and decrypting are the same operation. Variant Beaufort computes C = P - K and decrypts with P = C + K, which is the Standard cipher run backwards. All arithmetic is modulo 26.

No. It resisted attack for about 300 years, but Friedrich Kasiski published a general method in 1863 and later statistical methods such as the index of coincidence make it routine to break. Once the key length is known, each key letter is just a Caesar cipher that frequency analysis solves. Longer, random keys make it harder, and a random key as long as the message that is never reused (a one-time pad) is unbreakable, but at that point the Vigenère structure is no longer what protects you. For real secrets use AES or another modern cipher.

First it finds the key length: for each possible length it splits the ciphertext into columns and averages their index of coincidence. At the true length every column is a Caesar-shifted piece of English, which scores about 0.067, while wrong lengths score closer to 0.04. It then solves each column by trying all 26 shifts and keeping the one whose letter counts best match English (a chi-squared score). It needs enough text, roughly 20 or more letters per key letter, so short messages and long keys give guesses, not answers. The cracker is built for Standard Vigenère ciphertext.

The index of coincidence (IC) is the chance that two letters picked at random from the text are the same. English scores about 0.067, uniformly random letters about 0.038, and a Vigenère ciphertext falls in between, closer to 0.038 as the key gets longer. Entropy measures how evenly letters are spread, in bits per letter: English is around 4.2 and the maximum for 26 letters is 4.70. Both are computed from the letters only.

By default only the letters A-Z are changed: case is kept, and spaces, digits and punctuation pass through (and do not use up key letters). "Letters only" drops everything except letters and gives uppercase output. Letters outside A-Z, such as é or ñ, are left as they are. In the key, anything that is not A-Z is ignored, and the tool shows you the key it actually used.

No. Everything runs in your browser, the page's content security policy blocks outgoing network requests, and nothing you type is saved: there is no history and no local storage. The only thing the site remembers is your language choice for the menu.