Vigenère Cipher Calculator

What is the Vigenère cipher?

The Vigenère cipher is a polyalphabetic substitution cipher: instead of one fixed shift for the whole message, each letter is shifted by an amount taken from a keyword. The keyword is repeated along the message, so the letter A in the key means shift 0, B means shift 1, and so on up to Z for shift 25. Each key letter therefore acts like a separate Caesar cipher.

For about three centuries the cipher was nicknamed "le chiffre indéchiffrable" (the indecipherable cipher). Friedrich Kasiski published a general method for breaking it in 1863, and Charles Babbage had found the same idea earlier without publishing it.

Encryption and decryption formulas

Number the letters A = 0, …, Z = 25 and let the key have m letters K(0), …, K(m − 1). Encryption is C(i) = (P(i) + K(i mod m)) mod 26 and decryption is P(i) = (C(i) − K(i mod m)) mod 26.

Worked example: encrypt ATTACKATDAWN with the key LEMON. The repeated key is LEMONLEMONLE. The first letter is A + L = 0 + 11 = 11, which is L. The second is T + E = 19 + 4 = 23, which is X. Continuing in the same way gives LXFOPVEFRNHR.

Why it is stronger than Caesar, and why it can still be broken

Because the same plaintext letter can become different cipher letters, the letter frequencies of the ciphertext are flattened and a simple frequency count no longer reveals the key. The security grows with the key length, and a random key as long as the message that is used only once is the unbreakable one-time pad.

With a short repeated keyword, however, the cipher is weak. Repeated words in the plaintext can line up with the same part of the key, producing repeated groups in the ciphertext. The distance between such groups reveals the key length m (Kasiski examination). Once m is known, every m-th letter was shifted by the same amount, and each of these groups is an ordinary Caesar cipher that frequency analysis solves.

Tips for using the calculator

Enter the text, the keyword and choose encrypt or decrypt.

• The key may contain letters only; upper and lower case give the same shifts.
• The key advances only on letters. Spaces and punctuation are copied unchanged and do not consume a key letter. Other programs may advance the key on every character, so results can differ from theirs.
• A one-letter key turns the cipher into a Caesar cipher.