# Vigenère cipher decoder and encoder

The Vigenère cipher is named after Blaise de Vigenère, but it probably was invented by Giovan Battista Bellaso around 1553. The cipher is easy to understand and use, but it was considered unbreakable for three centuries. It is a polyalphabetic cipher, a series of Caesar ciphers, based on the letters of a keyword.

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## Decode Vigenere cipher

Note that you can type in either the ciphertext or plaintext area.

## Decryption/encryption settings

## Features

- The Vigenère cipher is probably the most popular polyalphabetic substitution cipher. The Enigma machine is another example of a (more complex) polyalphabetic substitution cipher.
- It was described in 1553 by Giovan Battista Bellaso in his book "La cifra del. Sig. Giovan Battista Bellaso". The name Vigenère comes from a later misattribution.
- A Vigenère cipher works as a series of Caesar ciphers. The secret key determines how many places each letter should be shifted. For example, if the secret key is ABCD, the shift will be 0, 1, 2, 3, 0, 1, 2, 3, etc. A more comprehensive example is available here.
- Cryptographer Friedrich Kasiski was the first to entirely break the cipher and published his technique in the 19th century. Kasiski's method (called the Kasiski test) takes advantage of repeated words in the plaintext, leading to repeated group in the ciphertext. It can help determine key length. Once the key length is known, frequency analysis can help find the actual key.
- The Gronsfeld cipher is a variant of the Vigenère cipher, that uses numbers (0-9) instead of letters in the key. It works identical to the Vigenère cipher with the only difference that the shift is between 0 and 9.

Vigenère ciphers, and variants of it, are frequently used in CTFs, geocaching mystery caches, and logic puzzles.

## Sample text

ZODGSVXRATMLSKCUTALSBETAWF

The ciphertext above represents "POLYALPHABETIC SUBSTITUTION" encrypted using the key KASISKI.