Four Square Cipher

The Four-square cipher encrypts pairs of letters (like playfair), which makes it significantly stronger than substitution ciphers etc. since frequency analysis becomes much more difficult. The four-square cipher uses four 5 by 5 matrices arranged in a square. Each of the 5 by 5 matrices contains 25 letters, usually the letter 'j' is merged with 'i'. In general, the upper-left and lower-right matrices are the "plaintext squares" and each contain a standard alphabet. The upper-right and lower-left squares are the "ciphertext squares" and contain a mixed alphabetic sequence.

The ciphertext squares can be generated using a keyword (dropping duplicate letters), then fill the remaining spaces with the remaining letters of the alphabet in order. Alternatively the ciphertext squares can be generated completely randomly. The four-square algorithm allows for two separate keys, one for each of the two ciphertext matrices.

Plaintext:

Keysquare 1:

Keysquare 2:

Ciphertext:

The following buttons can be used to manipulate the text into various forms such as uppercase, lowercase, reverse text, etc. The choice text chosen will be copied and a new page will be opened.