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.
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