PlayFair Cipher

The PlayFair cipher is a symmetric cipher based on grid polygram substitution.

Playfair encryption uses a grid, which can be generated by a key word.

Split the text into bigrams of 2 letters (complete with a letter if the message is of odd length).

Apply the following rules depending on the position of the letters in the grid:

— if the 2 letters are identical (or if there is only one remaining) insert another letter (for example a X or a Q) after the first letter and cipher the new bigram thus formed

— if the 2 letters are on the same row, replace them by the ones on their right (loop to the left if the edge of the grid is reached),

— if the 2 letters are on the same column, replace them by the ones directly under (loop to the top if the bottom of the grid is reached),

— else, replace the letters by the ones forming a rectangle with the original pair. The coded bigram begins with the letter on the same row as the first letter to cipher.

Playfair decryption requires a grid, which can be generated by a key word.

Split the text into pairs of letters (bigrams) and apply the following rules according to the letters positions in the grid:

— if the 2 letters are on the same row, replace them by the ones on their left (loop to the right if the edge of the grid is reached),

— if the 2 letters are on the same column, replace them by the ones directly above (loop to the bottom if the top of the grid is reached),

— else, replace the letters by the ones forming a rectangle with the original pair. Beginning with the letter on the same row as the first letter to crypt.

The ciphered message has an even number of letters.

The message may have 25 distinct letters at most (the number of letters in the grid)

It is impossible to find a bigram composed of 2 identical letters

It is impossible that a letter is crypted by itself. (Thanks to LeSingeMalicieux)

All references to games and fair play are clues.

PlayFair can be cracked using a known plaintext attack in order to discover a part of the grid.

Several squares can be used for the same decryption of Playfair, in fact, once a square grid of 25 is found, other squares obtained from the previous by rotating rows and / or columns gives the same result.

Multiple variants can be found when encrypting bigrams. When letters are in column or in rows, it is possible to cipher with the on on the right or on the left, or above or below. Again, when the letters are diagonally positioned, it is possible to write letter1-letter2 or letter2-letter1

1854 by Charles Wheatstone, after the name of one of his friends Lord Playfair.