The Playfair Cipher was invented by the physicist Sir Charles Wheatstone (1802-1875) in 1854. It was popularized by his friend Lyon Playfair (1819-1898), hence the name.

Although highly insecure by today's standards, Playfair was a practical and effective cipher in its day. It was probably first used by the British Army in the Boer War and was still in use in World War I.

Playfair Cipher uses a 5 by 5 square, in which the letters of an agreed key word or phrase are entered (suppressing duplicates), followed by the rest of the alphabet in order. I and J would usually be combined together.

The message to be enciphered is split into pairs of letters. If the two letters in the pair are in the same row, the letters to the right of each are used. If they are in the same column, the letters below each are used. Otherwise, the letters at the opposite corners of the rectangle are used. Try the utility above to see how it works in practice.

Special treatment is required for identical pairs of letters and a single letter left over at the end. Typically an obscure letter such as X would have been inserted to pad out the message.

Note that the term Playfair Code is not strictly appropriate as codes involve substitution at the level of words; 'cipher' is used for a letter by letter substitution.

For more background information on Playfair and many other codes and ciphers, see The Code Book by Simon Singh.

Playfair Cipher in Crosswords

The compiler Afrit was probably the first to use a Playfair Cipher in a crossword. It has since featured regularly in thematic cryptic crosswords.

Long words and phrases with no repeated letters, such as dermatoglyphics and ambidextrously, have made popular key words. If the key has repeated letters removed, this is normally stated in the preamble to a puzzle.

Compilers usually take care that answers to be enciphered are of even length, with no pairs consisting of the same letter twice. The complications of handling these are thus avoided (the above utility replaces letters that can't be enciphered conventionally with a question mark).

Playfair Crossword by Beetlejuice is an example puzzle using Playfair as a gimmick.

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