The Playfair cipher was the first practical digraph substitution cipher. It was invented in 1854 by Charles Wheatstone but was named after Lord Playfair who promoted the use of the cipher. The technique encrypts pairs of
letters (digraphs), instead of single letters as in the simple substitution cipher.
Any sequence of 25 letters can be used as a key, so long as all letters are in it and there are no repeats. Note that there is no 'j' so it will be combined with 'i'.
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