The Playfair cipher or Playfair square is a manual symmetric encryption technique and was the first literal digraph substitution cipher. The scheme was invented in 1854 by Charles Wheatstone, but bears the name of 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 and rather more complex Vigenere cipher systems then in use, The Playfair is thus significantly harder to break since the frequency analysis used for simple substitution cipher does not work with it. Frequency analysis can still be undertaken, but on the 600 possible digraphs rather than the 26 possible monographs.

More on Wikipedia: Playfair Cipher

More on Wikipedia: Playfair Cipher