Playfair cipher

Playfair cipher, type of substitution cipher used for data encryption. In cryptosystems for manually encrypting units of plaintext made up of more than a single letter, only digraphs (pairs of letters) were ever used. By treating digraphs in the plaintext as units rather than as single letters, the extent to which the raw frequency distribution survives the encryption process can be lessened but not eliminated, as letter pairs are themselves highly correlated. The best-known digraph...