Polygram substitution
The Polygrammsubstitution (also: Printing substitution ) is an encryption method of converting plaintext into ciphertext . In contrast to the monographic substitution, no individual plaintext letters are converted into corresponding ciphertext letters, but groups of letters (polygrams).
Often, pairs of letters are replaced. In this case one speaks of bigraphic (also: digraphic ) ciphers. Biographical encodings have the advantage of a much flatter frequency distribution compared to the monographic method (mostly with 26 letters) due to the higher number of possible characters (26² = 676 letter pairs) , which makes unauthorized decipherment more difficult and the code breaker now needs a significantly larger amount of ciphertext.
One of the most famous and oldest polygraphic ciphers is the Playfair cipher. This was developed in 1854 and in part (in a slightly modified form as a double-box key ) was still in use until the Second World War.