Introduction to Cryptography with Coding Theory, 3rd Edition

2.6 The Playfair and ADFGX Ciphers

The Playfair and ADFGX ciphers were used in World War I by the British and the Germans, respectively. By modern standards, they are fairly weak systems, but they took real effort to break at the time.

The Playfair system was invented around 1854 by Sir Charles Wheatstone, who named it after his friend, the Baron Playfair of St. Andrews, who worked to convince the government to use it. In addition to being used in World War I, it was used by the British forces in the Boer War.

The key is a word, for example, playfair. The repeated letters are removed, to obtain playfir, and the remaining letters are used to start a $5\times 5$ matrix. The remaining spaces in the matrix are filled in with the remaining letters in the alphabet, ...