A Brief History of Cryptology and Cryptographic Algorithms by John F. Dooley

Author:John F. Dooley
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

5.4 Ciphers in the Great War: The Playfair

While all the combatants in World War I reverted to trench codes for much of their tactical communications, ciphers were not totally forgotten. In particular, the British used a field cipher as their tactical communications system for at least the first two years of the war, and the Germans used a complex field cipher for their high-level communications till the end of the war.

Sir Charles Wheatstone, the physicist, mathematician, and engineer, invented the British system, known as the Playfair cipher, in 1854. It acquired its name from Baron Lyon Playfair, who spent years popularizing the cipher and attempting to get the British government to adopt it. The British Army finally adopted the Playfair in the 1890s as their field cipher. It saw its first use during the Boer War (1899–1902) and was still used as the field cipher down to the company level during the first years of World War I [3, pp. 198–202, 1, pp. 166–178].

The Playfair cipher is a digraphic substitution cipher that encrypts two letters at a time. Every plaintext digraph is encrypted into a ciphertext digraph. It is based on a five by five Polybius square that uses a keyword to map 25 of the 26 letters of the Latin alphabet (I and J are either mapped together in a single cell, or J is just dropped). The keyword is dropped in row-by-row, deleting any repeated letters, and then the rest of the alphabet is filled into complete the square. For example, if the keyword is MONARCHY, then the Playfair square looks like Fig. 5.1.

Fig. 5.1Example of a Playfair cipher square

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