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176 CHAPTER 9. DIGRAPHIC CIPHERS
be deciphered. 8
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9.4 Playfair
Sir Charles Wheatstone, like several other cipher inventors we have met, was
interested in many things. His name is attached to a method for measuring electrical
resistance, the “Wheatstone bridge” but he also constructed an electric
telegraph, invented the concertina, and wrote about acoustics. His accomplishments
led him to be elected as a fellow of the Royal Society and knighted. As the
Exposition Universelle in 1867 he displayed his “Wheatstone cipher machine,”
a disc cipher device of some complexity that is very simple to use.
But it is the Playfair Cipher we are currently interested in. Wheatstone
invented it in 1854 and showed it to his friend, Lyon Playfair, Baron of St.
Andrews, and in one of the more humorous episodes in cryptologic history they
brought it to the attention of British Government officials. As David Kahn tells
the story [Kahn, page 201]
Wheatstone and Playfair explained the cipher to the Under Secretary of
the Foreign Affairs. ... When the Under Secretary protested that the
system was too complicated, Wheatstone volunteered to show that three
out of four boys from the nearest elementary school could be taught it in
15 minutes. The Under Secretary put him off. “That is very possible”,
he said, “but you could never teach it to attachè’s”.
Playfair, reasoning that this reflected more on the diplomats than on the
cipher, remained enthusiastic about it.
The cipher was indeed used, perhaps first in the Crimean and Boer Wars. It
is easy to use, and was popular as field cipher because it does not need tables
or other equipment. The British attempted to keep it secret, but by the First
World War the Germans could routinely solve Playfair ciphers.
Wheatstone’s clever idea was to notice that in an rectangular arrangement
of the alphabet there are many smaller rectangles, with four letters forming the
corners. Exchanging two of the corners for the other two is a way to perform
the (plain 1, plain 2) → (cipher 1, cipher 2) substitution at heart of a
digraphic cipher.
8 This is the opening quote from Chapter 3.