A fascinating part of applied mathematics is the area of cryptography.The subject is probably older than Julius Caesar's famous use of a cipher but it is as fresh as today's public-key cryptography. Both of these examples, by the way, are fine illustrations of modular arithmetic, the one a very simple example, the other a much more sophisticated and elegant one. For the history of codes and ciphers one cannot do better than consult Secret and Urgent by Fletcher Pratt, Blue Ribbon Books, Garden City,N.Y., 1939 and The Codebreakers by David Kahn, Macmillan Co., New York, 1977. A history of codes and ciphers in America might include the smoke signals of native peoples as the very first instance. President Thomas Jefferson himself patented a mechanical coding device, and diplomatic codes have a long and persistent use in our international relations. There were various codes used in the American Civil War of course. And Samuel Morse is best remembered for his Morse code for the telegraph. Despite all these early examples, I consider the founder of American cryptography to be Herbert O. Yardley (1889-1958),a brilliant, self-reliant thinker but also quite a rogue. However the full flowering of American cryptography awaited the genius of William F. Friedman (1891-1969). All this may be explored through the links below.
A curiosity: Imagine my surprise when I came across this letter sent by Abe Lincon on Nov. 25, 1862 to General Burnside:
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