THE CODEBREAKERS by David Kahn. 1,164 pages. Macmillan. $14.95.

In his letters to Cicero, Julius Caesar employed a cipher in which each character was replaced by one standing three places down the alphabet—thus D stood for a, E for b, F for c, etc. Mary Queen of Scots wrote conspiratory messages in cipher; when intercepted and interpreted by England's first great cryptanalyst, Thomas Phelippes, they helped bring Mary to the chopping block. In the U.S., Benedict Arnold employed several codes, including one that was keyed to Volume I of the fifth Oxford edition of...

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