A brief history of cryptography—encoding and decoding messages—from ancient times to the present, including technical details of various systems used in the transmission of secret information. Kippenhahn (100 Billion Suns: The Birth, Life, and Death of the Stars, 1983), a former professor of astronomy and astrophysics at the University of Gîttingen and a former director of the Max Planck Institute for Astrophysics, in Munich, leads the reader into the arcane world of intricate number puzzles, secret keys, codebooks, and other devices often used by the military, undercover agents, and organizations such as the Ku Klux Klan to avoid detection by enemies and the public. Citing anecdotes in history, the author tells of the Greek historian Polybius, who devised cipher codes; of Herodotus sending secret messages warning of a Persian invasion at Thermopylae; of Scotland being betrayed when encoded messages from its supporters against Queen Elizabeth were found; of Jefferson’s key wheel, used by the US Army until 1920; and of the discovery of the famous Zimmermann telegram from Germany, which helped propel the US into WWI. Kippenhahn writes at length about the ingenious German code “Enigma——it was changed daily, growing more complicated over time. After many years of effort in the 1930s and later, the code was finally broken by three Polish mathematicians and Englishman Alan Turing. The Allies in WWII were then able to intercept military orders and plans the Nazis thought were beyond penetration. Today computers can process huge amounts of encoded data and do elaborate mathematical manipulations in a relatively short time. Will interest math mavens and computer junkies, but despite the fascinating anecdotes, the large mass of technical info may discourage reders who are less agile at manipulating numbers.