?Few false ideas have more firmly gripped the minds of so many intelligent men than the one that, if they just tried, they could invent a cipher that no one could break,? writes David Kahn in this massive (almost 1,200 pages) volume. Most of The Codebreakers focuses on the 20th century, especially World War II. But its reach is long. Kahn traces cryptology?s origins to the advent of writing. It seems that as soon as people learned how to record their thoughts, they tried to figure out ways of keeping them hidden. Kahn covers everything from the theory of ciphering to the search for ?messages? from outer space. He concludes with a few thoughts about encryption on the Internet.