Amazon.com (ISBN 0262041677, Hardcover)

There was a time when cryptography--the making and breaking of secret codes--was of interest only to spies, diplomats, and the occasional eccentric. Those days are over, and the reason, as Diffie and Landau explain, is that secret codes have become the key to preserving traditional notions of privacy at a time when technology is rapidly altering the nature of human communication.

When the vast majority of conversations happened face to face, keeping them private was a simple matter of stepping away from the listening crowd. But the growing number of conversations that take place over easy-to-intercept phone lines and e-mail channels requires more sophisticated safeguards. Above all, it requires online encryption tools of the highest grade, and this book does a good job of explaining how these tools work, both in principle and in practice. It does a better job, though, of explaining why the tools matter. The intense political battles that have surrounded digital cryptography in recent years are a testament to the profound political implications of privacy in the online era, and Diffie and Landau have delivered an admirably thorough overview of both the struggles and the stakes. If at times their thoroughness bogs them down in dry recitations of detail, their book at least generates more light than heat, and that can hardly be said of most contributions to the cryptography debate so far. --Julian Dibbell