Those days are over, and why, as Diffie and Landau explains, is that the secret code has become the key to preserving the traditional notion of privacy as technology rapidly changing the nature of human communication. When most conversations occur face to face, keeping them private is an easy step away from the crowds listened. Above all, it requires the highest value online encryption tool, and this book does a good job to explain the workings of the tool, both in principle and in practice. An intense political battle that surrounded the digital cryptography in recent years is evidence of a profound political implications in the era of online privacy, and Diffie and Landau have provided a comprehensive picture of the struggle and amazing good bet.