Telecommunication has never been perfectly secure, as the Cold War culture of wiretaps and international spying taught us. Yet many of us still take our privacy for granted, even as we become more reliant than ever on telephones, computer networks, and electronic transactions of all kinds. So many of our relationships now use telecommunication as the primary mode of communication that the security of these transactions has become a source of wide public concern and debate. Whitfield Diffie and Susan Landau argue that if we are to retain the privacy that characterized face-to-face relationships in the past, we must build the means of protecting that privacy into our communication systems. Diffie and Landau examine the national-security, law-enforcement, commercial, and civil-liberties issues. They discuss privacy's social function, how it underlies a democratic society, and what happens when it is lost. They also explore how intelligence and law-enforcement organizations work, how they intercept communications, and how they use what they intercept.