TABLE OF CONTENTS We think of digital technology as the backbone of our entire information industry. Today digital communication techniques are ubiquitous. The explosive growth of the semiconductor and microelectronics industries, coupled with the invention of the transistor in 1947, made modern digital communications possible at its present day scale. A key driver, the transformation of audio information into digital signals, is now a routine process incorporated into our telephone, television, and music equipment. Many of these capabilities were pioneered during World War II in a successful effort to provide secure voice communications for high-level government officials. This chapter explores cryptographic principles required to secure and exploit over-the-air (OTA) speech transmissions.
Before the United States entered WWII, the United States and the United Kingdom were using transatlantic high-frequency radio (HF) for voice communications between senior leaders. The analog voice privacy system in use, called the A-3, provided reasonable protection against the casual eavesdropper, but it was vulnerable to anyone with sophisticated unscrambling capability. This system continued to be used during the early part of the war, and government officials were warned that they could be overheard (just as we have warned politicians who ignore the advice). Germany set up a listening station in the Netherlands (because of excellent reception and LPD) that broke the U.S.-U.K. link conversations in real time. This situation was intolerable, but neither the United States nor the United Kingdom had a ready solution.1 The story of the German listening post given...
