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SIGSALY

The Start of the Digital Revolution: SIGSALY Secure:
Digital Voice Communications in World War II
J. V. Boone and R. R. Peterson

From the NSA web page


Introduction

Today, digital technology is the backbone of our entire information industry. As a part of this, the transformation of audio information into digital signals is now a routine process which is incorporated into our telephone, television, and music equipment (both recorded and live). Digital communication, measurement and data techniques are quite commonplace. This fairly recent situation was enabled by many things including the invention of the transistor in 1947 and the later evolution of semiconductor microelectronics techniques. However, the pioneering work for many of these capabilities was performed early in World War II in a successful effort to provide secure voice communications for high-level government officials. This brochure presents a brief overview of this revolutionary development.

Background and Overview

Before the full involvement of the United States in WWII, the United States and the United Kingdom were using transatlantic high-frequency radio 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. In fact, it was later discovered that a German station in the Netherlands was breaking out the conversations in real time. This situation was intolerable, but neither the U.S. nor the U.K. had a ready solution.

Fortunately, the technical groundwork for a solution was already in place. About 1936, Bell Telephone Laboratories (BTL) started exploring a technique to transform voice signals into digital data which could then be reconstructed (or synthesized) into intelligible voice. It was called a "vocoder," short for voice coder. An early demonstration of the voice synthesizer portion of the vocoder was even a part of the 1939 World's Fair in New York. The approaching war stimulated the investigation of true voice security. The BTL staff soon discovered that there were about eighty patents issued on the general topic, but analysis indicated that all of the methods were really unsatisfactory from a national security viewpoint. New technology was required. Spurred on by added interest from the U.K. and some early research results, the vocoder was selected as the basis of a new high-tech voice security system. BTL proceeded on its own to develop this much-needed capability and was soon able to demonstrate it to the satisfaction of the Army. A U. S. Army contract was awarded in 1942 for the production of the first two systems. This system eventually came to be called SIGSALY and was first deployed in 1943.