Even before the United States entered the Second World War the British Prime Minister, Winston Churchill, was constantly in touch with President Roosevelt. These communications were often sent via text message over a secure teletype system, by courier, or by diplomatic cable; but another, less safe, means of communication was also used – the telephone.

As well as censors monitoring all important transatlantic phone calls (including Churchill’s) an A-3 scrambling system was used in the early years of the war, but it was well known that these telephone calls could be intercepted and codes broken by an ordinary oscilloscope, (after the war it was found that a German listening post on the Dutch coast had complete transcripts of some phone calls, including ones between the Prime Minister and President). The Allied powers knew that a new and much more secure method of vocal communication was needed to help maintain communications secrets.

In the early years of the Twentieth Century, digital technology was in its infancy, being more theoretical than practical, and so Bell Laboratories began work on turning voice signals into digital data as early as 1936; research into a viable system, code-named ‘Project X’, began in 1940. This was cutting-edge technology and many patents were filed secretly – some only being disclosed in the late 1970’s. The pace of research increased when America entered the war after the attack on Pearl Harbor, and Britain sent Alan Turing from Bletchley Park to evaluate the prototype before six terminals were built. The project was code-named SIGSALY (SIG indicating its link to the Army Signal Corps), although those who operated them called the prototypes ‘Green Hornets’ because anyone trying to intercept the call would only hear a buzzing sound.

The US Army formed the 805^th Signal Service Company to operate SIGSALY, many of the technicians had worked for Bell Systems before the war. The system was operated for around eight hours every day, the other sixteen hours were needed for the complex testing and maintenance of the new technology.

The first transatlantic test took place in November 1942 using a One-Time Pad encryption. This meant that the voice was broken-down, digitally coded and mixed with an element from a random key-stream held on a record (the same as a music disc), this mixed sound was transmitted then re-assembled into speech at the other end, although it was difficult to recognise the voice of the speaker. Identical discs were held and used at each end so that the random sound could be matched-up and cancelled out; as each disc could only hold twelve minutes of data there were always two set up ready for use to allow for long conversations. Keeping the discs running at each end meant that if there was a short interruption in transmission the conversation could be picked up again immediately contact was restored. These discs were only used once before being destroyed so each phone call had a unique pattern which made it impossible for the enemy to de-code. The initial vinyl discs were replaced by acetate-coated aluminium records in 1943.

Each SIGSALY machine was huge, it needed 30,000watts to power the equipment which took up about 2,500 square feet and weighed 55 tons. These colossuses produced a huge amount of heat and had to be air-conditioned. Each terminal cost US$ 1 million in 1943, with twelve set up in various parts of the world this was a massive investment.

The first SIGSALY machine was set up in the Pentagon with a second in a basement of Selfridges department store on Oxford Street, London; these locations were chosen as they had space for the massive installations, and the deep Selfridges basement was safe from bombing. The US SIGSALY was linked via an extension line to the White House, while the British end was linked to 10 Downing Street, the Prime Minister’s Cabinet War Rooms, and the US Embassy. The first discussions between the two nations took place on 15^th July 1943 and are believed to have been about the Allied invasion of Sicily earlier that week; it is likely that the forthcoming invasion of Italy, planned for a few months later, was also discussed.

Security measures were needed to protect the confidentiality of calls so the cables between SIGSALY and its extensions were enclosed in pipes which were further protected by gas pressure and microswitches. Any tampering would cause the gas pressure to drop and an alarm would automatically be sounded. The whole system was Top Secret, with fewer than twenty-five of Britain’s most senior leaders knowing of its existence. As the war progressed and the Allies moved eastwards through Europe, SIGSALY terminals followed; one was set up in Paris, another in Frankfurt, and finally one in Berlin after Germany surrendered. The system was also used in the Pacific theatre, one was even on board a ship which was part of MacArthur’s Pacific Fleet. All in all, the twelve sites transmitted more than three thousand teleconferences during the latter years of the war, the majority of them amongst military commanders for planning purposes. Churchill also used SIGSALY in April 1945 to call President Truman after Roosevelt’s death; the two leaders then held a conference two weeks later in which they discussed tentative German surrender offers – that call was the longest ever made via SIGSALY at a little over two hours in length.

Any SIGSALY terminal could contact any of the other eleven; if distances were too great they could also be used to relay conversations, for example Washington was used to relay messages between London and Brisbane. The twelve locations were:

1. The Pentagon in Washington
2. Selfridges in London 
3. A second in Washington for dedicated use in the Pacific  
4. Pacific, on-board ship with the Pacific Fleet
5. Oakland in California
6. Fort Shafter in Hawaii
7. Brisbane
8. Algiers
9. Paris after the liberation of France
10. Frankfurt after VE Day
11. Berlin after VE Day
12. Tokyo after VJ Day

After the war the SIGSALY system was removed from service, it remained secret for another thirty years and was only disclosed two years after the codebreaking work at Bletchley Park in England was de-classified. Despite the secrecy Bell Laboratories won several awards, including Best Signal Processing Technology in 1946. Of course, as the work was still classified guests at the ceremony had to accept on trust that the award was justified!

SIGSALY was a vital part of the communication system during the Second World War, and a key part of the development of technologies we use today. So, next time you rely on digital encryption to protect your messages and data on social media don’t forget that, as the United States National Security Agency has said, “Today digital technology is the backbone of the information industry…. But the pioneering work for many of these capabilities was performed early in World War II.”