Even before the United States joined the Second World War in December 1941, British Prime Minister Winston Churchill wanted to be in close contact with its President, Franklin D Roosevelt. The technology of the 1930s allowed for phone calls that were private – but not entirely secret. Scrambler telephones, like the A-3 model, ensured a certain level of security but calls made on them could still be intercepted. Instead, the two leaders mainly communicated via encrypted telegrams. When they did speak by phone, censors listened in – and cut the call if they mentioned anything they shouldn’t.

Something more reliable was needed. In October 1940, a company in the US began a secret project that would enable secure phone conversations across the Atlantic Ocean. It came to be known as SIGSALY and, at the time, it was cutting-edge technology.

What was SIGSALY and how did it work?

Terminal of the Secure Radio Telephone Scrambler, known as "Project X", "Sigsaly" or "The Green Hornet".

© IWM(HU 45346)

The project, first called ‘Project X’, was carried out by Bell Telephone Laboratories in New York. The company developed a communication system that digitised the human voice, mixed this with a randomly generated noise and then sent the combined signal via radio waves across the Atlantic. Once received, the random noise was separated from the digital voice signal, which was then converted back into recognisable human voice using a vocoder (‘voice coder’).

The system was extremely secure, and German interceptors would only hear unintelligible noise. But SIGSALY wasn’t simple to use, as it relied on perfect synchronisation at both ends of the chain. The synthesised voice signal was combined with a unique random noise that was generated by a record on a large turntable. Operators at both ends of the line had to play an identical disc at exactly the same time, and each record lasted 12 minutes. Afterwards, the records were disposed of. This meant that calls had to be scheduled, and couldn’t happen on the spur of the moment.

Was SIGSALY top secret?

Main entrance to Selfridge's department store on Oxford Street during the Second World War.

© IWM(D 23005)

SIGSALY was a secret project, and America guarded its technology closely. It was also expensive and bulky. Each terminal cost $1 million (at 1943 values) and weighed more than 50,000 kg (50 tonnes). The first one was housed in the Pentagon, with an extension to the White House. A further 11 terminals were installed at different sites around the world, including one in the sub-sub-basement of Selfridges department store in central London. This became operational in July 1943, and extensions from it ran to the US Embassy and Number 10 Downing Street.

There was also an extension to the Cabinet War Rooms, which was kept in a small cupboard that was made to look like a toilet for Churchill’s use only. Even within this secret site, the existence of the Transatlantic Telephone Room was kept secret from the people who worked there.

When was SIGSALY used?

Prime Minister Winston Churchill and US President Harry Truman shake hands on the steps of Truman's residence, "The White House", at Kaiser Strasse, Babelsberg, Germany, on 16 July 1945.

© IWM(BU 8944)

Churchill and Roosevelt first spoke via SIGSALY in April 1944. They held several more conversations via this new transatlantic link, but still continued the bulk of their communications via encoded messages. Senior Allied military planners used the terminal underneath Selfridges much more often, however, to speak securely to Washington. In total, the 12 SIGSALY terminals carried at least 3,000 conversations during the war. The number of terminals in different places across the world – including Algiers, Australia and the Philippines – demonstrates how truly global the Second World War was.

In April 1945, Roosevelt died. Churchill used SIGSALY to call Roosevelt’s successor, Harry S. Truman, shortly afterwards to convey his condolences. A fortnight later, Churchill and Truman held their second conversation via SIGSALY. This one lasted for more than 2 hours, as the two leaders discussed what would happen after the imminent end of the war in Europe.

How did SIGSALY strengthen the ‘Special Relationship’?

President Roosevelt and Winston Churchill confer during a lunch break at the Livadia Palace during the Yalta Conference.

© IWM(EA 52857)

The SIGSALY terminal, and its extensions to both Downing Street and the Cabinet War Rooms, signify the importance of the wartime alliance between the United States and Britain. This was expensive, secret technology that was shipped over to Britain and required a team of US signallers to operate it. The fact that the US was prepared to devote such resources, manpower, time and money to set up SIGSALY in Britain at the height of the war goes some way to demonstrating the strength of the ‘Special Relationship’.

Churchill and Roosevelt developed both a positive working relationship as well as a real friendship during the war. They occasionally had differences of opinion, and there were some causes of tension between the senior political and military leadership of both countries, but they were still able to work together and to eventually triumph against their shared enemies. The existence of a cutting-edge technological link within easy reach of both leaders underlines how valuable – and valued – this alliance was.

You can find out more about SIGSALY and see the secret cupboard where Churchill made wartime calls at the Churchill War Rooms.

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