During the Second World War, the need for secure, private communication between the United States and the United Kingdom became a strategic priority. While the technology of the 1930s allowed for telephone calls, they were not entirely secret; even 'scrambler' telephones like the A-3 model could be intercepted by enemies. To ensure total privacy, leaders often relied on encrypted telegrams, but these were less immediate than a direct voice conversation. In response, a secret project began in October 1940 to create a secure transatlantic telephone link, eventually known as SIGSALY.
SIGSALY functioned by digitizing the human voice and mixing it with randomly generated noise before transmitting the combined signal via radio waves across the Atlantic. Upon reaching the other side, the random noise was separated from the digital voice signal, which was then converted back into recognizable speech using a vocoder (voice coder). This process ensured that German interceptors would only hear unintelligible noise, earning the system the nickname "The Green Hornet." However, the system required perfect synchronization. Operators at both ends had to play identical vinyl records containing the random noise at exactly the same time. Each record lasted 12 minutes and was destroyed after a single use to prevent future decryption, meaning calls had to be scheduled in advance.
The technology was massive and expensive, with each terminal costing approximately $1 million in 1943 and weighing over 50,000 kg (50 tonnes). The first terminal was housed in the Pentagon with an extension to the White House, while others were placed globally, including a secret location in a sub-sub-basement of London's Selfridges department store. This site provided extensions to Number 10 Downing Street and the Cabinet War Rooms, where a terminal was hidden in a cupboard disguised as a toilet for Churchill's exclusive use. While Churchill and Roosevelt used the system for several conversations, including a notable two-hour call in 1945 to discuss the post-war era, the system also served as a vital link for senior Allied military planners, carrying at least 3,000 conversations during the war. The deployment of such high-cost, high-security infrastructure underscored the depth of the 'Special Relationship' between the two nations during the conflict. (Source: primary, hn-50096110).
This image illustrates the massive and expensive technology described in the third paragraph, specifically the terminal hardware.
This image provides visual context for the specific secret location in London mentioned in the third paragraph.