During WWII, AT&T engineers design and build a top secret voice encryption system that secures transatlantic telephone conversations between the highest-level Allied commanders, including Franklin D. Roosevelt and Winston Churchill. The system, dubbed SIGSALY, is an evolution of the work done on the one-time pad cipher during WWI. The system, which is kept secret for decades, is the first voice encryption system.
WWII images and video courtesy of the National Archives and Records Administration and Archive.org.
AT&T archive videos and imagery courtesy of the AT&T Archives and History Center.