Engineers at Bletchley Park brought the Colossus Mark 2 into service, a dramatic improvement over the original machine that had first broken German Lorenz-cipher traffic months earlier. Using thousands of vacuum tubes to test cipher settings at electronic speed, Colossus machines gave Allied commanders intelligence that helped confirm German deployments ahead of the Normandy campaign, though the project remained classified for decades after the war.
Tommy Flowers, an engineer at the General Post Office's research station, had designed the original Colossus to crack the Lorenz cipher used for high-level German army and Luftwaffe communications — a far more complex system than the Enigma machine cracked at Bletchley Park's Hut 8. The first Colossus became operational in early 1944, and an improved Mark 2 model, five times faster, followed by mid-1944 in time to support the D-Day invasion.
Colossus used around 1,600 vacuum tubes to compare an intercepted enciphered message against possible key settings at extraordinary speed for the era, processing 5,000 characters per second. It is widely considered one of the first programmable electronic digital computers, predating ENIAC, though its existence remained a state secret until the 1970s and most of the machines were dismantled after the war on Churchill's orders.
Intelligence from Colossus-decrypted Lorenz traffic helped confirm to Allied planners that Hitler had been fooled by deception operations into believing the main invasion would come at Calais rather than Normandy. Because the program stayed classified for so long, the engineers and codebreakers who built and ran these machines received little public recognition in their lifetimes, and Tommy Flowers's pioneering computing work was largely written out of early histories of the digital age.
Key people: Tommy Flowers, Alan Turing