Colossus, the WWII codebreaker that birthed modern computing, named IEEE Milestone

In 1941, British listening stations picked up a high-speed binary teletype signal that turned out to be traffic from a new German cipher machine, built by the Berlin firm C. Lorenz and codenamed “Tunny” at Bletchley Park. Far more advanced than Enigma, the 12-wheel system was a black box until a lucky German operator error—retransmitting a 1,200-character message with the same settings—let John Tiltman recover matching plaintext and ciphertext. From that, Bill Tutte reverse-engineered the machine’s inner workings without ever seeing it, and Alan Turing devised “Turingery” and his delta-ing technique to deduce the wheel settings. Those methods alone cracked 1.5 million letters over a year, exposing high-level orders often signed by Hitler himself.

When German operators tightened security and the manual attacks stopped scaling, Tutte’s statistical method demanded impossibly large amounts of binary counting—months per message by hand. Post Office engineer Tommy Flowers, brought in on Turing’s recommendation, argued that roughly 2,000 vacuum tubes could automate the work if left powered on continuously rather than switched, defeating the era’s assumption that large tube arrays were too failure-prone to be practical. Bletchley’s advisors rejected the plan, so Flowers and a small team built it anyway in his London lab over ten months.

Colossus arrived at Bletchley Park on the back of a truck in January 1944, was running within two weeks, and decrypted its first message on 5 February 1944—making it the world’s first large-scale programmable electronic digital computer. Its role stayed secret for decades, obscuring Flowers’s place in computing history. The machine is now being recognized with an IEEE Milestone, with a dedication ceremony set for 29 September at Bletchley Park.