In the summer of 1941, British signals stations intercepted what operators described as “strange new music,” a fast, warbling stream of binary teletype unlike the Morse traffic protected by Enigma. The discovery set in motion the effort that produced the Colossus computer, a breakthrough machine that helped Allied codebreakers read Germany’s highest-level wartime communications. The Colossus computer is now being recognized as an IEEE Milestone, with a dedication set for 29 September at Bletchley Park.
Decrypting with the Colossus computer
The unfamiliar signals were eventually linked to a sophisticated cipher and transmission system engineered in Germany, far more advanced than the Enigma, which dated to 1920. British analysts later learned the machine’s maker was the Berlin firm C. Lorenz. At Bletchley Park, the device was codenamed Tunny, following the tradition of fish-themed codenames used for other German systems.
Early on, codebreakers suspected Tunny used multiple rotating wheels, much like Enigma. A repeated preface in intercepted messages offered a crucial clue: each transmission began with an uncoded list of 12 common German names, such as Anton, Bertha, Conrad, and Dora. Analysts inferred that the list signaled how to set 12 encryption wheels at the receiving end.
Progress accelerated after John Tiltman, a senior researcher at Bletchley Park, analyzed two roughly 1,200-character messages that started with the same sequence of names. One was a retyped version of the other with small variations, an error that allowed Tiltman, using informed guesswork, to pair ciphertext and plaintext across the length of both messages. That trove enabled mathematician Bill Tutte to deduce the internal logic of Tunny with striking accuracy.
Knowing the machine’s design did not instantly yield readable text. The codebreakers also needed the specific pin settings around each wheel, which flipped between 1s and 0s, and the initial wheel positions for each message. The initial positions were embedded in the 12-name preface, but the pin patterns varied over time.
Alan Turing devised a technique, later called Turingery, to infer pin configurations directly from intercepted ciphertext. His approach relied on a procedure he introduced as delta-ing, or differencing, which compares successive characters bit by bit to reveal wheel characteristics. Using Turingery and a British-built replica of the Tunny machine, Bletchley Park decrypted extensive traffic, including messages signed by Adolf Hitler and exchanges between Berlin’s Armed Forces High Command and front-line generals. Over the course of a year, the team solved some 1.5 million letters of ciphertext.
Security improvements on the German side soon reduced operator mistakes and the helpful name lists disappeared, limiting the effectiveness of Turingery. Tutte responded by developing a new, statistics-driven method that could extract wheel information from ciphertext alone, without relying on repeated settings or prefatory name lists. Implementing that method by hand would have taken months per message, which is why an automated solution became essential.
Building the Colossus computer
Engineers initially planned a relay-based machine with a modest number of vacuum tubes to accelerate counting, but design problems stalled progress. Tommy Flowers, an engineer from the Post Office Research Station who had previously supported Alan Turing’s work on Enigma-related equipment, was brought in to help. Drawing on his experience with large-scale electronic switching, Flowers argued that a fully electronic design using thousands of tubes could be both fast and reliable, provided the tubes were kept continuously powered.
Although advisors at Bletchley Park were skeptical, Flowers and a small team built the machine at his London laboratory over 10 months. In January 1944, they delivered what became the world’s first large-scale programmable electronic digital computer. Reassembled at Bletchley Park, Colossus processed its first German message on 5 February 1944.
Colossus read ciphertext from a loop of punched paper tape using photoelectric sensors. It computed statistical tests to expose Tunny wheel settings and printed results on a modified manual typewriter fitted with relays. Once Colossus produced enough information about the wheels, human cryptanalysts completed the decryption.
Flowers later recalled that colleagues “couldn’t believe it” until they saw the machine in operation. A 1945 Bletchley Park report described the system’s speed, the whir of the paper tape around polished pulleys, and the uncanny automation of the typewriter as it produced scores without human intervention.
The demand for more Colossi
With Colossus proving its worth, Bletchley Park requested more machines. A second version, completed in June 1944 just days before D-Day, expanded to about 2,400 vacuum tubes and handled roughly 25,000 characters per second. While Tommy Flowers compared Colossus’s relationship to modern computers to that of Stephenson’s Rocket to a 20th-century express, he noted it embodied key computing concepts including clock pulses, bit-stream generation, control circuits, loops, counters, shift registers, interrupts, and parallel processing.
As Allied forces advanced toward Germany, the Colossi produced continuous intelligence on German plans, strengths, and vulnerabilities. By war’s end, ten Colossus machines were operating in two reinforced buildings at Bletchley Park under the direction of mathematician Max Newman, forming the world’s first electronic computing facility. Historians note that even a six-month reduction in the war’s length would have saved millions of lives, though the exact impact cannot be measured.
After the war, most Colossus machines were dismantled under strict secrecy orders. Only two survived. Former operator Dorothy Du Boisson recalled that all that remained were deep holes in the floor where the cabinets had stood. Engineer Norman Thurlow later said staff were told they might one day be able to tell their grandchildren about Colossus and “the tapes that span on silver wheels.”
IEEE Milestone dedication at Bletchley Park
The IEEE Milestone plaque will be installed outside Block H at Bletchley Park, near Milton Keynes, England. According to the planned inscription, six Colossus codebreaking computers operated in the building in 1944–1945, designed by Thomas H. Flowers of the British Post Office, enabling the deciphering of encrypted radio traffic among German commands across occupied Europe, North Africa, and the Soviet Union. The text states that the resulting intelligence saved countless lives, helped shorten World War II, and marked the first successful large-scale application of digital electronics to computing, anticipating later developments.
The nomination was sponsored by the IEEE United Kingdom and Ireland Section. IEEE Milestones are reviewed by the IEEE History Committee and awarded by the IEEE Board of Directors to recognize significant technical achievements at least 25 years old. The program is run by IEEE’s history and heritage group, which also maintains an extensive archive of scientific and technical milestones.
