One summer day in 1941, a British radio operator monitoring German military frequencies heard something unexpected: a rhythmic warble of binary teletype code. A later report called it "strange new music[reference:148]."
Germany's engineers had developed a radically new encryption and transmission system — far more advanced than the Enigma machine[reference:149]. To break it, engineer Tommy Flowers built Colossus, the world's first large-scale programmable electronic digital computer[reference:150].
The Lorenz Cipher
The Germans called their new encryption machine the Lorenz. The British codebreakers at Bletchley Park codenamed it "Tunny," a British term for tuna fish[reference:151].
The codebreakers guessed that Tunny used 12 rotating wheels to encrypt messages[reference:152]. An extraordinary piece of fortune followed: two intercepted messages, both beginning with the same sequence of 12 common German names, allowed codebreaker Bill Tiltman to decrypt both using educated guesswork[reference:153].
That was enough for Tommy Flowers, a quiet young codebreaker, to deduce how Tunny worked. His description was uncannily accurate[reference:154].
Turing's Contribution
Alan Turing, fresh from his successes against Enigma, invented a method called "Turingery" that enabled codebreakers to deduce the positions of the pins on the wheels from ciphertext alone[reference:155]. The basis was a procedure called "delta-ing" — or differencing — that revealed information about the wheels[reference:156].
Tunny messages, often signed by Adolf Hitler himself, proved pure gold for the Allies. The machine was used by the German High Command to communicate with front-line generals. Once broken, the Allies could eavesdrop on the architects of Germany's battle plans[reference:157].
Building Colossus
Turingery was the only weapon against Tunny for a year, during which the codebreakers decrypted 1.5 million letters of ciphertext[reference:158]. But then the helpful lists of names at the start of each message disappeared[reference:159].
Tommy Flowers had been working on a different method based on Turing's delta-ing but taking a novel approach[reference:160]. His statistical method required scads of routine binary math and colossal counting of long binary sequences — impossible by hand[reference:161].
Flowers offered to build an all-electronic machine containing around 2,000 vacuum tubes. Bletchley Park's advisors rejected the idea, convinced it would never work reliably[reference:162]. But Flowers retreated to his London laboratory and quietly built it anyway[reference:163].
In January 1944, his engineers arrived at Bletchley Park with the world's first large-scale programmable electronic digital computer packed onto the back of a truck[reference:164].
How Colossus Worked
Colossus read input — Tunny ciphertext — photoelectrically from a large loop of punched paper tape[reference:165]. It processed at 25,000 characters per second. Its maximized timing-pulse rate was not far short of the performance of the first microprocessors from the 1970s[reference:166].
Flowers had pioneered clock pulses, bit-stream generators, control circuits, loops, counters, shift registers, interrupts, and more[reference:167]. He emphasized that Colossus "embodied all the basic features of a modern computer[reference:168]."
The Impact
There were 10 Colossi at Bletchley Park by the end of the war, housed in two vast steel-frame buildings, running day and night[reference:169]. They poured out wheel information, and the codebreakers provided the military with an unparalleled view of German strategies, strengths, weaknesses, and tactical intentions[reference:170].
If Colossus and the codebreakers shortened the war even by only six months, the number of lives saved was in the millions[reference:171].
The Secret Preserved
When the fighting ended, authorities decided that ultrasecrecy must be maintained. Orders were issued to break up the Colossi[reference:172]. Only two were spared[reference:173].
"All that was left were the deep holes in the floor where the machines had stood," one operator recalled[reference:174]. Colossus operator Norman Thurlow remembered being told that if the secrecy was ever lifted, he and his colleagues might be able to tell their grandchildren about Colossus and "the tapes that span on silver wheels[reference:175]."
On September 29, 2026, an IEEE Milestone plaque will be dedicated outside Block H at Bletchley Park, commemorating Colossus as an outstanding technical development that helped shorten World War II[reference:176].