Tommy Flowers returned to Bletchley just 11 months later with a working 1,800-valve Colossus Mk I, the world's first fully electronic, semi-programmable computer. But he was already working on the more powerful 2,400-valve Mk II, having learned of the increase in work that was needed. By the end of the war, 10 Colossi were in operation, the first being upgraded to a Mk II.
Roberts said one of the defining features of the machine was that it be flexible and programmable to allow the code breakers to test different theories. "To have invented a way to do this was, in itself, a remarkable achievement," says Roberts.
Furthermore, it had to be quick, so Flowers invented a way for Colossus to count up to five simultaneous data streams. This is believed to be the first instance of parallel processing by machine.
So secret was the existence of Colossus that, according to reports, Churchill ordered that all but two be destroyed to pieces "no bigger than a man's hand". Information now emerging suggests the order was either never made or never carried out. Most were dismantled, but two eventually went to the newly formed communications monitoring centre, GCHQ in Cheltenham, where they ran until the 1960s.
As astonishing as Colossus was, it only counted groups of characters. This gave only part of the solution, the so-called "de-chi-ed" script. The codebreakers still had to complete the decrypt by hand, but their workload was halved.
Roberts says Colossus was "absolutely fundamental" to the development of modern computers, providing speed and flexibility in terms of programmability.
After the war, the government gave Flowers an MBE and £1,000, which went some way to cover his personal costs in developing Colossus.
Roberts says he learned in 2002 that Tunny had been declassified. Tutte and Flowers both died with their secrets untold and largely uncelebrated.