Colossus was built to speed up the breaking of the Lorenz cipher, which carried the German high command's most secret messages. Designed by Post Office engineer Tommy Flowers and containing 1,600 thermionic valves, the Mark 1 machine had been delivered to Bletchley Park on January 18, 1944, and reassembled there. On February 5 it successfully attacked its first message, and it was in regular use at Bletchley Park through early 1944.
Alan Turing's electromechanical Bombe attacked Enigma, but Bletchley Park faced a separate and harder problem in the Lorenz cipher, used for the most sensitive strategic communications of the German high command. The mathematician Bill Tutte worked out the Lorenz machine's logical structure from intercepted traffic alone, and his statistical method could break messages, but applying it with the existing machines, the Robinsons, was slow.
Tommy Flowers, a senior engineer at the Post Office Research Station at Dollis Hill, proposed an electronic machine using between one and two thousand valves. The idea that so many valves could work together reliably was greeted with scepticism, and more Robinsons were ordered instead. Flowers and his team at Dollis Hill went ahead and built it, working from plans developed by the mathematician Max Newman. It worked at Dollis Hill on December 8, 1943, was taken apart and shipped to Bletchley Park, and on February 5, 1944, attacked its first message.
An improved Mark 2, five times faster, first ran on June 1, 1944, just in time for the Normandy landings. The existence of the Colossus machines was kept secret until the mid-1970s, so for three decades Flowers and his team received no public credit for building the first programmable electronic digital computer.
Key people: Tommy Flowers, Bill Tutte, Max Newman