Purpose and origins

The Colossus computers were used to help decipher radio teleprinter messages that had been encrypted using the electromechanical Lorenz SZ40/42 in-line cipher machine. To encipher a message with the Lorenz machine, the 5-bit plaintext characters were combined with a stream of key characters. The keystream was generated using twelve pinwheels. British codebreakers referred to encrypted German teleprinter traffic as "Fish" and called the SZ40/42 machine and the intercepted messages "Tunny". Colossus was used for finding possible Lorenz key settings – not completely decrypting the message. It compared two data streams, counting a statistic based on a programmable Boolean function. The ciphertext was read at high speed from a paper tape. The other stream was generated internally, and was an electronic simulation of part of the Lorenz machine. If the count for a setting was above a certain threshold, it would be sent as output to an electric typewriter.

The logical structure of the Lorenz machine was diagnosed at Bletchley Park without a machine being seen–something that did not happen until almost the end of the war. First, John Tiltman, a very talented GC&CS cryptanalyst derived a key stream of almost 4000 characters from a German operating blunder in August 1941. Then Bill Tutte, a newly-arrived member of the Research Section used this key stream to work out the logical structure of the Lorenz machine. He correctly deduced that it had twelve wheels in two groups of five, which he named the χ (chi) and ψ (psi) wheels, and the remaining two the μ mu or "motor" wheels. The χ wheels stepped regularly with each letter that was encrypted, while the ψ wheels stepped irregularly, under the control of the motor wheels.

In order to read messages, there were two tasks that had to be performed. The first task was "wheel breaking", which was the discovery of the pin patterns for all the wheels. These patterns were set up once on the Lorenz machine and then used for a fixed period of time and for a number of different messages. The second task was "wheel setting", which could be attempted once the pin patterns were known. Each message encrypted using Lorenz was enciphered at a different start position for the wheels. Knowing that in German, as in other languages, there is a non-random distribution of the different letters, and that the psi wheels did not advance with each character, Tutte worked out that trying two impulses of the chi-stream against the ciphertext would produce a statistic that was non-random. This became known as Tutte's "1+2 break in". The process of wheel setting found the start position for a message. Initially Colossus was used only to work out the start positions of the chi wheels, but later a method was devised for it to be used for wheel breaking as well.

Colossus was developed for the "Newmanry", the section headed by the mathematician Max Newman at Bletchley Park responsible for machine methods against the Lorenz machine. The Colossus design arose out of a prior project that produced a counting machine dubbed "Heath Robinson". The main problems with the Heath Robinson were the relative slowness of electro-mechanical parts and the difficulty of synchronising two paper tapes, one punched with the enciphered message, the other representing the patterns produced by the wheels of the Lorenz machine. The tapes tended to stretch when being read, at some 2000 characters per second, resulting in unreliable counts.

Tommy Flowers of the Post Office Research Station at Dollis Hill had designed the "Combining Unit" of Heath Robinson. He was not impressed by the system of a key tape that had to be kept synchronised with the message tape and, on his own initiative, designed an electronic machine which eliminated the need for the key tape. He presented this design to Max Newman in February 1943, but the idea that the one to two thousand thermionic valves (vacuum tubes and thyratrons) proposed, could work together reliably, was greeted with scepticism, so more Robinsons were ordered from Dollis Hill. Flowers, however, persisted with the idea and obtained support from the Director of the Research Station, W Gordon Radley.