The Colossus Computer

by GMoulden

The Colossus computers were computing machines devised by the British in World War II. They were used to decode German messages, and were the world’s first programmable, digital, electronic computing devices.

The Colossus machines were originally designed and built by the British Post Office Research Station by Tommy Flowers, with input by Allen Coombs, Sid Broadhurst, and Bill Chandler. The Colossus machines were used in the cryptanalysis of high-level German communications. These communications had been encrypted using the Lorenz SZ 40/42 cipher machine. The Lorenz machine outputs messages, printed with bits, generated by ten pinwheels. Apart from the stepping of the five irregular pinwheels, the Lorenz machine was actually five parallel pseudorandom generators.
British cryptographers cracked the code without ever having seen one of these machines-a German operator made a mistake in a 4,500-character message. The message was not received correctly on the other end, and, after an unencoded request for a retransmission alerted British cryptographers what was going on, the same message was transmitted again-in the same key settings. From these two related cyphertexts, John Tiltman and his crew were able to discover the plaintext and the keystream of the Lorenz.
Bill Tutte at Bletchly Park discovered that the keystream produced by the machine exhibited statistical biases that deviated from random. Therefore, these biases could be used to break the cipher and read messages from the German commanders. Unfortunately, before this could be done, two tasks had to be accomplished. The first was wheel breaking, or discovering the pin patterns on the wheels of the Lorenz machines. The second was wheel setting. Wheel setting was the process that found the start setting for each message. Tommy Flowers was presented with the problem and asked to fix it, creating a machine to decode the Lorenz messages.

The Colossus was the first ever to use shift registers and counters, and could perform Boolean calculations. There were two Colossus computers, Mark I and Mark II. Mark I had two racks of 1,500 vacuum tubes, while Mark II had 2,500. Mark II was five times faster than Mark I, and was much simpler to operate. The Colossus machines generated the wheel patterns electronically. The circuits were synchronized by a clock signal generated by the sprocket holes of the punched tape. The five-bit paper tape with the ciphertext was read by a type reading system, and the Colossus was electronically synchronized with the tape, in order to limit its speed. The machines were capable of reaching 9,000 characters per second before the tape disintegrated.

Unfortunately, the Colossus machines were used in the absolute utmost secrecy. Even years after the end of World War II, few knew that they had ever existed. Winston Churchill specifically ordered the machines to be dismantled to pieces ‘no bigger than a man’s hand’. Tommy Flowers himself deliberately burnt the diagrams showing the construction of the Colossus machines. Flowers and his crew of men were deprived of the recognition that they deserved, and the world of technology was deprived from the new knowledge that the Colossus computers provided.
Information on the Colossus did not begin to emerge until the 1970s, after the imposed secrecy was broken by Colonel Winterbothen’s book ‘The Ultra Secret’, which was on decoding machines of World War II. Finally, in 2007, a Colossus machine was reconstructed from what bits and pieces could be learned, and tested against an old Lorenz machine. The Colossus machine worked perfectly.

The knowledge that high speed, electronic, digital computing devices were possible, along with the technology of the Colossus computers, encouraged inventors to new hights, and had a significant impact on the development of computers in later years, both in Britain and the USA alike.


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