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The Enigma machine was invented by the German electrical engineer, Arthur Scherbus in 1918, and was similar to a compact typewriter, with an integral battery pack, electronic keys, and adjustable rotor wheels, which could create countless millions of permutations for encoding messages. The cryptographers were first helped in deciphering the enigma codes because some of the German radio operators were careless in not changing the rotor settings frequently and even worse frequently prefaced their messages with the greeting 'Heil Hitler!' which gave experienced cryptanalysts a fast way into deducing the most likely solution to deciphering the message.

The other code machines were even more complex than Enigma as they had more code rotors. The Lorenz SZ42 (code name 'Tunny') devised for German diplomatic codes and the Geheimschreiber ('secret writer', code name 'Sturgeon') proved to be exceptionally difficult to crack.

To begin with the codes were broken by hand, using hand methods, and strips of paper, utilizing mathematical logic, topology and statistical methods. However, this proved far too slow for the flow of messages that were coming in and meant that messages were never going to be read in anything approaching real-time, much reducing their intelligence value.

It was this fact, and pre-war theoretical papers by Turing and Neuman on possibility of using machines to predict the solution of statistical problems, which was to lead to the use of machines and then early computers in breaking the codes.

The first code breaking machine and prototype computer was applied to enigma codes. The so-called 'Bombe', was originally devised by the Poles to break the enigma codes, and developed by Alan Turing, which enabled the code permutations to be mechanically sorted. The Bombe first cracked codes in August 1940 and by 1943 there were a total of 150 Bombe machines.

The Bombe, however, was not sufficient to break the Lorenz codes, which first emerged in the radio traffic in 1942, and Geheimscreiber codes, which had many more possible permutations. The Lorenz codes were particularly vital as they were from the German High Command and were of high strategic importance. Working out these codes by hand (which took place in a section of BP called the Testery) proved to be frustratingly slow and it was realised that mechanical means would need to be used to break these codes.