Before World War II, Polish crypto-analysts had already designed an electro-mechanical machine to test Enigma rotor settings called a ‘Bomba’. However, in December 1938 the German military changed their system slightly thus thwarting the Poles’ ability to decrypt Enigma messages.

Before the war started, the Poles passed all of their information over to the Britain and France and two mathematicians working at Bletchley Park, Alan Turing and Gordon Welchman, were able to build on this research to develop the ‘Bombe’ machine. Turing and Welchman exploited the fact that enciphered German messages often contained common words or phrases, such as general’s names or weather reports and so were able to guess short parts of the original message. These guesses were called ‘cribs’. The fact that on an Enigma machine no letter can be enciphered as itself made guessing a small part of the text even easier. It also meant that the potential number of settings that the Enigma could be in on that day was greatly reduced.

A drum from a Bombe Machine

Before running the Bombe, the wiring at the back of the machine was connected in accordance with a ‘menu’ drawn up by the code breakers based on cribs. The Bombe found potential Enigma settings not by proving a particular setting, but by disproving every incorrect one in turn.

Over 200 of the Bombes were built by the British Tabulating Machine company at Letchworth, all of which were destroyed after the war. A Bombe machine is being rebuilt at Bletchley Park, further details can be seen here: Link to Bombe rebuild.
For further information on the Bombe Rebuild Project please visit