make the codes more difficult to break, each of the rotors could be taken out and replaced in a different order. Also, the rings on the rotors could be put in a different order each day – for example, on one day the first rotor could be set at B, the next day at F, and so on. The military version of Enigma was provided with a plug board, like an old telephone switchboard. This allowed an extra switching of the letters, both before they entered the rotors and after leaving them. The plug board had 26 holes. Connections were made with wires and plugs. With three rotors and, say, six pairs of letters connected with the plug board, there would be 105,456 different combinations of the alphabet. In December 1938 the Germans added additional rotors (up to six) and the number of combinations increased dramatically. The Germans believed that messages sent on their most sophisticated Enigma machines were so well coded that they could not be decoded. But Twinn and his colleagues proved them wrong. About 10,000 people worked at Bletchley. The core group was the small number of cryptanalysts trying to crack the Enigma machine; at the beginning, this group consisted of no more than ten people, with Knox and Twinn in charge. The British codebreakers had been working on the commercial version of Enigma, the easier of the two to break, during the 1920s and 1930s, and they had made much progress in breaking the military version. But Twinn and his colleagues were stymied because they could not work out the order in which the Enigma keys were wired up. In July 1939, a month or so before the war started, Knox and some others travelled to Poland. Polish cryptologists, some of whom were brilliant, handed over to their British colleagues key information about Enigma, including replica machines. The British discovered that Enigma machines were wired alphabetically: A to the first contact, B to the second, and so on. This was the order given in the diagram attached to the patent application. But Twinn and his colleagues thought it such an obvious thing to do that nobody considered it worth trying. In early 1940 Twinn made the first break into Enigma. This could have been done much earlier if only they had tried the alphabetical system detailed in the patent application. The ability to read German encoded