German Modifications

Four Rotor Enigma. (Wilcox 2002, 16).

The Germans gradually modified the Enigma machine, increasing its security and thus causing problems for the cryptanalysts at Bletchley Park. An important example of these modifications was when Germany had added a fourth rotor to the Naval Enigmas, which had left Bletchley Park unable to read any communications for months (Wilcox 2002, 21).

“[T]he Germans believed their forces had been spotted by the aircraft, not given away by Enigma. [German] Admiral Doenitz, however, was not satisfied. He intended to change the U-boat Enigma machines. He could not radically alter the machine itself as it had to continue to work with the rest of the German Navy. His change added a thin fourth rotor between the leftmost rotor and the reflecting plate. When necessary, the rotor could be set in a straight-through position, enabling it to act as a three-rotor machine.”
– (Wilcox 2002, 15-16)

General Challenges

Operations at Bletchley Park slowed down in 1941 due to a labor shortage. The problem was so severe that Alan Turing and some of his colleagues sent a letter to Winston Churchill on October 21st, 1941, detailing an urgent need for more staff to help the cryptanalysts.