was little they could do to prepare. Fortunately, the Germans made an error. In December 1941, before the change had been made official, a U-boat sent a message using the four-rotor machine. To compound the mistake, the same message was retransmitted using only three rotors. From this seemingly innocuous error, the cryptanalysts at BP determined the wiring of the fourth rotor. 

In February 1942 Admiral Doenitz officially changed the Enigma machines on his U-boats. Despite recovering the wiring to the fourth rotor, Bletchley Park had a lot of work ahead of them. In addition to changing the machine, the Kriegsmarine also instituted a new code; which Britain referred to as 'Shark.' BP now had two obstacles: break Shark and redesign the Bombe. The cryptanalytic