German attacks sank a majority of Allied shipping tonnage. Then, almost suddenly, it was the attacker who became the prey. Despite the assurances he received concerning the U-110, Admiral Doenitz suspected the Allies could read his fleet's Enigma messages. When he asked German High Command of this possibility, they assured him that the Enigma could not be broken. They proposed other reasons as to why his U-boats were less effective, including Allied direction finding capabilities (called Huff Duff by the British), aerial reconnaissance, or even a German traitor. In truth, even when the Navy Enigma messages could not be read, British direction finding combined with traffic analysis did have substantial successes.

Certainly the Germans' faith in the Enigma was not unfounded because of the astronomical mathematical possibilities. However, to encourage this unquestioned confidence, Britain went to great lengths to disguise how Enigma information, known as Ultra, had been obtained. The British took no action based on Ultra without first providing the Germans with a deceptive reason for the actions taken. Most commonly, British aircraft flew a reconnaissance mission over an area that Ultra had shown to be significant. When the Allies subsequently attacked that area, the Germans believed their forces had been spotted by the aircraft, not given away by Enigma.

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.

Bletchley Park learned of the impending change from decrypts and captured material, but until it was actually implemented there