At midday on 26th April, just as Gretton and ONS.5 entered the longitudes where the wolfpacks were assembled, the Germans made changes to the naval Enigma code settings that scrambled the messages sent between BdU, the U-boat headquarters, and the U-boats. In England, the Government Code and Cypher School, which intercepted and decoded these German messages, was abruptly blinded, unable to read the signals that, for months, had alerted ships to the whereabouts of U-boats and their plans. Without precise information, the best that the Submarine Tracking Room in London
