U-boat messages were enciphered using Enigma machines, which the Germans believed could not be broken. To send a message, an Enigma operator inserted three rotors and positioned them in a certain order. When a single letter was typed on the keyboard, the rotors—which were facing one another, like hockey pucks stacked sideways—would turn, transforming the letter over and over. A light on the top side, an ordinary flashlight bulb, would illuminate the letter as it emerged in its enciphered form; that letter would be radioed.
