U-boats were like terrorist cells in their ability to sow fear. They were invisible, ubiquitous, noiseless. To pluck off ships making the Atlantic crossing, the U-boats would place themselves across the convoy lanes and lie in wait. When a U-boat spotted a vessel, it would radio central command, which would alert other subs to close in. Some U-boat commanders were so daring that they would submerge and surface in the middle of the convoys, shooting at the Allies from the inside out.
The Battle of the Atlantic was a war of lives and of commerce. England needed food. The Allies needed troops and war materiel to press their campaigns in Italy and North Africa. American shipyards were churning out Liberty ships—low-cost cargo ships that were being mass-produced in unheard-of numbers—but the U-boats in 1942 were able to sink ships faster than America could make them. Making things worse was the fact that the Germans were reading the cipher the Allies used to direct their convoys, something the Americans suspected but the British were slow to admit.
To be sure, the Allies for some of this time were reading the German cipher, so the Battle of the Atlantic also was a battle of code-breaking prowess. 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. Each Enigma had more than three rotors to choose from, and each rotor could be set in twenty-six positions. The rotors were surrounded by movable outer rings, and there were plugs, called steckers, attached to a board. The upshot of all this gadgetry was that there were millions of ways a letter could travel through the encipherment process.
One major strength of Enigma was the setting order for the rotors and other movable parts, which was known as the key and changed each day. The Germans knew that commercial Enigmas had been circulating around Europe in the 1920s and early 1930s and that the Allies might have some idea how they worked. But they believed their enemies could not break the key, which was somewhat like guessing a computer password. Even if it was theoretically possible, the Germans figured, breaking the key would require a building full of machines to run through all the potential combinations, and they did not believe the Allies could produce a building full of machines.
Before the war, a team of Polish cryptanalysts had in fact figured out the workings of the Enigma. Small, vulnerable nations surrounded by big potential enemies—Poland is bordered by Russia and Germany—tend to be hypervigilant about their neighbors, and the Polish Cipher Bureau was remarkably good. The Poles broke the Enigma during the 1930s, in part thanks to a German who passed schematics and decrypted messages to French intelligence, who passed it to them, and to a commercial model they obtained. The Polish mathematician Marian Rejewski solved the wiring, and in 1938 they built six “bomby” machines that could detect possible daily settings.
In July 1939, before the Nazis overran their country, the Poles shared their discovery with the British and French. At Bletchley Park, Alan Turing and others refined the design, developing a method in which cryptanalysts could use a crib and write it beneath the cipher, then figure out, mathematically, what combination of rotor settings, wheel settings, and steckers might produce the cipher. This “menu” permitted the machine to check for possible settings, and was in some sense an early form of a computer program. The British built sixty “bombe” machines, which, beginning in 1941, were run by some two thousand members of the Women’s Royal Naval Service, or Wrens. The bombes would test a menu to see if it could be a viable key setting. If the bombe got a “hit,” then a smaller machine—an Enigma facsimile—was programmed with the setting and a message fed into it. If coherent German emerged, the code breakers knew they had the correct key setting for the day.
The British at first kept their bombe project secret, even from their allies, for fear the enemy would find out and change the codes. Churchill called his Bletchley code breakers the “geese that laid the golden eggs, and never cackled.” In February 1942, however, the hypercautious German Navy added a fourth rotor to the naval U-boat Enigma machines, increasing the possible combinations by a factor of twenty-six. The Allies called this new four-rotor cipher “Shark,” and initially it proved impenetrable. The Allies lost the ability to read U-boats. The whole system went dark. This crushing turn of events occurred just months after the United States entered the war, and it began an eight-month period of death and destruction and helplessness, a time when ship after ship went down and it felt very much as though the war could swing the wrong way.
This, then, was the demoralized atmosphere that the young women from the Seven Sisters colleges were entering when they traded their May Queen festivals, their end-of-year hoop rolls, their amateur theatrics, and other hallowed traditions for service in the hot downtown offices of the U.S. Navy. The Navy code-breaking program, as ever, operated separately and apart from that of the Army: While Arlington Hall across the Potomac River pursued its attack on diplomatic ciphers, the Navy, still in its downtown headquarters, wrestled with the task of breaking enemy naval messages in the two major oceans, with lead responsibility for code breaking in the Pacific. It felt, at the outset, like an undoable task. The women in the summer of 1942 were signing on with a Navy still reeling from Pearl Harbor and the swift Japanese victories that followed. Officers were being reassigned and lines of command reshuffled. America was losing the war on all sides—or so it felt—and the atmosphere was chaotic. In January 1941, naval code breaking consisted of just sixty people occupying ten office rooms in the sixth wing of the Navy building. By mid-1942 the number had increased to 720, with more arriving every day. The rooms were starting to overflow.