Seizing the Enigma (40 page)

With aids like these, the B-Dienst—by now totaling 5,000 persons, of whom 1,100 were in Berlin—mastered Naval Cypher No. 3 throughout most of 1942: by December it was reading 80 percent of the messages it intercepted. Dönitz planned nearly all his U-boat operations during this time on the information from these decrypts. On October 30, for example, the B-Dienst submitted a report that Convoy SC 107, then east of Cape Race, at the tip of Newfoundland, would steer 45°. At the same time, a U-boat sighted its exact position. Dönitz at once ordered his submarines to intercept it. “The timely arrival of the radio reconnaissance report on the route of the convoy,” he later wrote in an appreciation to the B-Dienst, “made it possible to pull the U-boat formation together so narrowly that within a few hours of the first sighting several U-boats made contact.” They sank fifteen steamers. Though this was an exceptionally striking case, the German cryptanalysts played an important role in U-boat attacks on convoys. Indeed, Dönitz estimated that 50 percent of his operational intelligence came from the B-Dienst.

Its romp slowed on December 15, when the British began enciphering their system indicators. But Tranow tripled the manpower working on the problem by shifting staff from other systems, and by February 1943 the B-Dienst was again solving a large proportion of
the traffic, sometimes reading directives to convoys ten and twenty hours before the movements they ordered took place. Its information helped Dönitz position his U-boats for what turned out to be the greatest convoy battle of the war. During three days in March, the U-boats sank dozens of ships from convoys SC 122 and HX 229. The Admiralty despaired, thinking that the Germans had virtually cut Britain’s lifeline to America. It was not until Naval Cypher No. 3 was replaced in June that the B-Dienst lost its grip on Allied communications.

Though these successes and failures tended to screen from Dönitz the Allies’ varying fortunes in codebreaking, secrecy was so vital to the success of German naval operations that almost any suspicion of its loss demanded an investigation. Thus in March 1942, the loss of two German auxiliary cruisers—the disguised and armed merchant vessels that sailed the seas to harass and destroy enemy shipping—prompted Admiral Fricke to look once again at operational secrecy. He analyzed the evidence and found no leak from communications. The movements of enemy ships contradicted any suggestion that they were reading the messages radioed to the auxiliary cruisers, he said. Similarly, when the battleship
Tirpitz
sailed to Trondheim and the
Gneisenau, Scharnhorst
, and
Prinz Eugen
dashed through the English Channel and the Straits of Dover to haven in German waters, intercepts of British messages gave no hint that the British were reading German cryptograms. And there were no indications that British information on German U-boats came from codebreaking: the implication was that it came from direction-finding or air reconnaissance. Fricke said that the Enigma was sixty times better than the British system and concluded that it was “unimaginable” that the enemy could read it. At the time, for M4, he was right.

A
VARIETY OF CLUES TOLD
N
AVAL
S
ECTION STARTING IN MID
-1941 that the Germans were going to replace their three-rotor Enigma on the Atlantic U-boat net with a new, four-rotor machine. The captured U-570 had yielded up a machine lid with four windows for the rotor key letters. Solved intercepts referred to the new machine, which had sometimes been used in error. Indeed, in December 1941 one such message, combined with a repeat enciphered in the proper three-wheel mode, enabled Hut 8 to recover the wiring of the fourth rotor even before it officially went into service.

All this made the cryptanalysts very apprehensive. The new rotor would multiply their work by a factor of 26, and they were barely keeping up as it was. Their apprehensions were justified. With the introduction of the four-rotor Enigma on February 1, 1942, a virtual U-boat blackout descended upon Hut 8. No longer could it read messages to or from Atlantic submarines on patrol or attacking or ordered to attack convoys. The Submarine Tracking Room admitted in its first weekly U-boat situation report after M4 came into effect that “the picture of the Atlantic dispositions is by now out of focus.” It grew increasingly indistinct.

Still, the situation was not quite as bad as during the no-Enigma-solution days preceding the captures of the
München
and the U-110 in mid-1941. Other sources of intelligence, such as aerial reconnaissance, were functioning better than before. The tracking room had
built up considerable knowledge in the half year it had been reading combat U-boat messages. Those solutions had told the British such details as the U-boats’ average speed of advance when proceeding to and from patrol and the endurance of the various types of submarines. Home Waters and Mediterranean Enigma and Dockyard messages, which continued to be solved at the astonishing combined rate of 14,000 per month, revealed the construction of new U-boats, their preparation and crew training in the Baltic, their addition to the fleet, their departures from their home ports. The Submarine Tracking Room thus knew the strength of the U-boat fleet and the number of boats at sea. It was further helped by radio fingerprinting, through which experienced intercept operators identified individual enemy transmitters by their distinctive methods of sending, or “fists,” and by
TINA
, a device that recorded and displayed on long strips of paper radio transmission characteristics to identify individual radiomen. Though not very reliable, these techniques could at least distinguish supply and minelaying U-boats from combat boats. Rodger Winn put into effect his “working fiction,” which maintained that the Submarine Tracking Room’s best estimates should be treated as facts and acted on until proven wrong.

For information on the U-boats’ locations in the Atlantic, the tracking room relied heavily during the M4 blackout on direction-finding. When a monitor in the big intercept rooms at Winchester or Scarborough heard a warship or a U-boat replying to the land station to which he or she was listening, the operator would yell out the frequency as loud as possible. At the far end of the room a controller would repeat it to the direction-finding stations on a direct landline.

The reported frequency went to the eight direction-finding stations from the Shetlands to Land’s End. Each had several listening posts attached to it, as well as subordinate control centers. The control center attached to the DF post of Wick, outside the Scottish village of Bower, was an H-shaped building that included a dormitory. Bearings
were taken in five 10-foot-square wooden huts isolated on the Caithness moors. Two were staffed by civilians and two by members of the Women’s Reserve Naval Service, known as Wrens; the fifth was a spare. No roads led to the huts; the operators were driven as close as possible and then had to walk a mile or two across the boggy ground. In winter a sailor helped shovel paths through the snow. At all times the women yielded the paths to the menacing local cattle. If the weather was too bad, they stayed in the hut from one watch to the next, sleeping in the bunk, melting snow for water, and heating cans of food in the kettle. But they preferred the trek back to the dormitory to staying in the hut and being jolted awake by the hut loudspeaker that announced a frequency on which a bearing had to be sought. None of the hardships fazed the young women. They felt that they were directly helping their men at sea and were proud to be doing the work.

Each operator sat at a desk listening to three telephone lines, one to the loudspeaker, one to each earphone. When the Wren heard the controller at Scarborough or another intercept post call out a frequency, she tuned the radio on her desk to that frequency. Early in the war, she would turn a wheel that rotated an antenna to reduce the signal to its lowest intensity; 90° from this was the source of the signal. Later she had a cathode-ray tube that scanned the horizon electronically. When it discovered the azimuth at which the signal was strongest, a spot in the center of the 9-inch circular tube stretched itself out toward the circumference in both directions. The operator spun the movable glass plate within a metal ring marked in degrees until a wire embedded in the glass lay above the glowing green line. When she put the tube’s cursor on the line, one half disappeared, so that it indicated a single direction—southwest, for example, rather than both northeast and southwest. The work had to be done very quickly, since many of the U-boat signals lasted mere seconds; sometimes the women got a bearing only on the last letter.

The operator noted the bearing in a log. When the control at Bower called on the intercom, she gave him the bearing; Bower would
pass this to the Submarine Tracking Room’s DF section, where the several bearings would be plotted on the chart that used the strings and pins.

During nearly all of 1942, Hut 8 solved only three
TRITON
keys: those for February 23 and 24 and March 14. The effect of this lack of
ULTRA
was becoming all too clear. In the last half of 1941, with Enigma solutions diverting Allied ships away from wolfpacks, U-boats sighted about one of every ten convoys. In the last half of 1942, with not very many more U-boats stationed on the North Atlantic routes, but with detours controlled only by direction-finding, U-boats sighted one of every three convoys. As a result, North Atlantic sinkings, which totaled some 600,000 tons in the last half of 1941, more than quadrupled to 2,600,000 tons in the last half of 1942. And each of the nearly 500 ships sunk in those six months meant more freezing deaths in the middle of the ocean, more widows, more fatherless children, less food for some toddler, less ammunition for some soldier, less fuel for some plane—and the prospect of prolonging these miseries.

Of all this the cryptanalysts of Hut 8 were acutely aware. But they could find no workable cribs, and without these they could not test for keys. Moreover, the three-wheel bombes were inadequate for solving the four-rotor Enigma: the three solutions of February and March, based on kisses, had each taken seventeen days on six three-wheel bombes. High-speed bombes to attack M4 were designed and put under construction. Nevertheless, at the highest levels, fears grew that the lack of intelligence, combined with the growing numbers of U-boats, would sink more ships than could be built. The tonnage left would not suffice to maintain rations and sustain industry in the United Kingdom, much less bring over troops, supplies, weapons, and ammunition to carry the war to Hitler.

So, while doing everything they could to accelerate ship construction and protect existing vessels, the supreme commanders looked with
growing impatience to B.P. for its contribution. On November 22, 1942, the O.I.C. urged B.P., with British understatement, to focus “a little more attention” on the four-rotor Enigma. The U-boat war was, it said in a silent scream, “the one campaign which Bletchley Park are not at present influencing to any marked extent—and it is the only one in which the war can be lost unless BP do help.” Unbeknownst to the O.I.C, however, that help was already on the way.

I
N SEPTEMBER
1941, H
ITLER ORDERED SIX
U-
BOATS TO THE
Mediterranean. They were to sink Allied vessels transporting supplies to North Africa and so relieve pressure on Rommel. Dönitz objected. He wanted to pursue the fundamental goal of cutting Britain’s supply lines. But Hitler told Dönitz that holding North Africa would keep Suez closed and force the British to go around the Cape of Good Hope, thus in effect costing them 3 to 4 million tons of shipping. In November he sent four more U-boats. They helped: Rommel, who had been driven back in 1941 almost to his starting position, was ready in January 1942 to launch an offensive. Eventually fifteen submarines, all of them with some of Dönitz’s most experienced crews, prowled what Hitler now called the “decisive area for the prosecution of the war,” the Mediterranean.