Seizing the Enigma: The Race to Break the German U-Boat Codes, 1933-1945

by Kahn, David

  The convoy was then heading east-northeast to Point F, where it was to turn northeast to Point G—a route that would have taken it directly through the concentration of U-boats. The message told it to head instead for a new point, WL, at 50° north, 50° west. After that it was to steer due north to WM, at 55° north, 50° west, and then east-northeast to WN, east-southeast of Cape Farewell, the southern tip of Greenland. This diversion would take it around the U-boats, leaving them to the east and south. When it received this message, the convoy had passed the 50th meridian; to get to WL, it had to backtrack a little. At 1850 Greenwich mean time, the commodore, van den Donker, altered course. The officers of Convoy and Routing had made an important move in the game.

  U-Boat Command continued to seek the convoy to the south. On Wednesday, April 21, it ordered the formation of a new wolf pack, WOODPECKER, for the boats detached from TITMOUSE and for more than a dozen others recently refueled by the tanker U-487. By noon the next day, WOODPECKER was to stretch some 300 miles north and south. The convoy was expected a few hours before the formation was complete, but surfaced U-boats could easily catch the convoy even if it arrived early. To prevent the Allies from learning of WOODPECKER and so possibly diverting SC 127, the U-boats were instructed to maintain radio silence until contact reports had to be made.

  While WOODPECKER was forming, one of TITMOUSE’s northernmost boats, the U-306, spotted the New York convoy HX 234. At 1:55 A.M. on Thursday, the U-306 torpedoed a 10,000-ton Danish freighter, the Amerika. Soon seven and eventually twenty-one TITMOUSE boats were operating against HX 234. This drew them away from SC 127, whose new route would have taken it almost through the center of the TITMOUSE patrol line.

  SC 127 was, however, having troubles of its own. On Wednesday morning, the third ship in the fifth column reported seeing a periscope between herself and the sixth column. The escort leader attacked with a pattern of fourteen depth charges what he thought was “a very doubtful contact”; he concluded that it was a false alarm. Immediately thereafter, the convoy, on its new, backtracking northwest course, spotted ice. One of the escorts thought the convoy could get through the loose pack and bergs and accompanied it; the escort commander followed a lead in another direction but could see no open water. He finally asked van den Donker to steer due east to work around the ice. The convoy had to turn around in a 4- to 6-mile gap in the ice, which the commodore and the other masters accomplished with great skill.

  But the escort commander, Lieutenant Commander C. E. Bridgeman, Royal Naval Reserve, could see that heading east would send the convoy right toward the area that the new course was intended to avoid. Instructed to head north, not east, he wanted to gain ground to the north by going northeast, and felt that the bright moon and good visibility would limit the danger from the ice. So after several hours of steaming east, he altered to the northeasterly course, sending one of the escorts ahead as an ice patrol; it stood by one berg with dimmed lights as the convoy paraded slowly by. During the night, the growlers and bergs thinned out, and early in the morning of Thursday the twenty-second, Bridgeman thought it safe to return to the original northwest course, which the convoy soon did.

  By then the Allies had resumed solving German U-boat messages. But they were not quite current: messages of the twentieth and the twenty-first were being sent to Knowles’s Submarine Tracking Room on the twenty-second. One of the solutions perhaps elicited some satisfaction among Norgaard and the others who had diverted SC 127. U-Boat Command had told WOODPECKER that an eastbound convoy was expected on April 22 in the approximate latitude of naval grid square BC 69, which would put it almost in the middle of the WOODPECKER line, an ideal location for a wolfpack attack. This was SC 127’s original route, now changed; the solution confirmed that Convoy and Routing’s diversion of SC 127 to the north had been wise.

  The intercepted German messages were reflected in Cominch’s U-boat estimates. The estimate for the twenty-first shrank the big rectangle in the west central Atlantic both in size and in number of U-boats and added two other infested areas halfway between Newfoundland and Greenland: “About ten [U-boats] estimated within 150 miles 56-00 46-00 from numerous DFs indicating probability that both HX 234 and ON 178 are being shadowed X Several within 150 miles of 54-00 50-00 from recent DFs possibly shadowing ONS 3.” The ten were the TITMOUSE boats, which had not appeared as separate units in the previous estimate. And the second area mentioned lay directly athwart SC 127’s new planned route: its center was 60 miles south of SC 127’s Point WM.

  That would not do, but other routes also entailed risks. On Thursday, Convoy and Routing, noting that some of the U-boats in the rectangle were, as the sub estimate said, “probably moving to the north or northwest,” changed SC 127’s course again. This time it moved the course south so the convoy would pass behind these boats and those of TITMOUSE, which were reaching for HX 234. It ordered the convoy to omit Points WL, WM, and WN and to “alter course forthwith for (WS) 050 deg 31′ north 044 deg 02′ west (WT) 56–58 north 34–57 west, thence (WO).” Point WS lay in the big rectangle that had been filled with U-boats, but that day the area did not appear in the U-boat estimate, its boats having dispersed. WT lay to the northeast, at the edge of a circle as big as New Mexico in which seven U-boats were thought to be present, moving west or southwest; the risk seemed small enough to take. WO, east-northeast of WT, was likewise within the circle.

  U-Boat Command continued the cat and mouse game. It kept WOODPECKER in its position south of TITMOUSE, but when the expected SC 127 was not contacted, it concluded that the convoy had taken a more northerly route. To ensure contact, it assigned U-boats arriving in the combat area to form, by Sunday, a new wolfpack: BLACKBIRD. Its eleven boats were to patrol a line trending north-northwest-south-southeast and moving westward. But its location was south of the new route for SC 127, and although the boats began to appear in the submarine estimate for April 23, Good Friday, as “about four within 200 miles of 56-00 27-00 moving westward,” the convoy’s course was not altered for BLACKBIRD.

  One reason for not changing course was that well over a dozen submarines had clamped onto HX 234. And they were sinking ships. The U-306 torpedoed a 7,000-ton American freighter on Good Friday, and an hour later the U-954 sent a 5,000-ton British vessel to a watery grave 1,500 fathoms deep.

  Subsequently, however, Cominch adjusted SC 127’s course again and again in response to perceived threats. On Saturday, for example, it replaced Point WT with WP, to the east-northeast. On Easter Sunday, the convoy having passed into the area of British control, the commander in chief Western Approaches in Liverpool ordered the convoy escort to report “if you consider you are being shadowed.” No reply was received, so it seemed that the escort believed that the convoy was not being followed. And U-Boat Command appeared to have abandoned its attempts to grapple with SC 127. Its order to BLACKBIRD to take up its patrol line and direction of movement meant, it noted, that BLACKBIRD’s “advance toward the SC convoy is at present no longer possible.” Nevertheless, Western Approaches twice ordered slight changes in SC 127’s course on Monday, April 26, as the convoy steamed southeast of Cape Farewell and began to approach Iceland.

  That same day, a Liberator arrived in the morning to give air cover: the convoy had successfully traversed the dreaded Greenland air gap, the “black hole” in the middle of the North Atlantic that could not be patrolled with the airplanes then available. On Tuesday a Liberator and a Catalina provided air cover throughout the day.

  Also on Tuesday, the solutions of two week-old messages dealing with SC 127 arrived in U-boat headquarters. They reported the locations of the convoy on the twentieth and the twenty-first and the courses it was given. The command observed that “It swung to the north quite early, probably to go around an assumed U-boat concentration.”

  On Wednesday, SC 127’s escort obtained an asdic bearing on a possible submarine; the escort searched the area with no result. On Thursday, the five ships destined for Iceland were detached, together wit
h SC 127’s only straggler, which blamed bad coal for its frequently being 4 to 5 miles behind. The convoy escort messaged Western Approaches: “Do not consider SC 127 seriously threatened by U-boats at present.” Air cover was provided throughout that day and the next. At 6 A.M. on Saturday, May 1, the seventeen ships headed for Loch Ewe were detached. The rest of the convoy arrived off Scotland’s Oversay at 6 P.M. on Sunday. And later there arrived in Convoy and Routing that most welcome of telegrams from the Admiralty, this one putting the seal of success on the transatlantic crossing of SC 127: “All arrived.”

  21

  THE CAVITY MAGNETRON CLUE

  SC 127 ELUDED U-BOATS AT ONE OF THE MOST DIFFICULT TIMES in the Battle of the Atlantic. The ocean was so full of U-boats that the first sea lord feared that “We can no longer rely on evading the U-boat packs and, hence, we shall have to fight the convoys through them.” In addition, the B-Dienst was at the height of its powers, solving 5 to 10 percent of its intercepts in time for Dönitz to use them in tactical decisions. Early information sometimes enabled him to move his U-boats so that a convoy would encounter the middle of the pack, enabling more boats to attack than if the convoy met only one wing of the patrol line.

  But the first signs of German weakness had begun to appear. Stronger Allied defenses—more escorts, more airplanes—kept the U-boats from attacking with the vigor and daring of the previous years. Dönitz’s exhortations grew shriller, complaining that anyone who failed to engage the enemy closely was “no true U-boat man.” The rate of success declined. The great convoy battle of March 1943, during which U-boats sank Allied ships at twice the rate at which they were being built, was followed in April by a fight that brought poorer results: the Germans sank twelve merchant vessels, but at a cost of seven U-boats. The situation worsened the following month.

  “In the Atlantic in May,” wrote Dönitz in his war diary, “the sinking of 10,000 tons was paid for with the loss of one U-boat, while not very long before that time one boat was lost for the sinking of about 100,000 tons.” He called such losses “unbearable,” and on May 24 he pulled the seventeen submarines on the North Atlantic convoy routes out and sent them to what he thought was a “less air-endangered area” to the south. From there they could operate against the convoys between the United States and the Strait of Gibraltar, through which supplies for the American forces in North Africa had to pass. But this was not the vital traffic whose loss would defeat Britain and keep the Allies from mounting an assault against Festung Europa. The move marked a major defeat for the Germans in the vital Battle of the Atlantic.

  The success of Allied convoy diversions in January and February 1943 had again raised Dönitz’s suspicions about the security of his ciphers. For two and a half weeks in January, U-boat sweeps had discovered no convoys along the North Atlantic routes to Britain; for the first time since the United States entered the war, merchant ship losses in all Atlantic areas fell below one a day. In February, the few convoys that were not sighted by chance were spotted only by single boats at the ends of patrol lines, suggesting that the convoys were going around the wolfpacks. Dönitz’s concern was intensified when Allied destroyers came upon the U-459 as it was refueling an Italian U-boat some 300 miles east of St. Paul’s Rock, the desolate traditional division between the North and the South Atlantic, far from any destroyer bases and far from the normal convoy lanes. And the B-Dienst’s solutions of Allied U-boat situation reports raised suspicions. On April 18, for example, an intercept of an Allied submarine situation report showed that the Americans suspected the presence of twenty submarines in the rectangle running from 48° to 54° north latitude and from 38° to 45° west longitude. And the report was correct: TITMOUSE was in the area with eighteen boats.

  Dönitz asked Maertens, the head of the Naval Communications Service, to investigate, as he had done in 1941. Again Maertens exculpated Enigma. The British U-boat situation reports themselves stated that the Allies’ information on submarine locations was coming from direction-finding, he said. Documents found in a French Resistance agent’s radio station showed that the Allies were obtaining information from the Resistance on departure times for U-boats and on whether they were headed for the North or the South Atlantic, enabling the foe, Maertens said, to estimate submarine movements with some accuracy. The British information about the wolfpacks DOLPHIN and FALCON was vague; if the information had come from cryptanalysis, it would have been exact. At worst, capture, perhaps of a cue word, which—contrary to all regulations—would have to have been written down, might have given the Allies insight into some messages. The chief of the Naval War Staff conceded that a capture was possible, and he approved Maertens’s plan to establish separate regional key nets.

  Maertens was supported in his position by the coincidental discovery on February 2, in a British bomber downed at Rotterdam, of a new type of radar. It was based on the cavity magnetron, a block of copper with eight cylindrical holes bored in it parallel to and around a central axis. These hollows enabled the radar to operate on a wavelength of 9.7 centimeters, much shorter than the earlier 1.5 meters. Because its wavelength was measured in centimeters, the device was called “centimetric radar.” It gave the British two advantages: it depicted objects—coastlines, buildings—on the radar screen, which the older radar could not do, and the U-boats’ radar warning receivers, which were tuned to the longer wavelength, could not detect it. With centimetric radar, British airplanes could thus locate surfaced U-boats from a distance without alerting the submarines and could attack them by surprise. The Royal Air Force Coastal Command had begun doing just this with some success against U-boats traversing the Bay of Biscay. Though Dönitz had as yet no evidence that centimetric radar was being used in the Battle of the Atlantic, the use of this powerful new weapon could not be excluded.

  So Dönitz accepted Maertens’s view that Kriegsmarine ciphers were secure and that the leaks were elsewhere. “With the exception of two or three doubtful cases,” he confided to his war diary, “enemy information about the position of our U-boats appears to have been obtained mainly from extensive use of airborne radar, and the resultant plotting of these positions has enabled him [the enemy] to organize effective diversion of convoy traffic.” And when SC 127 circumvented a wolfpack, he gave as the most probable reason that “the enemy has an extraordinary location device, usable from airplanes, whose effect cannot be observed by our boats.”

  Nevertheless, suspicion that the Allies were solving naval Enigma messages would not die. Dönitz tried to reconcile his concern with Maertens’s reassurances, but he was not always able to. On April 27, as SC 127 was slogging across the ocean, the Allies, in a U-boat situation report that the B-Dienst solved, reported five U-boats within a 150-mile radius of 50° north, 34° west. “For some time resupplying has been carried out here,” Dönitz noted. “It remains disquieting that they were suspected precisely in the area in which no radioing had been done for several days.”

  A few days later, Dönitz, for reasons that went beyond his fears about cryptosecurity, fired Maertens, sending him to Kiel to run a shipyard. He replaced him with the glass-eyed Stummel, Maertens’s chief of staff, promoting him to rear admiral. Stummel maintained, as always, that Enigma “had, on the basis of repeated and thorough investigations, proved itself up to the present as unbreakable and militarily resistant.” Dönitz apparently believed him, for in June he was telling the Japanese ambassador that U-boat losses were due to a new Allied direction-finding system.

  Despite his claims, Stummel began in 1944 to prepare a measure that would carry the Kriegsmarine’s basic cryptosecurity principle to its logical conclusion. By subdividing the navy’s cryptosystem into as many key nets as necessary, Stummel sought to reduce the number of messages in a common key. As the volume of traffic grew, Enigma key nets had expanded from one in the early 1930s to separate home and foreign key nets and to the addition of a U-boat net and many others by 1943, when traffic averaged 2,563 radio messages a day. Now Stummel proposed to g
ive each U-boat its own key.

  Individual keys were issued to some submarines shortly after D-Day, June 6, 1944; they began to be widely used in November, and by February 1945 they were carrying practically all the operational traffic of the U-Boat Command. In that month, Dönitz told Hitler that Allied knowledge of wolfpacks came from radar and betrayal. By then Stummel had also been ousted, but his program of individual keys justified his faith in Enigma: G.C.&C.S. solved only three keys for brief periods. Perhaps not coincidentally, sinkings rose steadily from November 1944 to April 1945 in the North Atlantic and North Sea, although the absolute number remained small. Solution of these individual keys would have required a great increase in personnel and in bombes, but G.C.&C.S. felt confident that it would have been able to do it. Germany’s surrender saved it from this test.

  Long before that happened, Dönitz mourned the loss of the source of information that he said gave him half of his intelligence: the B-Dienst. He had feasted on it for so long in part because the Germans had no monopoly on cryptographic failure. In this respect the British were just as illogical as the Germans. The surprise of the North African invasion confirmed the Admiralty’s belief that its cryptosystems were secure, just as Fricke had argued that the operations of British ships gave no indication that the British were reading German messages. And G.C.&C.S. retained confidence in its superencipherment (even though it had solved similar systems before the war) because it was encountering increasing difficulty in solving high-grade Italian codes after the summer of 1940 and fewer problems with nonnaval Enigma; this logic resembles the Kriegsmarine’s argument that Enigma must be secure because it was unable to break the American naval cipher machine.