Seizing the Enigma (47 page)

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 with 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.”

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 give each U-boat its own key.