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

by Kahn, David

  The cherished beliefs of the British were wrong. In December 1942, they learned from their Enigma solutions that the Germans were reading Naval Cypher No. 3, the main cryptosystem for convoy arrangements in the North Atlantic. And in Washington, in March 1943, Lieutenant McMahan of OP-20-G saw a German intercept that canceled an order by Dönitz of a few hours earlier and directed a radical change of course. McMahan thought that only a German solution of a message diverting an Allied convoy could have caused Dönitz to react like that. He went downtown to Convoy and Routing in Main Navy and, after some difficulty, persuaded them to let him see the messages to Allied convoys. His discovery of the very message that had ordered the detour brought together compartmentalized elements and confirmed the Allies’ recognition that the Germans were reading their traffic.

  In June, when Naval Cypher No. 5 replaced Nos. 3 and 4, the B-Dienst made no real progress against it. Concerns about the security in heavy traffic of the superencipherment, called the long sub-tractor system, had been raised as early as 1940; G.C.&C.S. devised a replacement—the stencil subtractor—by 1941, but the services did not decide to adopt it until after extensive trials that ended in March of 1942. Design and production of the devices and printing of the tables took the rest of the year, distribution for the Royal Navy until the middle of 1943, and distribution within the U.S. Navy until January 1, 1944—a record of cryptographic negligence that compares favorably with Germany’s. Still, from the middle of June 1943, the B-Dienst was effectively shut out from its vital Anglo-American intelligence. In May 1944, Hitler asked his naval codebreakers which English systems could be broken. They had to confess that although they were solving a number of secondary systems and a convoy system for stragglers, “The two main English systems cannot be read, the one [the main warship cryptosystem] since the start of 1944 and the other [the convoy system] since the start of June 1943.”

  * * *

  This admission unwittingly confirmed the Allied victory in cryptology. In August 1943, the British and the Americans had begun reading Enigma messages nearly always currently. The capture of the U-505 by an American task force on June 4, 1944, provided a copy of the Adressbuch that provided the keys for disguising grid positions; from then on the Allies read them as easily as the Germans did.

  But solving German messages did not always mean the successful diversion of convoys. It is true that in January and February 1943, when solutions were almost uninterrupted, the Allies suffered far fewer losses than in March, when for days no solutions were achieved. On the other hand, two convoys out of three escaped detection in August and September 1942, during the ULTRA blackout, while less than half avoided being spotted in the first five months of 1943, when solutions were frequent. The totality of other factors eclipsed ULTRA: the number of U-boats on patrol, the quantity of very long range aircraft the Allies had, centimetric radar, shipboard direction-finding, operational research, the arrival of escort aircraft carriers, the increase in escort vessels. But when ULTRA worked with these new Allied strengths, particularly after Dönitz withdrew his U-boats from the North Atlantic on May 24, the results could be spectacular. On September 21, 1943, Churchill announced to the Commons that, in the third of a year just ending, not one merchant ship had been lost to enemy action in the North Atlantic. The House erupted in cheers.

  22

  THE U-TANKERS

  THE EXTENSION OF THE U-BOAT WAR TO THE AMERICAN COAST in January 1942 created a problem for Dönitz. Because it took his submarines substantially more time to get to and from their rich target areas off New York or Virginia or Florida than it took to reach patrol lines off Newfoundland or Nova Scotia, their combat time was reduced. To prolong their battle period as much as possible, he resupplied the U-boats at sea, using special submarine tankers. These “milch cows,” twice as big as the standard Type VII combat submarines, carried 400 tons of fuel oil, 50 tons of provisions, a workshop, a physician, and personnel to replace injured or sick combat-sub crewmen. When the first U-tanker began work in March 1942, a U-boat averaged 41 days at sea. With one resupply, this time was extended to an average of 62 days and, with two, to a maximum of 81. Even for the more northerly operations, refueling was essential for efficiency. Experience showed that submarines had to spend from three to five weeks in the operational area before encountering a convoy in a favorable position. This meant that to make success likely, Type VIIC boats had to be refueled twice. After twelve months, by May of 1943, the U-tankers had completed 390 refuelings.

  It became clear to the Allies that sinking one milch cow would reduce the effectiveness of many combat U-boats. They were unable to plan such an attack during the codebreaking blackout of 1942 because they could not read the instructions for the refueling rendezvous, and U-Boat Command had taken care to have the submarines meet in remote locations, far from the convoy tracks and out of the range of Allied airplanes. The U-tankers maintained radio silence. Instructions were enciphered in the special officer-grade keys and then reenciphered in the general key; the grid encipherment disguised positions. In 1943, however, with codebreaking restored, it became possible for the Allies to attack tankers. The solved messages sometimes disclosed the date and place of a refueling rendezvous; when these specifics were not available, the O.I.C.’s knowledge of the departures and movements of the supply submarines and of favorite refueling areas could guide ships and airplanes to likely hunting grounds.

  But three factors saved the supply submarines for a while: the inability of Allied aircraft to reach the rendezvous, the need for surface forces to stay close to convoys, and the adamant refusal of the British to attack the isolated refueling points for fear that the Germans would guess that their cipher system had been solved. This refusal stemmed originally from the anxiety the British had experienced after the 1941 roundup of Bismarck supply ships, when two that were to be left alone so as not to raise German suspicions were accidentally attacked. Their decision was hardened by leakages that could be traced to Enigma solutions, by four cases in which Enigma solutions were repeated almost verbatim in British messages, and by a scare in March 1943.

  The Kriegsmarine, an intercept showed, had grown suspicious about British warships sighted in an area where they would have encountered a German convoy bringing supplies to North Africa had the convoy not been delayed. The first sea lord reprimanded the Mediterranean commander in chief, and Churchill threatened to withhold Enigma intelligence, or ULTRA, unless it was “used only on great occasions or when thoroughly camouflaged.” At the same time, the first sea lord emphasized in a personal message about ULTRA to his American counterpart, Admiral King, his anxious desire that “we should not risk what is so invaluable to us.” The next month he resisted American proposals for using Enigma U-boat solutions to attack U-tankers at their supply rendezvous, arguing that “if our Z [ULTRA] information failed us at the present time it would, I am sure, result in our shipping losses going up by anything from 50 to 100%.”

  But this risk declined late in May, when Dönitz pulled his submarines out of the North Atlantic. And a few weeks later an event showed how Enigma information, while still used with great care for its security, could greatly enhance the new offensive strength of the Allies at sea. This new strength consisted of the Americans’ introduction of task forces centered on small, “escort” aircraft carriers. These could bring airplanes to within striking distance of a refueling rendezvous. On June 12 the escort carrier Bogue, using information from both Enigma decrypts and direction-finding, sent out airplanes that, shortly after noon, spotted the 1,700-ton converted minelayer U-118 cruising placidly on the surface. The planes bombed and strafed her, drove her under, and, when she resurfaced, sank her. Her loss forced U-Boat Command to recall some submarines and delayed other combat boats in reaching their target areas. These disruptions, mentioned in Enigma intercepts, showed the Allies the value of attacking the U-tankers—a demonstration that was reinforced in a negative sense when the tanker U-488 refueled twenty-two boats to overcome the emergenc
y. As a consequence, the Americans pressed to use Enigma information against the supply subs. By this time the British fears of the loss of ULTRA were allayed because using aircraft to spot the submarines covered their reliance on cryptanalytic intelligence, so Britain concurred in the American proposal. Enigma solutions now enabled the escort carriers to carry the war to the enemy. For the first time, the Allies attacked U-boats not just defensively, as in fighting off wolfpacks, or fortuitously, as when a plane spotted a submarine, but actively—aggressively seeking out subs and hitting them. Enigma decrypts had changed from a shield to a sword.

  * * *

  Among the targets of these machete chops was the U-117, a sister ship of the U-118. She had taken her crew of fifty-odd on three supply cruises when, on July 22, 1943, she sailed from France under the command of Lieutenant Commander Hans-Werner Neumann as one of five supply submarines that Dönitz sent to sea in the last third of July. Three of these were sunk while crossing the Bay of Biscay before the end of the month; a fourth was destroyed west of the Faeroes. This put additional pressure on the U-117 to meet and refuel combat submarines that otherwise might not have been able to return home.

  One of these was the U-66, a veteran boat that had completed nine patrols in areas ranging from Cape Hatteras to the Mediterranean, had landed a saboteur on the northwest African coast, claimed to have sunk 200,000 tons of shipping, and had provided each of her two commanders with a Knight’s Cross to the Iron Cross. On this cruise she had been at sea the extremely long time of three months, during which time she had sunk two American tankers. On July 27, U-Boat Command ordered her to square CD 50, about halfway between Washington, D.C., and Lisbon, Portugal, to rendezvous with the U-117 for reprovisioning.

  The message was intercepted. But it had not yet been solved when the cryptanalysts read a message giving a new rendezvous for 8 P.M. August 3. The solution gave the location as “square 6755 of the large square west of” another square, which was disguised by the grid encipherment but which the cryptanalysts thought was CE. This would put little square 6755 in large square CD, making it 37° 57′ north latitude, or roughly east of Washington, D.C., and 38° 30′ west longitude, or north of the bulge of Brazil.

  At 1:05 P.M. Eastern War Time, August 1, 1943, the U.S. Navy’s codebreaking unit on Nebraska Avenue in Washington, D.C., teletyped a solved intercept to F-21, the Atlantic section of the Combat Intelligence section of Cominch, where the Submarine Tracking Room was located.

  The message was about twelve hours old, the time it took for Commander Engstrom’s back-room boys to crack it and the translators and evaluators to append to each U-boat commander’s name the number of his submarine and the latitude and longitude of its naval grid references. They put these insertions in double parentheses to show that they were not part of the original message. The first part of the text directed two submarines not to refuel but to proceed home. The second and more interesting portion, however, dealt with the U-117: “Neumann ((117)) head for Nav Sq 67 ((probably CD 67 = 37.57 N – 38.30 W)).”

  Fifteen minutes later, the teletypewriter tapped out another solved German message. Sent ten hours after the first, it instructed the U-66’s captain, Lieutenant Friedrich Markworth, where and when to get supplies: “Beginning 3 August 15 2000B Markworth ((66)) will provision from Neumann ((117)) in Sq 6755 ((probably CD 6755 = 37.57 N – 38.30 W)).… After execution Markworth report affirmative, Neumann wait in that area.”

  The day after these messages went to F-21, Cominch headquarters radioed the information to units at sea that could use it. It was included in the U-boat report for August 2. Not giving the source of the intelligence, the report stated: “Several [U-boats] area 3800 [north] 3830 [west].” The data were repeated in the next day’s report, with a cover source: “Several vicinity 3800 3830 by recent DFs suggesting refueling operations X.”

  One of the recipients was the U.S. Navy’s Combat Task Group 21.14, a convoy support group consisting of the escort carrier Card and three old destroyers. The U-boat situation reports told its commander, Captain Arnold J. (Buster) Isbell, where to look for subs to sink. He knew that if refueling, they could be caught at a particularly vulnerable moment—moving slowly on the surface, joined by a fuel hose—and that one of them would be a particularly valuable target. He headed toward the reported U-boat concentration while his planes scouted ahead and to the sides.

  * * *

  Late in the afternoon of August 3, as the Card was perhaps 150 miles from that area, two of his pilots, reserve Lieutenant (j.g.) Richard L. Cormier, in a Grumman TBF-1 Avenger torpedo-bomber, and his wing-man, reserve Ensign Arne S. Paulson, in a Grumman F4F-4 Wildcat fighter, making a routine submarine search, were flying southwest at 5,000 feet in clear skies when Cormier, with his binoculars, spotted a grayish white submarine off to port about 11 miles away. She was fully surfaced, cruising so slowly that no bow wave or wake was noticeable. It was the U-66.

  Paulson, on Cormier’s orders to strafe the submarine, gave his fighter full throttle and, 100 feet above the waves, raced directly at the U-boat. At 500 yards, he began firing and saw his bullets strike the conning tower, kicking up puffs of rust. He saw nobody; on the U-boat, however, a machinist who was topside smoking was wounded in both thighs. Cormier then swept in to depth-charge the U-boat, but the charges failed to release. As he circled to attack again, Paulson made another run. It was met at this time with inaccurate antiaircraft fire from the six or eight men now topside. The attack had, however, killed the submarine’s second watch officer and panicked the men in the conning tower into ringing the diving alarm. But Markworth, demonstrating anew why he had won the Knight’s Cross, bulled his way up the ladder, belayed the command to dive, and held his men to their guns.

  In his torpedo-bomber, Cormier sped toward the U-boat, skimming the water. He pressed his electrical bomb release and immediately pulled the emergency release. This time his acoustic torpedo and both depth charges dropped. Within seconds, while making a climbing turn, he saw a shock wave centered about 25 feet from the submarine’s starboard side and just forward of her conning tower. It swept to her port side and appeared to lift her from below and make her list to port. Then a heavy column of water about 100 feet high obscured the U-boat. When she reappeared, she was turning to starboard. Paulson attacked again. He saw half a dozen figures, some inert, on the conning tower. His shots killed one sailor who had kept firing despite several wounds, wounded another in the chest, and slightly injured six others. No further fire was returned. Cormier strafed, seriously wounding Markworth in the abdomen. Then it became clear to the men on the submarine that the planes had no more bombs and that it was therefore safe to dive, so the first watch officer gave the order. The bodies of the officer and the seaman had to be left where they were. Slowly at first, and then more rapidly, at an angle of 50 degrees, the U-boat submerged. Just as she was disappearing, Paulson made a final run, firing at the underside of the stern.

  Cormier dropped a marker and circled over the spot for forty-five minutes. Though neither he nor the pilots of the other planes that the Card sent saw any debris, oil, or air bubbles, the squadron commander claimed a sinking. He was wrong. Though the U-66 had two fatalities, several seamen wounded, and a captain suffering from a bullet in his guts, and though her ballast and fuel tanks were leaking, she had escaped.

  But she still had not met the U-117. She limped east toward home, with not enough fuel to make it and only two days’ worth of provisions. The next day, after midnight, she surfaced. Though the sailor’s body had washed overboard, the second watch officer’s body was, ghoulishly enough, still on the lower machine-gun stand. It and the body of a sailor who had died from his wounds were buried at sea.

  Meanwhile, Buster Isbell on the Card was being further tantalized by F-21’s U-boat situation reports to his carrier task group telling of combat and tanker submarines nearby. On August 6, for example, he was told, “One probably refueler locality 3915 3730 by DFs 052330 and 052350 probably moving NE.�
� By 2 P.M., he was steering for that area.

  That same day, the U-66 proposed a new rendezvous with the U-117 for that noon, some 54 miles north and 12 miles east of the August 1 meeting place. Dönitz acquiesced a few hours later. The Allied cryptanalysts could not read these messages as promptly as the others, and they remained a closed book. But the Tenth Fleet’s ULTRA-based knowledge of the rendezvous attempts, together with its background information that on July 30 the U-117 had been ordered to stand by within 100 miles of 38° 50′ north, 37° 20′ west, a circle within which the August 1 rendezvous was to have been effected, made it worthwhile to keep the Card in the vicinity.

  Shortly before noon on Friday, the U-117 and the U-66 finally met. After dark the combat submarine took aboard some provisions and a physician to treat Markworth. But, unable to refuel at night, the pair waited for morning. With daylight, the U-66 began to take on oil. At just about the same time, 6:49 A.M. Saturday, the Card flew off the same kind of airplane pairing as had attacked the U-66, an Avenger and a Wildcat. An hour into the patrol, however, the Wildcat had to return to the carrier because of engine trouble. The Avenger, piloted by reserve Lieutenant (j.g.) Asbury H. Sallenger, continued its routine submarine search. At 9:46, while flying west-northwest at 4,500 feet in a cloudless sky, Sallenger spotted a large white object 15 miles off his starboard bow. He thought at first it was a merchantman, but he soon realized that it was two submarines, painted white, close together, fully surfaced and proceeding very slowly southwest, with neither bow waves nor wakes. The refueling was still in progress.