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

by Kahn, David

  They played a major role in which the British critic George Steiner said about the entire codebreaking effort: “Increasingly, it looks as if Bletchley Park is the single greatest achievement in Britain during 1939–45, perhaps during this century as a whole.” And when the Queen herself visited Bletchley Park on 15 July 2011 and paid tribute to the secret men and women who had done so much to save freedom, the significance of their work was made clear to all at royal level.

  —NEW YORK, NEW YORK

  January 2012

  1

  A STAFF SCHOOL MEMORY

  FROM UNDER THE RUFFLED WATERS OF THE SPRINGTIME NORTH Atlantic, the captain of the German submarine U-110 peered his periscope at the oncoming convoy. He chose four ships in the second column as his targets, took aim, and, at 30-second intervals, fired three torpedoes from his bow tubes.

  His intended victims were members of Convoy OB 318, lumbering west toward America to be refilled with supplies for wartime Britain. An the center of the front line of the warships that surrounded the convoy steamed Escort Group 3’s flagship, the Royal Navy destroyer Bulldog. She was skippered by Commander A. J. (Joe) Baker-Cresswell, a fresh-faced, boyish-looking career officer, just turned forty.

  Baker-Cresswell had fixed his midday position by shooting the sun with his sextant through the thickening clouds when, to his astonishment, he saw a column of water rise near the merchantman Esmond, which was leading the starboard column. For a moment he was incredulous. The convoy, southwest of Iceland, was only 300 miles from the Greenland coast; no submarine had ever attacked that far west. But his surprise did not stop him from swinging at once to starboard, in the direction from which he sensed the torpedo had come.

  The local time was noon, the date, Friday, May 9, 1941. Great Britain and Nazi Germany had been at war for a year and a half. The United States was not yet involved, though that very day President Franklin D. Roosevelt’s son James had said that the country was at war in all but name. Adolf Hitler’s forces had overrun Poland, Denmark and Norway, Belgium, the Netherlands, and France. But England, buoyed by Winston Churchill’s trumpet-tongued defiance, had withstood the German threat. In the fall of 1940, Hitler abandoned his plan to invade Shakespeare’s “sceptered isle … set in the silver sea.” He had decided to force Britain to surrender instead. His bombs would destroy her war industry and the people’s will to resist. His submarines would cut her lifelines and starve her.

  And indeed, in the sea lanes between the British Isles and North America, U-boats were sinking American and Canadian ships supplying Britain, and the convoys’ escort vessels were fighting off the U-boats. This was the Battle of the Atlantic, the only battle in World War II that lasted from the first day of that war to the last.

  Paralleling this visible battle was an invisible one—the code war. British codebreakers were intercepting German radio messages and trying to crack German naval codes. The Allied naval commands used whatever information they could gather about U-boat movements to divert convoys and sink subs. Unlike the men in combat, the codebreakers did not face death, but they labored under intense pressure. The U-boat cipher was the most difficult of all the ciphers used by the Kriegsmarine, the German navy, and they had been unable to crack it. They were perfectly aware that their efforts would determine not only the fate of men and ships, but even how quickly the war would end.

  The transatlantic convoy system was a great chain of waterborne buckets bringing food, military supplies, and raw materials to Britain and taking money-earning goods to her allies. OB 318 was a typical outward convoy—outward from Britain—another in the OB series, which had begun sailing from Liverpool on September 7, 1939, four days after Britain declared war. The O in the designator meant “outward”; the B distinguished this series from the identically numbered OA convoys, which left from London. On average four OB convoys a month sailed for North America.

  On April 25, 1941, seventeen of the many ships swinging at anchor in the Mersey River estuary at Liverpool were issued orders that would form them into the core of OB 318. Vessels at Welsh and Scottish harbors were ordered to rendezvous at certain times and places with the Liverpool group. During the week before the sailing, the masters of the vessels met with one another and with the senior officer of the warships escorting them to answer questions and to get acquainted.

  Not all of the ships nominated for the convoy were ready on time, and an extra one joined, so in the end OB 318 consisted of thirty-eight vessels. They varied in size from a little trader of 890 tons to a passenger-cargo steamer, the Ixion, of 10,263. The loaded merchantmen carried clay, coal, chalk, wood, pulp, Scotch whisky. Sixteen had brought supplies to Britain but had been unable to find cargoes for their outboard voyage to America, so they were now in ballast, loaded with rock or sand for stability at sea. Most of the ships were heading for the United States or Canada, but some were destined for the Caribbean or South America. Their company markings had been painted over with gray, and most had a 4-inch gun at the stern as well as some lighter, dual-purpose (antiaircraft and low-angle) weapons. Though some of the vessels could steam at 12½ knots, the convoy was limited to the speed of its slowest ship, 9 knots. In practice, because of the difficulties of steaming in formation, that speed was reduced to about 8 knots.

  The Liverpool core departed in the afternoon of Friday, May 2, and proceeded up the west coast of Britain, annexing the other elements along the way. At 10:15 P.M. on Sunday, the full group sailed out between the Butt of Lewis, the northernmost point of the Outer Hebrides, and the tall majestic cliffs of Cape Wrath, at the northwestern tip of Scotland, to pass into the long swells of the open Atlantic. Here the warships of Escort Group 7, which would accompany the convoy to the waters off Iceland, joined and took command, and here OB 318 assumed its ocean formation of nine short columns about 500 yards apart, with the ships in each column about 200 yards from those ahead and astern.

  The next morning, as OB 318 swung west, the sky was clear and the weather warm. Overhead an airplane patrolled.

  To defend against the U-boats the British had to know where they were. Asdic, the underwater sound-ranging system, could detect submerged submarines within a half mile on average. Radar could spot them on the surface 2 to 3 miles away if the weather was calm; the range dropped in rough seas. But both of these systems were little more than crude aiming devices. There was no way to locate the U-boats in time for ships and airplanes to concentrate against them. The few airplanes available could cover only a tiny fraction of the ocean, even when lying from British-occupied Iceland.

  Radio intelligence was little better. At that time it relied chiefly on direction-finding: rotating a radio antenna until an enemy signal was heard most loudly, much as one turns a portable radio to get the best reception. The direction from which the signal came was its line of bearing. Two or more antennas took bearings on a signal from different places, the lines were drawn on a map, and the point at which they crossed gave the location of the transmitter. The direction-finders of the time were not precise, however. Although the margin of error averaged 25 miles, it could range up to 60 miles 500 to 1,000 miles offshore. Moreover, at that time ships did not have direction-finders. Thus the area from which a U-boat had radioed could be only generally fixed. The vagueness of direction-finding meant that the British could be directing a convoy straight at a U-boat infestation instead of past it. Besides, direction-finding told only where the U-boats were at that moment, not where their orders were directing them.

  Only one source could provide more exact information in time for the convoys to detour around the U-boat groups. That source was the solution of coded German naval messages. And Britain had not then solved enough to help them. For the U-boat messages were well protected by the cryptographic armor of an electrical cipher machine called the Enigma, with which the Germans put radio messages into secret form.

  The Enigma operator pressed the keys of a typewriter-like device as one does in hunt-and-peck typing. When a key was depressed, curre
nt flowed through a set of wired codewheels to illuminate an output letter on a glass panel The output letters comprised the cryptogram of the original message. This was transmitted by radio to a U-boat. There the radioman-cipher clerk, using an identical machine identically set up, typed in the letters of the cryptogram. On the illuminable panel flashed the letters of the original German text.

  Cracking the Enigma was the job of the British agency called the Government Code and Cypher School, or G.C.&C.S. (the name was a disguise). Formally a branch of the Foreign Office, G.C.&C.S. was housed at Bletchley Park, an estate in the town of Bletchley, some 50 miles northwest of London. Here more than a thousand people-mathematicians, linguists, chess champions, clerks—labored to read not only the Enigma messages of the Kriegsmarine but also messages enciphered in the Enigmas of the Luftwaffe, the army, the SS, and other German organizations, as well as nonmachine, or pencil-and-paper, cipher systems.

  With the Luftwaffe’s Enigma, which used a simpler keying system than the navy’s, and with some of the Kriegsmarine hand ciphers, G.C.&C.S. often succeeded. These solutions were translated, supplemented by information from previous solutions, and forwarded to the armed forces. The output of G.C.&C.S.’s Naval Section—weather reports, reports of damage to German merchant shipping, local messages about the departure and return of U-boats, and data on troop transports—was sent via teleprinter to the Operational Intelligence Centre, a branch of the Naval Intelligence Division. The O.I.C. was bunkered in an almost windowless, forbidding concrete annex behind the Admiralty building off London’s Trafalgar Square. Among its other duties, O.I.C. tracked the U-boats in the North Atlantic that were working to sever Britain’s supply lines. But its intelligence was inadequate. Bletchley Park was unable to read the naval Enigma, which guarded the U-boats’ orders, except on rare occasions. In the 613 days of war up to the moment when torpedoes struck the Esmond, Naval Section had solved less than 70 days’ worth of Enigma messages. And most of those were too old to help. As an example, G.C.&C.S. had last teletyped a solved U-boat intercept to the O.I.C. two days before the U-110 attack—and that was of a message transmitted eleven days earlier.

  The submarines were sinking more ships than Britain and the United States were building. And the sinkings were accelerating—from 126,000 tons in January 1941 to 249,000 tons in April Rationing was beginning to hurt. In March 1941, two months before the attack on Convoy OB 318, meat rations were cut for the fourth time. Cheese rationing was begun in April Butter and sugar had long been restricted, and fresh fruit and eggs were rapidly becoming a memory. It was said that people were beginning to think with their stomachs. Churchill had estimated that 31 million tons a year of nontanker imports were necessary to keep Britain’s population healthy, her factories running, her armed forces fed, equipped, and fighting. In the first four months of 1941, the annualized rate was under 28 million tons.

  The codebreakers understood that unless they could solve the naval Enigma and ascertain the U-boats’ movements in advance, the British were in grave danger of losing the Battle of the Atlantic, with possibly fatal consequences for the nation. Different measures that might turn the tide, such as building more merchant ships or adding more escort vessels, would take a long time; solving the naval Enigma offered the only possibility of immediate aid. Naval Section urged itself on.

  Other intelligence sources sometimes provided help. At 9:38 A.M. on Monday, May 5, the navy’s Western Approaches Command, probably alerted by direction-finding, diverted Convoy OB 318 to the south of its planned track “to avoid possible U-boat threat.” That afternoon, course was altered again, a little more to the south. The next day, as the convoy furrowed the calm sea in its painfully slow progress, airplanes patrolled above it; around teatime, a Sunderland flying boat lumbered overhead, then departed, leaving the convoy without air escort. That morning, the four merchantmen from Iceland that were to join OB 318 had sailed under the protection of three corvettes and an armed trawler, part of Escort Group 3. And the next morning, Wednesday, May 7, the destroyers that formed the powerful core of that group steamed out of Reykjavik harbor to protect OB 318; when it arrived, Escort Group 7 would return to Britain.

  Escort Group 3’s commander, Joe Baker-Cresswell, was born in London on February 2, 1901, while most of his family was watching the funeral procession of Queen Victoria. After attending Gresham’s School in Norfolk, he joined the navy and served in a variety of ships, from submarine to battlewagon. He then attended the Naval Staff College in Greenwich. At the start of the war, he was on the Middle East Joint Planning Staff, but he wanted a command at sea. In London one day, as he entered the Admiralty to plead his case, he ran into Admiral Sir Percy Noble, commander in chief Western Approaches, under whom he had served earlier. He explained his wish. Noble said, “Leave it to me,” and in January 1941 Baker-Cresswell was appointed commander of Escort Group 3, which he soon molded into an efficient force.

  Baker-Cresswell tested various colors as camouflage in the northern Atlantic waters, found pale mauve the best in spring and summer and white the best in winter, and painted his ships those colors. He had blue and yellow squares painted around the tops of his ships’ funnels—the color and pattern of the naval signal flag meaning “3.” He flew his pennant from the Bulldog, an eleven-year-old, 1,360-ton destroyer whose original armament of guns and torpedo tubes had been reduced to enable her to carry more depth charges.

  As Baker-Cresswell was heading south on May 7 with his three destroyers, the British intercepted a U-boat radio message. Although they could not read it, they recognized that it was a sighting report, from the characteristic Morse symbol that preceded it as prescribed by German radio regulations. The Admiralty apparently obtained only a single bearing on this transmission and was therefore able to determine only the line from which it originated and not the point. Thus, though the U-boat was telling of a convoy far to the east of OB 318, the Admiralty guessed that OB 318 was the one sighted. So at 3:04 P.M. on Wednesday, the Admiralty notified Escort Groups 3 and 7 of its suspicions and ordered the convoy to make an evasive turn to starboard and then to steer due west. This directed it straight at the U-94, which with the U-110 was one of the nine U-boats in the North Atlantic north of 55° north latitude (approximately the latitude of Belfast in Northern Ireland and of the middle of Labrador). The commander of the U-boats, Rear Admiral Karl Dönitz, had ordered six submarines to area AK on the Kriegsmarine’s gridded chart of the world’s oceans; the chart used pairs of letters to compress location specifications and to keep them secret. AK lay smack in the middle of the North Atlantic, athwart many of the convoy routes.

  Baker-Cresswell arrived at the rendezvous point 150 miles south of Reykjavik (and within AK) a bit ahead of time. After a brief search, he found the convoy only 7 miles north of the rendezvous point and fifteen minutes early—much closer to plan than usual. It was about 4 P.M. A little later, the U-94, on the surface, spotted the convoy’s funnel smoke.

  She headed for the convoy as Escort Group 7 transferred responsibility for OB 318 to Baker-Cresswell. He arranged his destroyers forward and aft of the merchantmen, with his own Bulldog ahead of the starboard columns. The weather was fine; one of the merchant captains remarked that never in his long life had he known the Atlantic so calm so far north. At 7 P.M., the Bulldog’s asdic contacted an object 200 yards ahead, then lost it. Baker-Cresswell reversed his course and, knowing that U-boats liked to get between two convoy columns and fire torpedoes from fore and aft tubes, steamed back down between the seventh and eighth columns. He found nothing.

  But the U-94 had submerged and sneaked between two other columns. Between 7:09 and 7:11, she fired three single shots from her bow tubes and one from her stern tube. The latter missed, but the bow torpedoes struck, almost simultaneously, the big British steamer Ixion and the Norwegian Eastern Star, a 5,600-tonner, both carrying Scotch whisky. The two ships burst into flames and began sinking. When the U-94’s skipper came to periscope depth to see what he had done, a
lookout on the sloop Rochester spotted the periscope in the smooth sea at almost the same time that the destroyer Amazon gained asdic contact. The Bulldog led the depth-bomb attack, which damaged the U-94’s hydroplanes and some gauges. But despite a lengthy hunt, the submarine escaped. The crews of the sunk steamers were rescued, along with a number of cases of Vat 69 Scotch which were perhaps later issued to the seamen for medicinal purposes.

  By 11 P.M. the convoy was back to its normal routine, as were the screening vessels, their watches broken only by the continuous pinging of the asdics and the periodic ringing of the zigzag clocks, which told the helmsman when to zig and when to zag to confuse the U-boats. Baker-Cresswell spent the next day, Thursday, in an unsuccessful search for the U-boat. He felt sure he had driven it off, and the Admiralty sent no more warnings.

  In U-boat headquarters, however, Admiral Dönitz was planning another move. He knew that during the spring of 1941 the range of convoy protection had expanded to the west as the number of escort destroyers grew. He had consequently sent his submarines farther west to attack convoys where they were still unprotected. Dönitz estimated the limit of escort protection at between 25° and 30° west longitude. Thus, in the first eighteen months of the war, U-boats sank only eight Allied ships west of 25°, but in the next two months, March and April 1941, they sank twenty-four. Now, on Thursday, May 8, Dönitz moved some of his northern boats west. They joined the U-110, which had already arrived south of Iceland. Its commander was twenty-eight-year-old Lieutenant Fritz-Julius Lemp.