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

by Kahn, David

  One of the major determinants of Europe’s weather is the polar front: the boundary between the warm moist air of the south and the cold dry air of the Arctic. Another is the North Atlantic Drift, the remnants of the Gulf Stream. To determine their locations and movements, the Germans favored two observation areas. One was some 300 miles east and slightly north of Iceland, a watery square 54 miles on a side designated as AE 39 on the Kriegsmarine grid. The other was the square AB 72, in the Norwegian Sea some 550 miles northeast of Iceland. Its southernmost boundary was at 71° north latitude, the parallel that cuts the North Cape, the northernmost point of Europe. The weather ships worked these areas for three to five weeks, excluding the time required to go to and return from their stations. The Coburg, for example, sailed on February 10, 1941, and returned on March 28; the Ostmark left port on April 2 and returned on May 6.

  While the Ostmark was still at sea, the München received orders to relieve her. This was to be the München’s second cruise and the second time she had replaced the Ostmark in AE 39. And conditions would be better than on the first cruise, which had put the vessel in the freezing North Atlantic from the middle of February to the middle of March. She sailed in the evening twilight of Thursday, May 1, from Trondheim, the fortified seaport in whose ancient cathedral kings of Norway are crowned, steaming slowly in the gathering darkness down the wide fjord. She had 40 miles to go to reach its mouth, and another 30 until, after passing the chains of islands that paralleled the coast, she rocked to the swell of the open sea.

  Her crew of about twenty consisted of some seamen who had worked on her as a fishing trawler and who ran the ship and some navy sailors who had been assigned to her for weather, radio, or weapons duties. One of them was the young weather observer Fritz Rebelein, who had been plucked from his barracks in Wilhelmshaven apparently at random to be trained in meteorological observation. Another was draftee Heinrich Wiggeshof, not yet 20. A ham radio operator in peacetime, he had been sent to the navy’s communications school at Flensburg and then assigned to the München for his first duty afloat. Both had completed the first cruise, but this time Rebelein had a premonition that he would not come back.

  Once the München had reached her assigned square east of Iceland, Rebelein and the other observers took measurements twice a day. They encoded the results in the Short Weather Cipher and gave them to one of the radiomen. He enciphered this in the Enigma, for which he, like all Kriegsmarine vessels in the Atlantic, had the Home Waters cipher net keys for May. And because the München’s replacement might be delayed, or the ship for some other reason might have to stay at sea longer than the scheduled twenty-five days, she also carried the keys for June.

  The Germans were not the only ones paying attention to the weather ships. The solutions of naval Enigma traffic made possible by the documents from the Krebs included messages from the weather ships and acknowledgments of their reception. The messages often told of the weather ships’ movements to and from their stations.

  In his little office in Hut 4, Harry Hinsley, who analyzed the cryptanalysts’ output especially for trends and for information on specialized (nontactical) matters, read these intercepts along with all the others. One day in the spring of 1941, a “passing thought” brought to his consciousness something that had remained beneath the surface of his mind: the weather messages were enciphered in Enigma. He asked himself whether the Germans would be so foolish as to put the cipher machine used for the whole German navy and its accompanying key documents on an isolated, vulnerable weather ship. But then, in a “mad moment,” he concluded, “Good God, this must be right!” It further dawned on him that if the weather ships were out at sea for a long time, they had to carry Enigma keys for several months. Perhaps with Ian Fleming’s unfulfilled idea of seizing cipher documents from a lured-out air-sea rescue boat and the success of the Krebs episode in the back of his mind, Hinsley thought of capturing Enigma papers from a weather ship.

  The young intelligence officer discussed his idea with Clive Loehnis, Bletchley’s liaison with the Admiralty, particularly its Operational Intelligence Centre. The O.I.C. was impressed by the idea, but it recognized the chief risk. If a weather ship radioed that it was being boarded, the Germans would probably change the keys, perhaps including the rotor wirings, and possibly even abandon the Enigma for another system. On the other hand, the O.I.C. and B.P. observed that, despite the destruction of two weather ships through British action, the Germans had not seemed to fear that their cryptographic documents had been compromised and had left their systems in place. Perhaps their complacency would continue. So the O.I.C. and B.P. concluded that the prize was worth the risk. Even if no cryptographic papers were captured, the Germans would be deprived of a source of weather reports. In addition, Hinsley knew, there were two weather ships on northern stations at a time, doubling the chances for capturing cipher documents and wiping out German weather ship information.

  This clinched it for the intelligence types. With these arguments in hand, they approached Captain Haines, assistant director of the O.I.C. for radio intelligence. Operational questions like these came first to him. No doubt after satisfying himself of the necessity of such a “pinch” and the possibility of its succeeding, he discussed it with operational officers of the Admiralty. They concluded that such a seizure, while tricky, was not impossible.

  Perhaps under the supervision of the director of the Operations Division (Home), Captain Ralph Edwards, Haines planned the attack. He determined the forces, the time of day, and the angle of approach that gave the best chance of success. He studied the three-page report on German weather ships that Hinsley had compiled on April 26, 1941, from various sources. The report made clear to any naval officer the isolation of the weather ships. Perhaps from later intercepts, the British concluded that the München was to sail May 1 to relieve the Ostmark in grid square AE 39, east of Iceland.

  The same source may have revealed the presence of the other weather ship that Hinsley knew about, perhaps the Sachsen, in the other preferred observation area, AB 72. Edwards figured that seven ships would be needed to search the grid square. Since AE 39 extended about 1° of latitude from north to south, or 69 statute miles, and since ships would be able to stay in sight of one another at a spacing of 10 miles, seven ships would adequately sweep the area. Presumably one warship, quickly reinforced by others, could swiftly capture a surprised and lightly armed weather ship.

  So it was decided. Edwards detached three cruisers and four destroyers from the Home Fleet for the assignment. In command of the forces was Vice Admiral Lancelot E. Holland, a gunnery specialist, head of the Eighteenth Cruiser Squadron. A short, slim man with sharp features and nearly white hair, Holland, intensely ambitious, was regarded as one of the most competent senior officers in the service. He flew his flag from the Edinburgh; the other two cruisers were the Birmingham and the Manchester, both armed with twelve 6-inch guns and capable of speeds greater than 30 knots. Also aboard the Edinburgh was Haines, who knew what the cryptanalysts needed and was assigned to search for it aboard the seized vessel.

  The destroyers were the Nestor and three of the large, fast, and tested Tribals: the Eskimo, the Bedouin, and the Somali. All three had, barely two months before, participated in the Lofotens raid, in which the Somali had sunk the Krebs and captured Enigma rotors and keys. Now they and the cruisers, together with the other gray warships of the Home Fleet, held in readiness as a strategic force at Scapa Flow, tugged impatiently at their anchor chains.

  On Monday, May 5, the seven ships raised anchor and steamed out of the basin. The destroyers took one route, the cruisers another to their rendezvous next morning north of the Faeroes. That afternoon the flotilla practiced a battle action. The Manchester acted as an enemy pocket battleship, with the Nestor laying a smoke screen to help it escape. During much of the day the force pretended to cover for some minelaying west of the Faeroes. At 3 A.M. on Wednesday, May 7, the group altered course from north-northwest to southeast. This wa
s to disguise its intentions in case it was seen by that morning’s German meteorological flight. At 6 A.M., the flotilla again altered course, this time to a little east of north. It was heading for its starting position for the sweep, which it reached a little after noon. At 12:55 Holland turned his ships east and eight minutes later spaced them at 10-mile intervals along a north-south line a little bit west of AE 39. Visibility was only 7 or 8 miles, which made it a little harder for the ships to remain in contact but enabled them to approach the weather ship more closely before being detected.

  They steamed at 17 knots for two and a half hours, then Holland increased speed to 20. The sky was clear, the sea, slightly ruffled by a light breeze, calm. The temperature stood around freezing. The sun declined slowly on its shallow oblique to the sea. At 4 P.M., the Birmingham sent up an observer aircraft. Its pilot and the lookouts on all ships strained their eyes for the trawler’s smoke or for the needle of her mast above the horizon. Aboard the Somali, the physician, Surgeon-Lieutenant Dr. M. G. Low, retired to the wardroom to read Country Gentleman while awaiting the action.

  Suddenly, a few minutes after 5 P.M., someone aboard the Edinburgh saw smoke off the starboard bow between Edinburgh’s course and Somali’s. At about the same time the man who had shot open the locked drawer on the Krebs, Lieutenant Warmington, on the bridge of the Somali, spotted the smoke. The rattlers sounded action stations. Low threw down his magazine and ran out on deck. He, too, saw the smoke. The Somali increased speed to her maximum 32 knots and raced toward the ship that appeared to rise above the horizon.

  Aboard the München, Radioman Wiggeshof was transmitting a weather report when the cry rang out, “Mastheads on the horizon!” The little ship put on way and ran. The crew began rushing about. The wireless noncom came into the radio shack, grabbed the Enigma machine, put it and the current keys into a canvas bag with a lead bottom, added papers and instruments that Rebelein brought up from below, and slung it overboard. At 5:28, Wiggeshof transmitted on 7769 kilocycles, in the clear, “Werde gejagt” (Being chased).

  By then the München had emitted a dense white smoke screen and was dodging to and fro trying to stay behind it. On the Somali, Low felt a pang of sympathy for the little ship and her crew, suddenly seeing the bow waves of a flotilla of enemy warships heading toward her.

  When she was about 3 miles from the weather ship, the Somali opened fire with her 4.7-inch guns. The Eskimo followed. The München, with only a machine gun, did not return the fire. None of the British shells struck her, but, rumbling over the trawler and sending up towers of water next to her, they had the desired effect: the crew members were seen abandoning ship in two boats. Not all escaped that way, however. Some of the lifeboats could not be dropped to the water, and Wiggeshof, as soon as he had transmitted his warning signal, drew on another pair of pants and jumped overboard. Others remained on deck.

  The trawler’s crew made no attempt to scuttle, in part because some of the seacocks had been cemented in when the sand ballast was replaced by concrete. The impression gained aboard the Edinburgh that the München was settling rapidly by the stern proved to be an optical illusion. The Somali readied her boarding party: Warmington; the second in command, Henry Stuart-Menteth, now a commander; and two seamen. She sped to the trawler at top speed, reversing to full speed astern to come to a thumping stop alongside. The boarding party, revolvers at the ready, jumped down from the destroyer onto the deck of the weather ship. One of the seamen shot himself in the toe with his pistol as he landed. Stuart-Menteth was nervous that the Germans had set explosives, but the party found none when it searched the ship. She was intact.

  Warmington went to the radio shack, where he picked up a few bits of paper. He found nothing he thought worth taking; the Germans had apparently thrown everything valuable overboard. Haines came over from the Edinburgh with the prize crew. He seemed to know just what he was looking for and that some cipher documents were kept in officers’ quarters separate from cipher documents in the radio shack. So he didn’t bother with that room but disappeared into the depths of the ship, apparently looking for something specific. Within a few moments he seemed to have found it, for he emerged from below carrying papers.

  The Edinburgh and the Somali, meanwhile, were picking up the enemy crew. “What’s the name of that ship?” Captain Caslon bellowed down to Low, who spoke German. “Wie heisst deine Schiff?” shouted Low to the Germans. “München,” they replied. Wiggeshof had lost consciousness in the 35-degree water; the next thing he knew, a British seaman was slapping his face and asking whether he wanted tea or coffee. Wiggeshof, who had never in his life had tea, must have nodded his head at the wrong time because the sailor brought him a steaming mug of it.

  After about twenty minutes the boarding party returned to the Somali, and the prize crew took over. By 6:45, an hour and a half after the trawler’s smoke had first been sighted, the München, escorted by the Somali, was on her way to a British port. Haines, meanwhile, transferred to the destroyer Nestor, which at 11:45 was ordered to bring him and his papers to Scapa.

  Upon his arrival in Bletchley on Saturday, May 10, Haines turned over to Peter Twinn the Short Weather Cipher and the inner and outer Enigma settings for the home water keys for June. Holland had called them “rather undistinguished documents,” but they bore within them seeds of great power.

  That their weather ship with its documents might have been seized seemed never to cross the Germans’ minds. They had received Wiggeshof’s frantic signal about being chased, and the Naval War Staff concluded two days later, when the München failed to respond to a request to report her position, that she had been lost. The Germans picked up the intentionally misleading British official communique: “One of our patrols operating in northern waters encountered the München, a German armed trawler. Fire was opened, and the crew of the München then abandoned and scuttled their ship. They were subsequently rescued and made prisoner.” But though the German navy duly noted this in several war diaries, nothing was said of the possibility that secret documents had been captured, and no changes in German naval cipher procedure were ordered.

  How could they not have thought about this? How could they have not taken ordinary precautions? In the first place, the previous losses of weather ships had led to no dire consequences. Second, the Naval Group Command North was getting ready for its role in Hitler’s vast ideological attack on the Soviet Union, less than seven weeks away. The group had to lay thousands of mines in the Baltic before the assault began and had to keep track of Soviet warships. It had to escort freighters carrying nickel from Finland, ward off raids like that on the Lofotens, protect U-boat departures—all duties linked to Norway’s being what Hitler called his “zone of destiny” in the war, and it would not be wise to fail the Führer. Finally, the high command had larger concerns. Only three weeks before the München was captured, Hitler and Admiral Erich Raeder, the head of the navy, discussed the perennial problem of the United States’ hindrance of the U-boat war and the newer issue of Japan’s planned attack on Britain’s major Asian base at Singapore.

  In these realms of high strategy, it was easy to forget a single trawler, one of many tiny specks on the chart of the North Atlantic, doing a secondary job. Her fate was seen as insignificant in a struggle in which ships were sunk daily and many men died. For all these reasons, the Kriegsmarine ignored the loss of the München. Wrongly.

  Britain’s target: an Enigma cipher machine of the German navy. The Enigma put German messages radioed to U-boats into secret form; an identical machine, identically set up, reconverted the ciphertext to plaintext in the submarines.

  How the Germans set up an Enigma machine for enciphering messages.

  From the eight rotors available, the three specified by the key list for current use are chosen. The alphabet ring on each rotor is turned to the position given in the key list and locked into place with a pin on a leaf spring.

  The three current rotors are assembled on the shaft from left to right in t
he order given in the key list.

  The rotors are inserted into the machine.

  The jacks on each end of several short cables are plugged into the sockets specified by the key list of a plugboard on the front of the machine.

  The rotors are turned until the letters specified by the key list appear in the windows of the Enigma’s lid.

  When the machine has been set up, encipherment begins. Each plaintext letter of the German message is pressed on the keyboard, and the letters that light up on the illuminable panel are written down. This is the ciphertext. After the entire message has been put into cipher, the cryptogram is radioed from headquarters to a U-boat or from a U-boat to headquarters.

  Russian sailors look at the wreck of the Magdeburg, partly blown up near the island of Odensholm in August 1914.

  Arthur Scherbius, the inventor of the Enigma.

  Hans-Thilo Schmidt, the spy who revealed some secrets of the Enigma.