Codebreakers Victory

by Hervie Haufler

  So desperate for new captures of naval Enigma keys did Turing and his team become that they supported a wild scheme put forward in December 1940 by Ian Fleming, a naval intelligence officer later to become the creator of James Bond. Fleming's idea was to seize one of the Enigma-equipped rescue boats the Germans manned in the Channel to pick up downed Luftwaffe fliers. To do this, Fleming advocated, and gained permission to implement, dressing an English flight crew in German uniforms and putting them aboard a captured German bomber. After the next raid on London, when the bombers headed home, the bogus bomber would sneak in among them. Over the Channel it would begin to emit fake smoke from its tail, send out an SOS and ditch. The crew would float in a rubber dinghy until deliverance appeared. "Once aboard rescue boat," his plan continued, "shoot German crew, dump overboard, bring boat back to English port." For the "word-perfect German speaker" among the English fliers, Fleming nominated himself.

  The scheme almost came off. Fleming secured a plane from the Air Ministry, recruited "a tough crew" and settled in Dover to await his chance. The stumbling block was that neither reconnaissance flights nor radio monitoring turned up a suitable boat to be raided. The mission had to be aborted, much to the disappointment of all concerned.

  BP did hatch an ingenious and successful stratagem for securing usable cribs. The codebreakers had discovered an interesting discrepancy in the Germans' use of naval codes when they dealt with mines sown by the British. German minesweepers were not equipped with Enigmas but relied instead on a manual code called the Dockyard Key, which BP had broken. When the British created new minefields, German warships had to be alerted, and they were equipped to handle Enigma traffic, not Dockyard transmissions. Oftentimes, when the RAF dropped mines, exactly the same message would be sent out in both ciphers. The new Bletchley scheme, appropriately called "gardening," was to have the Air Ministry sow a new minefield so that BP could reap the sudden sprouting of Dockyard and Enigma alarm messages. From the Dockyard decrypts, Hut 8 derived substantial cribs to be used in unlocking the naval Enigma. These cribs from two identical messages transmitted on different cryptographic systems were called "kisses."

  In March 1941, Turing's team received another gift of captured materials. Once again off Norway, a trawler was disabled and driven ashore. This time the boarding crew seized the Enigma tables for the whole month of February. The resulting trove of decrypts was much larger. Moreover, Turing and his Hut 8 associates were learning enough about the naval Enigma to keep on reading the navy signals through the month of May. Then a new set of keys stymied them, and the flow of decrypts once more dried up.

  None of this intelligence was of much use to the Admiralty. Messages that revealed important information were deciphered only long after it could have helped. However, the new batch of decrypts did yield one disclosure of supreme value. Young Harry Hinsley, studying the traffic, observed that some of the messages had been sent to small ships the Germans had stationed out in the North Atlantic to transmit weather reports back to headquarters. Although the weather trawlers were not enciphering their reports on Enigmas, they had to have the machines if they were to decode the Enigma messages transmitted to them. And since each ship was out there for stints of several months, they must have aboard a succession of naval Enigma tables.

  Certainly, Hinsley told the Admiralty, if one of the weather ships was attacked, the crew would quickly jettison their Enigma and the current key tables. But wasn't it probable that future keys would be stored in a safe? And wasn't it also likely that in the stress of being attacked, the crew would forget about those hidden-away tables and they could be recovered by a boarding party?

  The Royal Navy cooperated. A carefully planned foray on May 7, 1941, carried out by a flotilla of seven cruisers and destroyers, captured the trawler München east of Iceland. Sure enough, the weather boat crew, before surrendering, placed their Enigma and secret tables in a bag and threw it overboard. But also, as Hinsley had predicted, in the captain's quarters British sailors found the Enigma settings for the month of June.

  Two days later, Bletchley analysts were handed an unexpected gift. A North Atlantic convoy escort group under Commander Joe Baker-Cresswell forced the German submarine U-110 to the surface. The U-boat skipper, certain that the sub was sinking, ordered his crew to abandon ship. But she didn't sink. Baker-Cresswell saw his opportunity. Remembering from his naval training how in World War I the capture of a German warship had enabled the British to seize codebooks and break the German navy's codes, he launched a similar mission aboard the U-110. He had the sub's survivors picked up and stowed out of sight belowdecks before his own crewmen boarded the U-boat and removed two boatloads of useful papers as well as the sub's Enigma.

  The documents retrieved from the München and the U-110 gave Hut 8 a heartening, if temporary, breakthrough. Before, analysts had taken days to obtain Enigma decrypts. Now, with the June keys in hand, they lowered their decryption time to an average of six hours. The information they passed on to the sub trackers was current enough to locate U-boat wolf packs and deflect convoys to safer routes.

  However, this heady time soon ended. The bigram tables they had so painstakingly compiled were replaced in mid-June, after BP had been reading Enigma traffic for only two weeks. Turing figured he needed a month of decrypts to work out the new tables. Could Hinsley persuade the navy to capture another weather ship?

  He could. The navy would cooperate in tracking down the Lauenberg, which Hinsley selected as the most promising target. She had left her Norwegian base at the end of May to take over the North Atlantic patrol from a sister ship. Hinsley figured that she would be out there for at least a six week tour of duty and would consequently be carrying the Enigma keys for both June and July.

  This time when the Royal Navy task force steamed off to find the weather ship, GC&CS staffer Allon Bacon went along. The operation came off smoothly. Informed about the little trawler's location by BP decrypts, the British flotilla found her behind an iceberg. The British fired shells that came close but deliberately avoided hitting the trawler, forced her crew to take to lifeboats, took the Germans aboard and sequestered them so that they could not see what the boarding party was doing. The Germans had ditched their Enigma and tried, only partially successfully, to burn the confidential papers in the ship's stove. The boarders presented Bacon with thirteen mail sacks of documents. Sorting through them, he found just what he hoped for: the Enigma settings till the end of July.

  Once again, Hut 8 needed only a few hours in which to decipher the German navy's Enigma traffic. Of much larger significance, the learning curve had been completed. Turing's team could now proceed on its own, without need for further captures.

  An additional benefit came from the U-110 materials. Besides the regular German naval Enigma, Donitz used an "Offizier," or "Officer," code for higher-level communications. It was doubly enciphered, first in the special code and then again in the regular Enigma cipher. The U-110 papers included the special Offizier settings for the month of June 1941 and so allowed Offizier messages to be broken for that month. But when the settings were replaced, BP was again shut out—that is, until Rolf Noskwith, a German Jew who had emigrated to England with his parents in 1932, studied the earlier decrypts and hit upon a crib that he used to crack open the code. Offizier was broken for the remainder of the war.

  For the last half of 1941, and until February '42, the cryptologic battle of the Atlantic swung sharply in favor of the British. While B-Dienst was still breaking the Admiralty's codes, Bletchley Park had an ally whose contributions more than made up the difference. This inadvertent aide was Admiral Donitz himself. Early in the war the German admiral had decided, as he wrote later, that "I myself could quite easily direct the whole tactical operation against a convoy from my headquarters ashore." The result was a flood of communications back and forth as he instructed the U-boats where he wanted them to be and what he wanted them to do, while also expecting that they transmit regular situation reports back
to him. The masses of advice BP was teleprinting to the Submarine Tracking Room far exceeded in value whatever B-Dienst was disclosing.

  The statistics of shipping losses reflected the change in cryptographic fortunes. From April to June 1941, another 150 Allied ships were sunk. The July-to-September period showed a decrease to ninety, and from October to December the score declined to seventy. Factors other than the code-breaking helped account for the decline. The support by convoy escort vessels was strengthened, especially by the deployment of fifty World War I destroyers that the U.S. had taken out of mothballs and turned over to the British in a nominal exchange for ninety-nine-year leases on British bases in the Americas. Longer-range planes gave the convoys air cover for larger stretches of their journeys. Technological developments such as shipborne radar improved the convoys' chances. But the codebreakers had the most impact on the change. Their decrypts enabled the convoys to be rerouted, often in midcourse, to swing clear of the wolf packs. By pinpointing the U-boats' locations, BP decrypts caused more of them to be sunk. From a seemingly hopeless struggle, the Battle of the Atlantic, for this sunny period, swung in favor of the Allies.

  BP Decrypts Help Sink Surface Raiders

  While the U-boats posed the most serious threat to Allied shipping, they were by no means alone. The marvel warships the Germans had begun building in the 1930s stood ready to be released as hunters and raiders. The Sigint forces at Bletchley Park paid special attention to them because they were new and fast and, therefore, especially dangerous.

  The first major engagement with the raiders came too soon for BP to have much effect. In December 1939 the pocket battleship Admiral Graf Spee was set loose in the South Atlantic, sinking three British merchant ships in five days. She was tracked down by three Royal Navy cruisers, whose guns scored some fifty hits, forcing the raider to take refuge in the harbor of Montevideo, Uruguay. Seeing no possibility of escape, the ship's captain ordered her to be scuttled. He then shot himself.

  In the aftermath, though, British intelligence did score a notable success. Accompanying the Graf Spee was a supply ship, the Altmark. Before scuttling the battleship, the German captain ordered that some three hundred captured British merchant seamen be put aboard the Altmark, which was to be dispatched to Europe. British and French agents tracked her to Norway, where a Royal Navy destroyer intercepted her and liberated her prisoners before sinking her.

  Admiral Erich Raeder, the German navy's commander in chief, next planned for his surface raiders to attack Atlantic shipping. He would unleash his newest battleship, the Bismarck, accompanied by the heavy cruiser Prinz Eugen. They would be joined by the battle cruisers Scharnhorst and Gneisenau. His plans were partially frustrated when Scharnhorst needed to stay behind for engine repairs and Gneisenau was severely damaged when a gallant Coastal Command torpedo plane pilot got off his missiles before being shot down. These setbacks did not deter Raeder from sending out the Bismarck and Prinz Eugen.

  Since naval Enigma messages were then being broken only days and weeks after being intercepted, they were usually too late to be of use. However, an April message read in May did disclose that the Bismarck was to be loosed as a raider. Aerial recon found that she had first moved to the Norwegian port at Bergen and, subsequently, that she had left Bergen.

  A British naval squadron located the Bismarck and engaged her, but she shot her way out, sinking the battleship Hood and damaging the Prince of Wales. She herself suffered a bomb blast that caused her to leak oil. In foul weather, the Royal Navy lost contact. When the German warships separated, British attention focused on the Bismarck. Where would she be headed? After escaping past the north of Britain toward Iceland, would she double back to Norway? Or keep on going in the Atlantic? The British could only guess.

  Harry Hinsley weighed in with his opinion. He lacked decrypts for an answer, but he could see from direction-finding reports that messages being sent to the Bismarck, previously originating in Wilhelmshaven, were now being sent from Paris—a sure sign, to him, that the ship was bound for a French haven.

  Admiralty authorities shook him off. His information was too tenuous to be relied on. August minds at the top were convinced that a more northerly route was most likely.

  BP provided the clincher. Normally Gordon Welchman's breaking of the Luftwaffe's Red code would not have supplied any useful naval information, but this time it so happened that a Luftwaffe general in Athens had a close relative on the Bismarck, and he inquired as to the ship's destination. The word came back and was promptly decoded in Hut 6: the Bismarck was bound in a southeasterly direction for the French port of Brest.

  She never made it. Even though one British flotilla misinterpreted the signals that intelligence transmitted to it and headed off in the wrong direction, the battleship Ark Royal, hurrying up from Gibraltar, found her. One of its aircraft torpedoed her, jamming her rudder so she could only travel in circles. Her captain ordered her to be scuttled.

  As with the Graf Spee, supply ships had been dispatched to the Atlantic, both to serve the needs of the Bismarck and to aid the U-boats. By early June, Hut 8 had begun reading naval Enigma messages almost currently, and decrypts revealed the positions of eight supply ships. To avoid arousing the Germans' suspicions that the codes were being broken, it was decided that only six of the eight ships would be destroyed. As luck would have it, though, other Royal Navy ships, without advice from BP, happened upon the remaining two and sank them as well. Concern about Ultra was soon eased. The Germans continued to blame the sinkings on superior British direction finding or French agents or some other cause rather than accept that the Enigma could be compromised.

  The Scharnhorst and the Tirpitz, the last two of Germany's battle cruisers, were hiding out in Norwegian harbors, ready to sail out and ravage convoys headed for the USSR. At Christmastime in 1943 the Germans decided to unleash Scharnhorst for an attack on convoy JW55B. Donitz, succeeding Raeder as commander in chief, was sensitive to the plight of German soldiers now being battered on the Russian front and wished desperately to stanch the flow of supplies to the Soviets. He believed he could commit the precious battleship because he had lulled Britain into more relaxed convoy protection by allowing recent sailings to pass through unmolested. Bletchley decrypts, however, warned of his plans. Although key German messages relating to the Scharnhorst were in the Offizier code, whose doubled encipherment slowed decryption, and were of little tactical value, BP did inform the escort fleet when the Scharnhorst put to sea. Further, the decrypts guided the British warships in placing themselves to intercept the German battleship when she did try to attack the convoy. In the early hours of December 26, a star shell fired from a British cruiser lit up the Scharnhorst instants before a salvo from the fleet's battleship, HMS Duke of York, slammed into her. Left burning, she was finished off by torpedoes from swarming destroyers. Of her crew of more than two thousand, only thirty-six men were rescued from the Arctic waters.

  The final chapter of warfare against Germany's surface raiders was not written until November 1944. This was the struggle against the Tirpitz. Fear of this capital ship had caused the British to attempt repeatedly to put her out of commission. In March 1941 a commando raid against Saint-Nazaire succeeded in seriously damaging the only dry dock on the Atlantic coast capable of handling repairs of the massive battleship. In October 1942 the Admiralty sent across the North Sea a Norwegian naval officer in a fishing boat that had two manned torpedoes slung beneath it. The idea was to enter the fjord where the Tirpitz was berthed and aim the torpedoes at the ship. The scheme went awry when the boat encountered a squall and lost both the torpedoes. In September 1943 a fleet of six midget submarines was towed into position to glide in close to the Tirpitz and her escort vessels and detonate explosives beneath their keels. Three of the tiny subs made it through and, as Enigma decrypts later confirmed, made hits on the Tirpitz, so badly damaging her that the Germans did not expect to have her ready for action until mid-March of 1944.

  When, repair
ed, the ship again became a danger, she was attacked and once more damaged, this time by carrier aircraft. A second carrier-planned sortie in July, meant to finish her off, met with increased antiaircraft fire and made no hits. But on September 15, 1944, RAF bombers operating from north Russia again put her out of action.

  With Ultra decrypts carefully monitoring every stage of the big ship's latest repairs, the finale came when Tirpitz was once more reported ready for raiding. On the twelfth of November, thirty-two British bombers, each carrying a single twelve-thousand-pound bomb, took off from Scotland. At least two of the bombs hit the Tirpitz and capsized her. German sailors, hopelessly trapped within the inverted hull, were heard singing the German national anthem, "Deutschland Uber Alles," to their last breaths. "What a tragedy," one observer commented, "that men like that had to serve the Nazi cause."

  Removal of this last threat of the surface raiders enabled the Admiralty to dispatch warships to the Pacific for the war against Japan.

  Bletchley Copes with Shark

  In February 1942, when everything had been going so well, Hut 8's intelligence feast abruptly ended. Up until then, both the surface ships and the U-boats had used a common cipher, called Dolphin by GC&CS. Now the U-boat command gained its own cipher, which the British dubbed Shark. It was not only a new code; it involved a change in the design of the U-boats' Enigma machines. This was a thinned reflector that allowed a fourth rotor to be added in its slot. Applied to the machines Donitz used in his communications with his Atlantic and Mediterranean U-boats, Shark plunged Bletchley into a ten-month blackout. During those long months, Hut 8 penetrated the new cipher on only three days—and each of these times only because of a German error in sending a message in both the Dolphin and Shark ciphers. Banburismus no longer worked. Turing's EINS catalogs were of no avail. The Submarine Tracking Room reported, "Little can be said with any confidence in estimating the present and future movement of the U-boats."