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

by Kahn, David

  The convoy was scheduled to head east to aim once again for Britain after its diversion, and the U.S. Navy radioed Task Unit 4.1.7 to recommend that “convoy proceed at utmost speed during next 48 hours.” As the convoy turned northeast to pass north of the sub-infested area, it raised its speed to 10 knots. Over the next two uneventful days, it made a succession of turns to starboard until, on Friday, it was steaming due east. On Saturday, a few hours after a fog started to lift, the American escort met the British at the MOMP south of Iceland. The Americans departed for home. HX 155, having dodged the U-boats, continued on toward the British Isles. On November 3, the Admiralty messaged: “HX 155 met as arranged. All ships now arrived. No stragglers.” The grain in the Margarita Chandris, the sugar in the Coulberg, the fuel oil in the British Chemist, the steel and copper and gasoline and tobacco had reached the island kingdom—thanks in part to the back-room boys, the boffins, of Hut 8.

  16

  WHEN SAILORS LOOK FOR LEAKS

  COMMUNICATIONS WERE ESSENTIAL TO DÖNITZ’S CONTROL OF wolfpack warfare, and they ran smoothly. But those communications had to be secret, and this worried him.

  From 1940 to 1943, his headquarters lay in three summer villas in Kerneval, a suburb of the port of Lorient on the southern coast of Brittany. Communications, under the effective Commander Hans Meckel, were centered in a bombproof bunker where the air always seemed bad.

  Sending a message to a U-boat at sea began with Dönitz or a subordinate writing it out. A watch officer then took the message to a command transmissions officer, who time-stamped it and gave it to the radio watch officer for encipherment and transmission. The chief radio watch officer for much of the war was Lieutenant Helmut Kühne, who worked in the bunker, along with fifteen to twenty radiomen per shift.

  Kühne handed the message to a radioman for enciphering on an Enigma. Radiomen handled both transmission and reception as well as ciphering, but Kühne, when he could, put those with better Morse touch and hearing on radios and those who typed better on Enigmas. Since only officers could prepare the machine’s inner settings, which remained in force for two days, the radio officer on watch set all but one of the several Enigma machines with the new inner key before every other midnight; later in the war, this was done before every other noon. One Enigma was left set with the previous day’s key to decipher late-arriving messages.

  After the officer was finished, an enlisted radioman plugged in the jacks of the plugboard. The machine was thus ready for the radioman handling the message to establish the message key. When he had completed that intricate process, he turned the rotors to the key. As he finally did the easy part—pressing the letters of the plaintext on the keyboard—a colleague wrote on the message form the ciphertext letters as they appeared on the illuminable panel. The message was divided into four-letter groups. The two four-letter indicator groups at the beginning were copied at the end. The message was given a date-time number, a number indicating how many four-letter groups it had (excluding the repeated pair at the end), and a serial number.

  But before the ciphertext was transmitted, it was given to another radioman, who, using only the indicators that it carried, determined the message key and deciphered the cryptogram as the radioman on a U-boat at sea would do. If he could not decipher it, the error was sought and corrected. Only when the cryptogram was successfully deciphered was it ready to be sent.

  The radioman determined the operating area of the U-boat and whether it was attacking a convoy, which told him the traffic network, or circuit, to transmit on. Eventually U-boat Command had several geographical nets—Amerika A (northern area), Amerika B (southern area), Afrika 1 and 2, Ireland, and others—and two nets for submarines attacking convoys. One, named for the goddess of the hunt, was called DIANA; the other, named for the patron saint of hunters, was called HUBERTUS. Each geographical net had three radio frequencies, chosen to give the best reception in its area in the daytime, at night, and at twilight. HUBERTUS had six frequencies, among them 4601 kilocycles from midnight to 7 A.M., 7645 from 9 to 11, and 12950 from 1 P.M. to 8 P.M. Sometimes a wolfpack was given its own frequency.

  The radioman in Kerneval chose the proper frequency, tuned his radio to it, and tapped out his dots and dashes. From the bunker, the pulses coursed by wire to radio towers in Lorient, whence they were pumped into the ether. They were received not only by U-boats but by the station of the then-defunct French Colonial Office in Sainte Assise, a few miles southeast of Paris. This station repeated the cryptogram immediately with greater power to give the U-boats a second and better chance at hearing it. It also repeated the message two, six, twelve, and twenty-four hours later and, if very important, forty-eight hours later. On an average day, U-Boat Command sent some twenty to thirty messages this way. Each one took some fifteen to thirty minutes to transmit from the time the command transmissions officer got it.

  Aboard each U-boat, one radioman was always standing watch, listening through his earphones for the ethereal chirping that linked him with home. Radio transmissions were received when the vessel was on the surface, nearly always when she was at periscope depth, and sometimes even when she was 40 feet down. The radioman wrote down all messages, whether or not they were addressed to his boat. The serial numbers told him whether he had missed any. Some radiomen missed no messages; some missed sixty on a cruise. The skipper determined whether missed messages meant that the radioman was negligent, in which case he would be arrested, or whether the messages were lost because the boat was frequently submerged.

  When the radioman heard the peeps, he stored the first few letters in his memory as he put down his book or turned down the phonograph, then wrote them on a message form and began taking the rest of the text as it was transmitted. He replicated the complicated procedure that yielded the message key, turned his rotors to it, and deciphered the message. He then gave it to the skipper. If the message was addressed to his boat, the captain followed its orders; if it was not, he read it to know what was going on.

  The captain drafted messages; his radiomen enciphered them and transmitted them to headquarters. U-boats used five main kinds of messages:

  1. Convoy contact reports. These were usually encoded first in the Short Signal Book and then enciphered in Enigma. They were preceded by dot dot dash dot dot to clear the wavelength of less urgent signals; because that Morse symbol stood for the Greek letter alpha in German signals and a French accented e in the British, these messages were called alpha signals by the Germans and E-bar by the British.

  2. Short transmissions of one to six groups, likewise encoded first in the Short Signal Book and then enciphered in Enigma. These reported, for example, a U-boat’s position and fuel situation in answer to a query from headquarters. They were preceded by dash dot dot dot dash, beta in German Morse, called B-bar by the British because it added to the Morse symbol for b the dash for t.

  3. Weather reports, encoded first in the Short Weather Cipher and then enciphered in Enigma.

  4. Other messages, such as those dealing with rendezvous with refueler submarines, enciphered in Enigma.

  5. The radio cipher conversation, in which headquarters and a U-boat established a key and used it for an exchange of messages, each picking up the encipherment from where the other left off. It was faster than setting up a new key for each party’s successive messages.

  Kerneval rebroadcast the U-boat messages it received both to acknowledge their receipt and to make sure that other boats heard them. Kühne’s group deciphered the messages. But since the radiomen aboard ship did not always have the time or the desire to check their work by deciphering it before sending a message, and because their messages were harder to hear, unreadable cryptograms were more common in messages from U-boats than in those to them. To unscramble them, Kühne had a “puzzle group” whose members, he thought, were all but codebreakers.

  * * *

  The Enigma was, the navy said, “the main cipher method of the Kriegsmarine.” All secret communications were t
o be enciphered with it, “except when the use of other cipher methods and systems is prescribed.” It was considered superior to them, in speed as well as in strength. As a 1941 field report said, “Equipping with the Enigma all vessels that must send reports about the enemy quickly is viewed as absolutely necessary, since all other systems are too cumbersome.” Moreover, many subordinate systems depended upon the Enigma for security, among them the Short Signal Book and the Short Weather Cipher.

  Because Enigma protected the central element—control—of the central part—the U-boat offensive—of the naval war, the navy sought in a variety of ways to ensure its security. Deciphering ciphertexts to make sure they were correct before transmitting them, for example, reduced the number of wrongly enciphered messages that had to be redone correctly, thus cutting the number of different cryptograms with identical plaintexts—a chink in a cipher’s armor.

  The navy monitored its own communications. When one of the monitoring posts spotted an error that could jeopardize cryptosecurity, even though it did not know whether the enemy had actually exploited it, the post notified the commands involved. In the spring of 1941, boat NS 25 off the Norwegian coast requested a weather report with the standard service abbreviation QOB. Because the enemy would know that the reply was a weather report and thus would have a crib, the monitoring post reported that this “made an answer impossible” and was “a serious violation.” The culprit, Radioman Fourth Class Wilhelm Lemcke, was to be told of his error, restricted to quarters for three days, and sent to Stavanger in occupied Norway for additional instruction. The commands themselves sometimes blundered. At 8:29 P.M. on April 26, the headquarters at Stavanger radioed an Enigma message to the Admiral West Coast and to the ship Seefalke. The Admiral West Coast knew that the Seefalke had only the Dockyard Cipher, so, after deciphering the Enigma message, his headquarters reenciphered the text in the simpler Dockyard and at 8:56 transmitted it. This violated cryptographic security since solving the easier system would provide a kiss, as Bletchley called it, to the more difficult one.

  The navy also minimized Enigma’s physical exposure. In many cases, vessels undertaking operations that made them particularly susceptible to capture by the enemy, such as operating in shallow waters off the enemy coast, carried as few documents as possible. Thus when, on October 14, 1939, Lieutenant Gunther Prien’s U-47 slipped between concrete-laden blockships into Scapa Flow to sink the battleship Royal Oak in one of the most daring escapades of the war, his sub carried no Enigma or Enigma keying documents, but only a hand cipher. And when, in December 1939, the pocket battleship Admiral Graf Spee sailed from Montevideo, Uruguay, to scuttle herself in shallow water before she could be sunk by three British cruisers awaiting her at the 3-mile limit, she left her Enigma with German diplomats on shore.

  When a ship was lost where the British might be able to recover cryptographic material, either by salvage or by picking up flotsam, the German navy demanded a report. For example, when Submarine Hunter 173 was sunk by bombs on May 8, 1941, off the Norwegian west coast in 150 feet of water, her command detailed the destruction of the cryptographic material. At the time of the attack the regulations for use of the Enigma, the Dockyard Cipher, and cipher message sheets were in use. Also aboard were an Enigma, the general- and officer-grade keys for April and May, and the instructions for cue word PERSEUS for emergency change of Enigma settings. But, the report said, a bomb detonated under the radio shack, collapsing it, setting it on fire, and destroying the documents lying out. Radioman Koch smashed the Enigma machine. No papers would float. The report clearly implied that there was no danger of compromise, and reports of other events suggested the same thing.

  Most important of all, the navy investigated situations in which the Enigma might have been compromised. Many of these probes were undertaken by Captain Ludwig Stummel, a career signals officer with a glass eye and a limp who was in effect chief of staff of the Naval Communications Service. He had joined the navy during World War I and later fervently supported Nazism, though his enthusiasm faded as the excesses of the regime increasingly offended his strong Catholicism. He hated sloppiness and required those under him to follow the book strictly. If a probe convinced him that corrective action was warranted, he ordered it.

  Several events early in 1940 impelled Stummel to make his first investigation. Patrol Boat 805 was lost under obscure circumstances in the Heligoland Bight, off the northwestern coast of Germany. The U-33 was sunk in February in the Clyde estuary, whose waters were shallow enough for divers to reach the submarine. Four days later, the Altmark, a supply ship for the Admiral Graf Spee that was carrying British prisoners of war, was cornered in the neutral waters of a Norwegian fjord and boarded by the British, who, shouting “The navy’s here!” freed the prisoners.

  Though he did not conclude that a leak had occurred in any of these cases, Stummel, as a precaution, changed the indicator for weather messages, fake messages, and officer-grade messages to the indicator for general-grade messages; the German radiomen would see upon deciphering them what they were, but the enemy intercepters would not be able to differentiate and sort them out and so take even the first step toward solution. After several weeks of study, to which his cipher specialists contributed, Stummel reported comforting conclusions:

  The many components of the Enigma system offered security even if some components were lost to the enemy.

  Water-soluble ink protected most important documents.

  Solution could be achieved only through superimposition. But the frequent changes of keys—more frequent, he boasted, than in the enemy’s systems—precluded this.

  In April, Stummel investigated the British sinking of eight German destroyers near the end of a Norwegian fjord and concluded that “it cannot be thought” that a compromise occurred. Then, on June 17, Dönitz telephoned. The rendezvous point of some Allied convoys, against which some of his U-boats had been directed by radio, had been moved. Could this be attributed to British recovery of cipher documents from the U-13?

  This time Stummel’s superior, Admiral Eberhard Maertens, the head of the Naval Communications Service, mollified Dönitz. The enemy’s reading of U-boat messages could be seen as likely only if all of four unlikely events had taken place: (1) the U-boat crew, threatened with capture, had not undone the Enigma machine’s keys by changing the rotors, ring setting, and plugboard, (2) the water-soluble ink had not made the key list illegible, (3) the enemy could detect the difference between the actual settings and those the key list called for—this a consequence of the changes called for by the cue word, and (4) the enemy could solve the German messages and pick out and determine the meaning of those that ordered the submarines to the convoy rendezvous. Maertens said that individually these events were unlikely and together even more so.

  The most serious fears about cipher security arose in 1941, triggering two major investigations.

  The first followed the sinking of the Bismarck. That battleship, together with the heavy cruiser Prinz Eugen, had been assigned in May to disrupt as much British shipping as possible. To resupply the two ships on their three months of raids, the Naval War Command had dispatched five tankers and two supply ships to prearranged points in the empty wastes of the Atlantic; two scout ships also sailed to discover targets and warn of enemy warships. These vessels were at sea when the Bismarck was sunk and the Prinz Eugen escaped.

  At the end of May, the British, who were reading Enigma messages with a delay of from two to three days, learned the support ships’ locations and attacked them. On May 29, the Admiralty ordered its cruisers to search for an enemy supply ship in the area 57° to 59° north, 46° to 48° west. They found the Belchen on June 3, at 59° north, 47° west, and sank it. The British intercepted a message to the Esso Hamburg, which had refueled the Prinz Eugen, setting up a rendezvous with a supply ship for U-boats, the Egerland, to which she was to give torpedoes. On the evening of June 4, the Royal Navy cruiser London sank the Esso Hamburg and then, the next morning and in
the same area, halfway between the bulges of Africa and South America, the Egerland. Other German ships were destroyed by British warships in other locations. The Admiralty had ordered that the tanker Gedania and the scout Gonzenheim be omitted from the plan of attack. It feared that if they were sunk, the Germans would conclude, correctly, that only codebreaking could have led to the loss of so many widely dispersed ships. But Royal Navy warships happened upon the Gedania and the Gonzenheim and, on June 4, destroyed both.

  Thus, by June 21, all five tankers, the two supply ships, and one of the scouts had been sunk or captured, five of them on three successive days.

  And what the British feared occurred. The losses, coming so close in time and so far apart in space, triggered German concerns about security. Admiral Kurt Fricke, chief of the Naval War Command, investigated the matter thoroughly. He advanced several theses to explain this improbable loss. One was coincidence: the ships could have been spotted accidentally, especially those in the busy area west of the Bay of Biscay. A spy could have betrayed the orders, though the frequent instructions radioed to the supply ships and tankers to go to new positions rendered this hypothesis a little thin. The British could have followed the ships’ movements through direction-finding of their many signals. French agents could have tapped the navy’s telephones. Perhaps British radar had a greater range than German. Or the enemy could have read the coded German messages.

  Fricke gave this thesis his greatest attention. He presupposed that, even with an Enigma machine and all the rotors, solution was not possible without either parallel plaintext and ciphertext or all of the daily keys and the indicators. Could the enemy have obtained these? Fricke ruled out spying: the documents looked alike and underwent “daily, more than daily, monthly, or more than monthly changes, so the entire system is extraordinarily difficult, even for a man who has been well instructed in these things.”