Seizing the Enigma (39 page)

At about this time Dönitz sensed a trend that affected his tactics in the most fundamental way. He had come to suspect that his wolf-packs were finding convoys less often than individual U-boats were. The inference was that the British were steering the convoys away from the packs. Moreover, the Germans were finding it more difficult to locate the British supply ships. None of this, Stummel implied in a response to Dönitz, meant that the British were reading weak German ciphers. It could all be explained by Britain’s full exploitation of her vast reconnaissance capabilities, including air reconnaissance of the supply route and of U-boat departure and arrival routes; possibly by airborne radar spotting that U-boats could not detect; and by extraordinary British direction-finding. In addition, a spy, a chatterbox, or laxness in locking up maps or cryptomaterial or other documents could not be ruled out. What could be ruled out was

a current reading of our messages by the enemy. … Without any contradiction, all specialists, in particular those of the B-Dienst headquarters and of the most important specialists of the High Command of the Armed Forces, have determined, in comprehensive work, that the Enigma system is viewed as by far the most resistant of all known methods for secrecy in military communications.

Stummel may have consulted Captain Henno Lucan, the signals officer who in 1930 had proposed some useful improvements to the Enigma. The specialists in the High Command of the Armed Forces worked in the
Chiffrierabteilung
, or Cipher Branch, the descendant of the ChiStelle whose staff had, in the 1920s, adopted the Enigma for army use. The Cipher Branch’s Desk IVa tested German cryptosystems. Dr. Karl Stein, the desk’s head, a professor of mathematics, analyzed the Enigma theoretically, calculating limits of security and thereby complementing the pragmatic investigations of the B-Dienst
cryptanalysts. Maertens summarized Stummel’s report by saying, in his covering letter to Dönitz, that “despite great stresses, including, among others, losses, the resistance of the most important cryptosystems seems not to have been impaired.”

The German cryptologists were not fools. Experience had taught them that cipher systems were usually broken because of laziness or errors on the part of cipher clerks or the capture of documents in wartime; they had not forgotten the
Magdeburg.
The system they had designed blocked both these avenues and at the same time rendered pure cryptanalysis all but impossible, they believed.

To prevent cipher clerks from choosing rotor settings that might lead to overlaps in the machine’s cipher-alphabet sequence, which might permit solution, they prescribed the settings for a key net. And to nullify capture, they set up the system so that in most cases the capture of even three of the four elements—the machine, the machine-settings list, the indicators list, and the bigram tables—would still (they thought) preclude solution. The machine was assumed to be in enemy hands. But even if the British captured the machine-settings list and the indicators list, the cryptologists said, they would not be able to divine the indicator for a particular message because they did not have the bigram table that would link the unenciphered indicator to the enciphered one that was transmitted. Likewise, if the British had the indicators list and the bigram table, they would not know which rotors had been used in which order, nor their starting positions, nor the plugboard arrangement. Only if they seized the machine-settings list and the bigram table could they reconstruct the indicator and thus be able to read a message. But for this eventuality the Germans had devised the cue-word system. By immediately changing the rotor order and settings, the cue word rendered the captured machine-settings list useless. In addition, the Germans changed the bigram tables from time to time, and new machine-setting lists were issued each month. All these safeguards pretty much eliminated any dangers from captures and from cipherers’ errors, they thought.

The cryptologists believed that trying to solve cryptograms on the basis of letter frequency was laughable: the Enigma generated far too many alphabets, and messages were kept too short, for this procedure to have any hope of working. The more promising method of the probable word, in which a presumed text was matched against a cryptogram, would founder on the vast number of possibilities that had to be tested, and the plugboard would make such a match even more difficult. Moreover, many messages had codewords from the Short Signal Book as their plaintext. And while solving one message would reveal the rotor order of other messages, it would not disclose rotor starting positions.

All in all, the German cryptologists had looked at the situation from both the theoretical and the practical points of view. They had evolved a system that apparently assured nearly perfect security for their messages, in which even a capture would give the enemy insight into German messages only for the brief and limited period before either new keys came into service or a cue word in effect created new keys immediately. Dönitz’s directives to his U-boats, they assured him, were safe.

Nevertheless, the cryptologists did not rest on their laurels. They sought to further secure the system by facilitating the work of the encipherers and thus reducing human error. One measure sought to reduce the pressure—around 5 pounds—needed to depress each of the typewriter keys. The basic resistance came from the rubbing of the rotors against one another. Though this could not be cut down because it would have entailed too extensive changes in the machine’s construction, other changes did lessen the pressure to about 4 pounds, which led to “a palpable lightening of operation.” In another change, a larger illuminable panel was attached to make the letters more easily visible, particularly by the man who was writing them down. A third measure was to print the output, thereby eliminating this second man and his errors. The navy’s first effort, a two-typewriter
device called the MS, which weighed more than 100 pounds and cost 5,000 reichsmarks ($12,000 in 1991 dollars), failed. The manufacturers then sought to connect the Enigma to electric typewriters or punched telewriter tape in a succession of “partial solutions” called the MZSB, MZSE, and MZSS devices. But metal shortages as the war went on had led the manufacturers to use plastic instead of metal in some parts, notably the indented thumbwheels for the hand turning of the rotors. This substitution required an increase in tolerances. And despite the Enigma’s heavy construction and simple mechanics, the machine turned out to be extraordinarily sensitive to inaccuracies in manufacture. With an accidental accumulation of variations, the machine with a typewriter-printing attachment became unreliable. As a consequence, though some 700 machines were fitted with the tape printer, none of these “partial solutions” was deemed satisfactory, and the manufacturers, which now included a firm called Konski & Krüger and the Olympia typewriter company, took over the development. Delivery of their printing versions was to start in the fall of 1944.

In addition, the
Kriegsmarine
sought to reduce the number of machine breakdowns and to meet expanded communications demands by producing more Enigmas. With the original firm, Heimsoeth & Rinke, apparently producing at its maximum, the navy contracted with Olympia. On June 23, 1943, from its factory in Erfurt, southwest of Berlin, the typewriter firm delivered its first twenty basic Enigma machines. By December, it was delivering seventy-five a month.

Most important, the cryptologists also took steps to block the dangers from the constant increase in traffic. In 1939 radio messages averaged 192 a day; by 1942 volume had soared to six times that number, or 1,200 a day; in 1943 it was on its way to doubling again. The experts recognized that this volume gave enemy codebreakers more opportunities. As they put it in one case, “according to cryptanalytic knowledge the permitted limit of the daily total of radio messages had been overstepped.”

One way to lessen the danger was to reduce the number of messages enciphered with the same rotor and ring positions by creating additional key nets. (The participants in each net shared the machine-setting list that specified these positions for each day.)
TRITON
, the Atlantic U-boat net, was one of these new key nets. Others were created throughout 1942, such as
NEPTUN
, for the operations of the main fleet, and
MEDUSA
, for the Mediterranean. By January 1, 1943, the
Kriegsmarine
was utilizing eleven key nets. As traffic grew, it kept adding others.

Another way to reduce the dangers of high traffic volume was to add another rotor to the machine. The advantages of a fourth rotor, which had been in development since 1940, were that it would raise the number of possibilities a cryptanalyst would have to test and would lower the likelihood of key overlaps that a cryptanalyst could exploit. But two serious practical difficulties blocked the implementation of the idea. First, adding another rotor would change the dimensions of the machine, requiring redesign of and retooling for parts of the machine not otherwise involved and making it impossible, on many ships, for the machine to fit into the space designed for the smaller version. Second, a fourth rotor would prevent communication with other branches of the service that used the three-rotor machine.

To get out of these difficulties, the navy came up with the idea of a thin rotor that could be fitted in next to a new, thinner reflector (the nonrevolving half-rotor that sent the current back through the three revolving rotors). The thin rotor would not revolve during encipherment because no stepping mechanism existed at the leftmost rotor. However, to create a key, the stationary thin rotor could be turned to any one of twenty-six positions. This multiplied by twenty-six the number of possible keys. And the fourth rotor could be wired so that in a certain position, it, with the new thin reflector, replicated the wiring of the old thick reflector, permitting communication with three-rotor machines. It took more than a year to resolve the problems of the extra rotor and to produce, test, and distribute the new
machines. Finally, however, on February 1, 1942, the new model, called M4, went into service on the U-boats’
TRITON
key net. This was the most significant event in German cryptography during World War II.

If the advent of the M4 had been followed by a decrease in diversions of Allied convoys and an increase in sinkings in the mid-Atlantic, Dönitz might have guessed that the Enigma had earlier been penetrated. But the decline in Britain’s cryptanalytic fortunes was concealed from him in part by American stupidities. The entry of the United States into the war against Germany on December 11, 1941, voided Hitler’s concerns about sinking American vessels, and Dönitz sent his U-boats to the rich hunting grounds off the East Coast. Here freighters and tankers, trawlers and barges, marched individually up and down the coast, disdaining the lessons of convoy so painfully learned by the British over two world wars. And they did so before a blaze of city lights, foolishly kept burning by chambers of commerce afraid of losing business during the tourist season. As a consequence, for six months the U-boats enjoyed what they called a “happy time,” sinking dozens of ships, sometimes within sight of crowds on shore, with barely a loss of their own. Finally, reason took over, convoying was introduced, and the U-boats quit the coast.

Their success kept Dönitz from seeing that the Allies had lost much of their U-boat intelligence. And later in the year, when the U-boats returned to the central Atlantic, he could attribute their increased sinkings, not to an Allied cryptanalytic failure, but to a German success: breaking Allied codes.

For the
Kriegsmarine
not only made codes, it sought to break them as well. The 900-man English-language section of the B-Dienst was headed by former radioman Wilhelm Tranow, who had been brought in to investigate the hermeticity of the Enigma. A tall, erect man, with firm features and a compelling way of talking, so bursting with
energy that he seemed to skip instead of walk, he was that rarity in a bureaucracy: a man who both performed the technical aspects of his job in exemplary fashion and administered the men under him effectively. It has been said of him that “If one man in German intelligence ever held the keys to victory in World War II, it was Wilhelm Tranow.” Tranow, who had cracked Royal Navy messages in World War I, achieved an important breakthrough in 1935.

He and his assistants had solved the Royal Navy’s most widely used code, the five-digit Naval Code. But they had had less success with the more tightly held and more important Naval Cypher, which despite its name was a code; it used four-digit codegroups that were superenciphered to provide an extra layer of secrecy. The superencipherment consisted of adding to the four-digit codegroups random-seeming numbers from tables of 5,000 number groups. In the fall of 1935, a British naval squadron patrolling the Red Sea to keep watch on the Italian invasion of Abyssinia superenciphered its Naval Code messages, using, however, the Naval Cypher number tables. Since Tranow had solved the Naval Code, he could easily determine the superencipherment. He could then strip it from the Naval Cypher. His knowledge of the names of the ships and their movements cracked the bared Naval Cypher. And these systems were still in use when the war began.

The B-Dienst of that time employed 500 persons, some of them in its sixteen intercept posts, some at its headquarters in the main navy building, the brown sandstone structure at Tirpitzufer 72–76 in Berlin. By April of 1940, the B-Dienst was reading a third to a half of the messages it intercepted in Naval Cypher, which it called
FRANKFURT
. On August 20, 1940, however, the British replaced the Naval Code and Naval Cypher with new editions of each, reducing B-Dienst successes against them. In June of 1941, Naval Cypher No. 3 came into use for communications between the Royal and the United States navies. Many of the messages concerned convoys, stragglers, and the U-boat situation.

To strip the superencipherment from a message—the first step in solving it—required a “bite” of two or more messages with overlapping superenciphering numbers. This phenomenon was not at all uncommon. Indeed, in any one hundred messages the chances were better than half that two would not merely overlap, but would start at the same point in the number tables. Tranow’s six tabulating machines prepared lists that discovered the overlaps. Like the British, the Germans exploited cribs. For example, the senior naval officer in Newfoundland radioed a comprehensive convoy report for the North Atlantic at the same time every day that invariably began, “SNO Halifax
BREAK GROUP
Telegram in [a number of] parts
FULL STOP
Situation. …” Hut 8 would have recognized the technique.