Read Seizing the Enigma Online
Authors: David Kahn
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:
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.”
The orders for the destruction of cryptomaterial were clear and simple; the crews had been taught them, and in all previous cases the men had fulfilled their duty to guarantee security. The fundamental documents were in water-soluble ink and were kept separated: the indicators with the radiomen in the radio shack, the key lists with the radio officer elsewhere. A capture of one or the other would not suffice for solution; both would have to be seized, and this would be possible only if a British warship had, unnoticed, come alongside the German ship and a boarding party had surreptitiously entered her. Aside from the basic improbability of this scenario, Fricke maintained, his survey of the circumstances of the sinking of each German vessel showed that it seems not to have happened. He decided, without stating his ground, that “seizure of cryptomaterial is unlikely.” He made the same determination in every case, either because—as with the tanker
Belchen
—the ship had been sunk by gunfire and the papers and machine went down with her, or because—as with the
Gonzenheim
—the crew had had enough time to destroy the material, or because—as with the
Esso Hamburg
—surprise entry was excluded.
What about pure cryptanalysis, unaided by booty? “After a renewed, very comprehensive examination,” Fricke reported, “all specialists unanimously agreed that a reading [of German navy messages
by the enemy through solution] is impossible.” In the end, Fricke found “no palpable, unequivocal” cause of the roundup, though he tended to favor coincidence as the answer.
Despite his failure to reach a definite conclusion, the navy instituted a number of measures to restore or improve security. All orders were to be printed and charts marked in water-soluble ink. All radio messages pertaining to operations were to be top secret. Because documents recovered from the sea might have enabled the British to read German cryptograms for the period that the same keys remained in service, a new cue word—
PERSEUS
—put new and uncompromised keys into effect on June 22. In August Dönitz began addressing his submarines by the names of their captains instead of by their boat numbers.
The method of defining meeting points by latitude and longitude in the new Short Signal Book was regarded as compromised, so Dönitz sought to disguise their positions on the
Kriegsmarine
’s grid of the oceans.
This grid was divided into quadrants 486 nautical miles on a side. Each was designated by a two-letter group; quadrants were designated from west to east in alphabetical order. Thus CA covered the East Coast of the United States from about Portsmouth, New Hampshire, south to Cape Fear, North Carolina, and from inland (though this was useless) to about the longitude of northeasternmost Maine; CB, adjoining it to the east, covered the Atlantic south of Nova Scotia; CC, CD, CE, and CF moved east across the ocean, putting CG on the coasts of Portugal, Spain, and North Africa, including the Strait of Gibraltar. South of the C row came the D row. Some quadrants were slightly irregular.
Each quadrant was divided into a nine-by-nine matrix of eighty-one smaller squares. Each of these was in turn divided into nine squares, and finally these again into nine. The squares of the first subdivision, represented by two digits, extended 54 nautical miles on a side; those of the fourth and final subdivision, represented by four digits, were only 6 miles on a side. A vessel could thus give its position
with precision using two letters and four digits. For example AK 2799 marked the watery square at 57° 21′ north latitude, 32° 00′ west longitude, a spot in the middle of the North Atlantic where a U-boat might well be in wait for a convoy. The system saved enciphering and transmitting time.
Since the grid served the entire German navy, and since some high headquarters held the Home Waters U-boat Enigma key, these posts could follow the movements of the submarines. Dönitz feared that this situation endangered security. So he took yet another step to protect his U-boats: in addition to forbidding all but a few units to map U-boat locations, he ordered that these locations be disguised by replacing the grid digraphs for the North Atlantic with substitutes, known only to the U-boats and their commands, from Table B of the digraph substitution booklet
FLUSS
(“river”), also used to encipher indicator groups for the naval Enigma. The table consisted of a 26-by-26 square of letter pairs with single letters at the head of each column and row. This was modified for the grid encipherment. Atop each of the 26 columns the cipher clerk wrote one of the 26 most-used grid digraphs (omitting those for the Pacific, for example) in a sequence specified by U-Boat Command. The encipherer replaced the grid digraph with any one of the 26 digraphs under it. Thus grid digraph AL might become cipher KS, or LK, or OM, or any one of 23 other digraphs.
Dönitz enciphered these instructions in officer-grade Enigma and radioed them in six parts totaling 504 four-letter groups on September 10, 1941, to all U-boats. They went into effect immediately. And at the end of November the navy complicated the location system still more. Dönitz instituted the use of not one but many digraph tables, indicating each by a name and a street address, such as “Gottfried Becker, Bluecherplatz 30,” which came into force at midnight, December 28. The number in the address was the key to the disguise of the four-digit location number. The 30, for example, meant that 3000 was to be added to the true number. Thus the grid
digits 6268 would be enciphered as 9268 for transmission. Errors seemed to be rare, though one submarine was told with some asperity that according to its grid letters it was transmitting from the middle of the Andes.
In between these changes, on October 5, the high command segregated U-Boat Command communications from other users of the Home Waters key by modifying the settings. This new key net was called
TRITON
, after a sea demigod famed for sounding a conch shell as a horn. Not only would fewer people be able to read U-boat messages, but fewer messages would be sent in the basic daily key. This would reduce the chances of overlaps leading to superimposition solutions and of errors that would permit special-case solutions.
Four months after Fricke’s investigation, however, a series of events led to another probe. One was an Admiralty announcement that the British had captured a U-boat all but undamaged at the end of August 1941. This was the U-570, which had unluckily surfaced south of Iceland at the very moment that a British patrol plane was passing overhead. The aircraft dropped four depth charges, which straddled the submarine, shook her severely, smashed many of her instruments, let in sea water, and persuaded the captain that the fight was over. When his men tried to climb out of the conning tower, the plane, to prevent them from manning guns, opened fire. The crew showed a white flag, and the airplane circled watchfully. Eventually a British destroyer arrived and took the U-boat in tow.
On October 18, Maertens opened his analysis of the security consequences of the capture by saying that “a current reading of our messages is not possible.” On the next page, however, he conceded that if the enemy had found the Enigma undisturbed and all the key documents, a current reading was possible. But then he concluded that this was unlikely—that there was time to drench the documents, making their water-soluble ink unreadable—and in the end he left the impression that the British were not solving Enigma messages. Even
if they were, the new keys that were to go into effect on November 1 would restore full security.