Codebreakers Victory (12 page)

To put British shipping out of harm's way, Cunningham canceled a southbound convoy that was due to leave the Greek port of Piraeus. He arranged that a convoy bound for Piraeus from Alexandria should reverse course at nightfall. A British cruiser force already at sea was ordered to be south of Crete at dawn on March 28. After ordering his fleet to get up steam, he himself carried out a neat piece of deception against the Japanese consul general.

He knew that the Japanese official regularly played an afternoon round of golf at Alexandria's Country club. On the afternoon of March 27, Cunningham made himself unmistakably visible with his clubs and an overnight bag. That night he slipped away to his flagship and led his battle squadron to sea. Sure enough, a BP decrypt confirmed that the Italians believed Cunningham's battleships and aircraft carrier had remained in Alexandria.

Instead, the British ships were in position waiting for the Italian fleet to sail into their trap. Mindful of the need for preserving the codebreakers' security, Cunningham sent an aerial patrol to spot the Italian ships—and to make sure they were
seen
spotting the Italian ships. Then he proceeded to dispatch the enemy. In the Battle of Matapan, three Italian cruisers and two destroyers were sunk, and the flagship
Vittoria Veneto
was heavily damaged. By contrast, the British lost only one torpedo plane. By so crippling the Italian navy, Cunningham eliminated the danger of attacks on British convoys between Egypt and Greece and gave Britain control of the eastern Mediterranean, proving crucial to the evacuations following defeats in Greece and Crete. What was left of the Italian fleet retreated to its base in Taranto and did not come out in force again until-two years later, when the ships sailed to surrender to Admiral Cunningham at Malta.

By their early breaks of the Enigma and their solving of Italian ciphers, Britain's codebreakers had delivered to their commanders a golden flow of information that led to the successes at Matapan and in North Africa. But these gleams of hope were soon quenched. The Italian defeats made up Hitler's mind to come to the aid of his Axis partner and to secure his south-em frontier. An entire corps of German planes was transferred from the Russian front to Italy and Sicily. Packs of U-boats with expertly trained crews shifted to the Mediterranean out of the North Atlantic. Erwin Rommel arrived in North Africa with disciplined, well-equipped troops to supplement and stiffen the backs of their Italian allies. It was a turn of events that was to prove in the period ahead a hard truth about secret intelligence: even the best of it cannot prevail unless there is sufficient force to exploit its revelations. In the sands of North Africa, that degree of force would be slow and long in coming.

 

 

 

5

 

Battle of the Atlantic: Cryptologic Seesaw

 

 

When the Germans went to war in 1939, their navy was almost completely unready to take on the Royal Navy of Britain. German naval officers had considered an outbreak of hostilities with the English an unlikely prospect and had planned accordingly. Their aim was to give Germany a small, balanced fleet that could play a useful subsidiary role in a war against another continental power—France, say, or the USSR. The few state-of-the-art battleships they had designed were meant as much
to be
showpieces of German technology and expressions of German pride as to serve as meaningful war machines. As for submarines, which had been so lethal a force in the Great War, the German navy had failed to build even as many as permitted in postwar armaments agreements, and the ones they did commission were small vessels with limited range.

Almost
completely unready. One important exception has already been noted: the Germans' cryptographic branch, B-Dienst, was reading a variety of the Royal Navy's codes. Britain's tradition-bound Admiralty had rejected Lord Louis Mountbatten's recommendation that it change over to code machines, as the RAF and army had done. Instead, it was clinging to the age-old practice of relying on codebooks whose listings of naval terms and their cipher equivalents had to be enciphered and deciphered by operators using manual methods. The naval officers believed that by employing superencipherment tables and changing them frequently, they could thwart cryptanalysis. It was a vain hope, especially when German divers recovered current codebooks from sunken British warships. By the summer of 1940, B-Dienst cryptanalysts were deciphering a high percentage of the traffic in the codes used by both the Royal Navy and the Merchant Navy. They were reading a total of some two thousand messages a month.

The German advantage came to the fore immediately after the war began. B-Dienst's decrypts more than made up for the U-boat fleet's limitations. The subs sent their first British ship to the bottom only days after war was declared, and between September 1939 and the end of the year they, together with other ships of the German navy, sank 150 merchant vessels, the passenger liner
Athenia,
the battleship
Royal Oak
and the aircraft carrier
Courageous.

They had another advantage in the competence of the man commanding the
Unterseeboot,
or U-boat, command, Admiral Karl Donitz. Donitz was convinced that his U-boats could strangle Britain into submission, as they had almost done in the 1914-18 war. Germany's neglect of U-boat construction during the interwar years dismayed him; he needed at least three hundred boats—and preferably one thousand—to do the job. But he set to work skillfully with the fifty-six operational craft available to him at the war's outset, while thirteen boatyards began turning out newer and better designs of U-boats in a stepped-up building program.

To these positives should be added the German navy's development of the most advanced and secure code systems of all the Enigma users. As mentioned earlier, the navy's Enigma was equipped with three extra rotors—eight in all. Although only three were used at any one time until later in the war, to have eight available meant that they could be arranged in 336 different ways and not merely 60 as with the army and air force Enigmas. Further, the navy's method for indicating message settings was entirely different. Instead of having the Enigma encipher the keys—a practice that had betrayed other services' settings—the navy instructed code clerks to determine their settings using a two-letter "bigram" table that was, in effect, an extra code unto itself. The bigram substitutions made the system "operator-proof": no choosing girlfriends' initials or sequences copied from the keyboard. The navy's operators weren't likely to take those shortcuts anyway; they were much more disciplined in observing strict security measures than the code clerks handling Luftwaffe and army traffic.

Further, the German navy used its Enigmas for a bewildering variety of codes. The list of merely the principal codes compiled by British naval intelligence officer Patrick Beesly in his book
Very Special Intelligence
runs to thirteen, ranging from the Atlantic U-boats' Triton to the Bertok code used for communications between the naval attache in Tokyo and the navy high command.

Dilly Knox and his team at Bletchley Park were inclined to give up on the navy Enigma. They had concluded that its system was impregnable.
 

Coupled with this spirit of cryptologic helplessness was the fact that the British had woefully neglected the resources needed to conduct antisubmarine warfare. Control of the Admiralty had been taken over by Great War gunnery officers who believed that the scant funds available to the navy in the 1930s should be spent on battleships rather than convoy escort vessels or reconnaissance aircraft.

At the war's beginning, consequently, it appeared that Donitz and his U-boat crews held the winning hand. They might well fulfill their promise to Hitler to turn Britain's protective moat into a garrote around the nation's neck.

This early air of pessimism did not apply to Alan Turing and his colleagues in Hut 8. He refused to be discouraged. As Stephen Budiansky has expressed it, "During those two months Turing had performed a feat that if anything surpassed Rejewski's. He had laid out the entire theoretical framework for tackling the Enigma in all its variations." Turing also recognized the importance of seizing any opportunity, or even
creating
opportunities, for the capture of naval Enigma materials. Nevertheless, except for small temporary successes, Donitz's system defied Turing for the first twenty months of the war.

While his first bombe—the one without Welchman's diagonal board—was still being built, Turing went at the naval Enigma using manual methods. His approach, as ever, depended on applying cribs. What cribs could he hope to use to get at the naval Enigma messages?

One he borrowed from the Poles. They had found that often a message was a continuation of another and that this second message began with
FORT,
"forward from," followed by the time of origin of the preceding message. When it became known from a German POW interrogation that the naval Enigma procedure was to spell out numbers, the crib was extended. A message originating at 2330 could be enciphered as
WEEPY.
So the time that a message was sent could lead to identifying the plaintext letters. This method of cribbing was consequently called FORTY WEEPY. Using it, Turing and his small team managed to break messages from five days of Enigma traffic during the preceding November, before a more complex system of plugboard connections had gone into effect.

To break subsequent messages from those days, Turing invented what became known as the
EINS
catalog. He had observed that the word eins, German for "one," occurred in about ninety percent of all messages. By compiling a catalog of all possible encipherments of
EINS,
he was able to search out equivalents in the ciphertexts—another small opening into securing the plaintexts.

BP's "Prof" got his first boost from captured materials in February 1940. A German U-boat,
U-33,
tried boldly to lay mines in the Firth of Clyde, on the west coast of Scotland. The U-boat was discovered, depth-charged and driven to the surface. Mindful of the need to protect his Enigma, the skipper passed out the code wheels and insisted that when his crew members jumped into the sea they were to make sure that the wheels sank. The scheme didn't quite work. When the British rescued most of the crewmen, three code wheels were found in the pocket of a nearly drowned sailor. Two of the three were of the new code wheels added to the naval Enigma, and from them Turing learned the wiring of all but one of the new wheels.

Another helpful capture came in April 1940 when a British destroyer off the coast of Norway disabled a German trawler posing as a Dutch fishing vessel. Before surrendering, the German crew threw overboard two bags containing confidential documents and cipher materials. A British crewman dived into the icy water to rescue one of the bags, and other papers were found aboard the trawler. Included were the Enigma keys for the four previous days. With this gift Turing and his team were able, in May, to read some of Donitz's April traffic and to advance their understanding of the naval Enigma. But they could still not decipher further messages.

Stimulated by the captures, however, Turing's inventive mind came up with another development that he could put to use when his first bombe arrived. This was a technique which, used on intercepts for which no cribs could be imagined, drastically reduced the number of bombe runs needed to determine the arrangement of the Enigma's rotors for a specific time period. It was an essential step, since to run through all 336 possible rotor orders would take as much as a week of full-time operation. His limiting method became known as Banburismus because the long sheets of paper used in it came from the town of Banbury. With Banburismus, Turing exploited a flaw in the naval Enigmas. The Germans thought they were adding to their system's security by having each rotor notch that caused its neighbor to turn located at a different letter of the alphabet. Thus, rotor one had its notch cut at the letter
R;
number two at
F;
number three at
W;
number four at
K;
and number five at
A.
The BP analysts created a meaningless but memorable mnemonic to keep the sequence in mind: "Royal Flags Wave Kings Above." Turing saw these disparate notch settings as a way to identify each rotor—if the Germans had cut their notches all at the same letter the Banburismus method wouldn't have worked. Later the Germans recognized the flaw. When they added three new rotors, all were notched identically.

The Banbury sheets were made of stiff white paper about ten inches from top to bottom but varying in width from two to five feet. On them were printed vertical rows of alphabets side by side. The narrower sheets accommodated shorter messages. The young women of Hut 8 took each intercept and punched holes in the alphabets for each of its letters. By sliding the sheets over each other above a dark table, analysts could spot the places where the same letters appeared in a pair of messages. By keeping score on the repeats, Turing and his colleagues could, by a series of logical—if formidable—deductions, apply the Royal Flags formula to determine the lettered notches at which the rotors in use that day were located. With luck, Banburismus could reduce the number of rotor orders to be tested by the bombes to as little as six runs. A detailed explanation of Banburismus is included as an appendix in Sebag-Montefiore's
Enigma
book.

 

 

Ascendance of the Wolf Packs

 

For all of Turing's brilliance, Hut 8's breaks in the naval Enigma remained, throughout 1940 and into 1941, too few and too slow to be of tactical use against Dönitz's U-boats. It was a time of havoc for British shipping. Merchant ship losses for 1940 totaled nearly four million tons, when only one million tons of new shipping was under construction in British shipyards. From B-Dienst, Dönitz knew the convoys' routes and sent his U-boats out in "wolf packs" to waylay them. To make matters worse for Britain, with the fall of France in mid-1940 both the Luftwaffe and the U-boats gained the great advantage of relocating in western France. The German air force added its own substantial totals to drowned tonnages of Allied shipping. For the U-boats, the new French ports markedly increased the times they could spend prowling the North Atlantic. They enjoyed what their crews thought of as a "happy time." The anguish in Hut 8 was deepened by press reports of victorious U-boat crews returning to their Bay of Biscay bases to the blare of brass bands and the popping of champagne corks, not to mention the presence of the fiihrer himself, there to pass out Iron Crosses.