The slips of paper came from the army’s intercept station at Chatham, a town in southeast England facing across the Channel toward Europe. There, operators with wireless sets and headphones scanned shortwave frequencies for the clicking of Morse code. An intercept operator had to be able to hear rapid dots and dashes and automatically write them as letters. Highly skilled operators could hear the pattern of Morse when anyone else heard silence or static. The very best ones could recognize the personal rhythm of the hand of an individual German radio man on the telegraph key, the way you’d recognize a voice you heard over and over while eavesdropping at the door of a room, even without knowing the name or face that belonged to the voice.
They had their ears pressed to the door of the Third Reich. All around Germany and what had been Austria and Czechoslovakia, and in a piece of Poland that grew daily as German armies advanced, radio operators were sending messages up and down chains of command.
In his first days at Bletchley Park, Welchman picked up from Kendrick and others how the Enigma system worked and what was known about the preambles. Once a month, the German code clerks got instructions on how to set their Enigma machines each day. Different parts of the military got different instructions. You could only read a message meant for the air force if you had air force settings—the “key,” as the tiny Enigma team at Bletchley Park called the daily settings for a particular part of the military.
Part of the preamble was a three-letter code, called the “discriminant,” that told what key a message was in—for instance, that it was in the air force’s settings. For security, the Germans used four different discriminants for each key, like using four different codenames for the same spy, and changed them every day.
If this sounds confusing, it was meant to be—for an enemy trying to listen in.
The key, Welchman learned, told the clerk which wheels to insert in the three slots in the Enigma each day, and in what order. The Germans now had five different wheels to use in the machines. In each one, the maze of wiring was different and would scramble the message differently. Today the order could be wheels A, B, and E; tomorrow, it could be D, C, and A. Altogether there were sixty possible wheel orders.
Each wheel had a metal ring around it, inscribed around the outside with the letters of the alphabet. The operator could rotate the ring around the wheel, so that any letter could be lined up with a starting mark on the wheel. The key also told him where to set the rings. With twenty-six positions on each of three rings, there were seventeen thousand possible ring settings. Multiply that by sixty wheel orders, and you had over a million combinations.
The Germans decided that this wasn’t safe enough. To the original Enigma design, they added a plugboard. It looked like a telephone switchboard, but with a socket for each letter of the alphabet. If the Enigma operator connected the A and G sockets with a cable, then A would be enciphered as if it were G and vice versa. The daily key included a list of ten pairs of letters to be connected. The number of ways you could create ten pairs on the board came to an astounding 150,000,000,000,000. Multiply this by the number of wheel orders and ring settings, and there were 150 quintillion ways to set up Enigma.
The Germans reasonably figured that even if a machine and all five wheels fell into enemy hands, no one would ever be able to test all those settings to find the right one.
And yet, the Germans added another security measure. When the clerk inserted the day’s three wheels in the proper order, with the letter rings set to their proper place, the top letter on each wheel showed through a window. Every time a code clerk sent a new message, he chose three letters and turned the wheels so that they showed through the windows. This became the starting position for the turning of the wheels. To decipher the message, you needed to set the whole machine to the same key—and then put the wheels in the same starting spots.
Somehow, though, the sending clerk had to communicate his wheel settings to the receiving clerk in a way no uninvited listeners could understand. This is how the Germans solved the problem: the sending clerk would use Enigma itself to encipher his starting wheel positions. First, he chose two sets of three letters—let’s say “ABC” and “XYZ.” As the last part of his unencoded preamble, he sent ABC, known as the indicator. Then he set his wheels to the starting position ABC and typed in “XYZ XYZ.” The pulses ran through the maze of wire. With each letter he typed, one or more wheels moved, and the maze shifted shape. A garbled series of six letters lit up on the lamp board of his Enigma. It could be “TRABQW” or “VGDZGL” or anything else. He’d send those six letters at the start of his enciphered message.
At the other end, the receiving clerk saw the “ABC” at the end of the preamble, and set his wheels with ABC showing at the top. Then he typed in the first six letters in the message. On his lamp board “XYZ XYZ” lit up.
Aha, he’d say, that’s the starting position of the wheels for the rest of the message. Since the first three letters and the next three matched, he knew he hadn’t messed up. He moved his wheels so that “XYZ” showed at the top and typed in the rest of the enciphered message. In place of nonsense, words lit up on his lamp board, one letter at a time.
To this, the Germans added a standard precaution. Code clerks had to break long messages into shorter pieces and send them separately. For each piece, the operator chose a different wheel setting.
Welchman wasn’t really supposed to concern himself with the wheel settings; he’d been banished from working on Enigma. His assignment was the unencrypted stuff that came before the wheel settings.
Welchman sat in the empty classroom, puffing his pipe and studying the preambles. The first mystery was what kind of pattern he was even looking for. Most people, faced with a mountain of unintelligible detail, get anxious, get headaches, look away. Welchman got curious. He began “making lists of this and that.” Things connected. Like vines, patterns started growing.
One showed up in multipart messages. When a German code clerk at an airfield cut a long report into pieces to send it to the air force command, he usually used a different discriminant in each piece to identify the air force key. It seemed so obviously safer not to repeat the same one. Welchman realized that if he followed a four-part message, he’d often find all four of that day’s discriminants for a key. Then he could identify every message in that key that Chatham had intercepted on that day. The security precaution turned out to be a security breach.
The Germans were not stupid. They were human. They’d come up with a system. Each time they checked their logic, it came out reassuringly correct. They’d built a fortress in their minds, with high walls, a moat, and a bolted gate. Every time they looked at it, they saw it was impregnable.
Welchman wasn’t one of them. He looked at the fortress: it shimmered in front of his eyes and came into focus, and he could see that the gate was a drawbridge lowered across the moat, inviting him in.
Enigma appeared unconquerable. Its fundamental flaw was that human beings built it, and other human beings could see it differently.
The same law governs codebreaking and safecracking: any lock that one human mind can design, some other human mind will eventually see a way to break. The first mind snaps shut around its solution. The second mind must be unlocked and open.
On his daily lists, after he figured out which three-letter discriminants belonged to which key, Welchman marked them with colored pencils. From then on at Bletchley Park, the names of Enigma keys were colors. He used his red pencil a lot, along with blue and less often green. Red was the key used by the Luftwaffe, the German air force.21
Curiosity doesn’t stop where it’s told. Welchman also made lists of the wheel settings, the first six letters of the enciphered text. Again, patterns bloomed. Sometimes, the “X” in “XYZ” would be enciphered by the same letter both times, so that the message might begin GBD GWL. The same stutter could happen with the second letter, or the third.
The mathematician began calculating. Only about half the comb
inations of wheel orders and ring settings could make one of those repetitions happen. If another repeat showed up the same day in the same key, it cut the number of settings by half again. With twelve such pairs, the potential settings dropped from a million to just 250—few enough to check individually.
He designed a method for taking the patterns and finding the daily settings. The machinery would consist of hundreds of sheets of thin cardboard, each punched with holes in precise spots on a grid. The holes represented settings that could produce a particular repeat. It might take months to punch the holes, a couple million of them altogether. But once the full set of sheets was ready, they could be used daily, stacked in accordance with the repeats in each day’s messages. Where the holes lined up, light would shine through, indicating potential solutions. You could try them on an Enigma machine with that day’s intercepts. In a matter of hours, you’d have the day’s settings.
It would work only because the German clerk had to type his wheel setting twice. Again, the safety measure opened the gate to the Enigma fortress. Welchman rushed to the cottage and told Dilly Knox his brainstorm.
“Dilly was furious,” Welchman would write. “What I was suggesting was precisely what he was doing… Dilly reminded me that I had been told to study discriminants and call signs” and not to break Enigma.
What Dilly did not say was that Welchman had just kicked his self-respect where it was already bruised.
IN FACT, DILLY told Welchman nothing about his secret journey to Warsaw at the end of July—just a few months earlier and in an entirely different era, when he and Alastair Denniston could still take a train across Germany.
The invitation came from the Polish Cipher Office. More than anyone, Polish codebreakers could taste how close war was. They decided to risk sharing their secret in order to share the impossible burden of their work. The matchmaker between the Poles and GC&CS was a man who often appeared in British documents under the alias Godefroy—Captain Gustave Bertrand, director of cryptological services in French military intelligence.22
Britain’s commitment to defend Poland was born only in March 1939, when Hitler seized Czechoslovakia. The alliance of France and Poland against Germany, the dangerous country that lay between them, dated to 1921. It led to cooperation—cautious and selective—between their codebreakers.
In 1931, a German named Thilo Schmidt walked into the French embassy in Berlin and offered to sell secret documents. Germany’s economy had collapsed. Schmidt had a wife, children, and mistresses and was not getting by on his salary from the job that his brother had landed him at the Defense Ministry Cipher Office. The French intelligence agent assigned to him generously helped out. Schmidt showed his value by filching German army manuals on Enigma procedures. After that, he provided lists of settings that the army was using for Enigma.
Bertrand went from ecstasy to angst. His staff understood much better how Enigma worked—but still didn’t understand a single day’s messages. He took his Enigma papers to Warsaw, gave them to his Polish counterpart, and went home with a promise that the Poles would share any breakthroughs.23
By then mathematician Marian Rejewski had come up with equations that would reveal how Enigma wheels were wired. But there were too many variables; solving the equations threatened to be very slow work. The French treasure changed that. In those days, the Germans used the same settings throughout their military, and only changed the wheel order every three months. The September 1932 settings stolen by Schmidt gave the summer wheel order; the October settings gave the order for autumn. Rejewski could fill in several variables, and he quickly solved the equations.24
Rejewski and his two even younger colleagues, Jerzy Rozycki and Henryk Zygalski, were soon deciphering Germany’s perfectly protected military secrets daily. Long before Welchman heard of Enigma, they noticed the German practice of enciphering the wheel setting twice and the clues it provided to discover the daily settings. Rejewski came up with a machine that looked like two Enigma machines tethered together. It could rapidly check settings to see which ones could produce the patterns they found. They called it a bomba, a bomb, perhaps because it ticked, or because it dropped weighted pendulums when it found a likely setting, or because the three of them were eating the molded ice cream treat called a bombe, a bomba in Polish, when Rejewski got the idea. When the Germans started making more connections on the plugboard, they unknowingly defeated Rejewski’s bomba—but not an invention by Zygalski. He came up with using cardboard sheets with small holes cut in them, and lining them up to find the daily setting.25
For six years, their agency never told Bertrand of their success.26 The simplest explanation is fear: what you share with an ally, you also share with whoever is spying on your ally. If the French didn’t know, a French traitor couldn’t tell the Germans.
And then, in December 1938, the Germans added two new Enigma wheels to the original three, creating sixty wheel orders in place of six. Rejewski figured out the wiring. But now they would need ten times more of Zygalski’s sheets. The three men on the team were cutting the holes in the sheets themselves with razorblades. The job was impossible. German radio networks chatted war preparations in a language that had gone back to being incomprehensible.27
The Poles asked for help.
Bertrand and one of his codebreakers checked into the Bristol Hotel in Warsaw on July 24, 1939, along with Denniston and Dilly Knox.28 The next morning, they were picked up at 7:00 and driven to a clearing in a forest near the village of Pyry, where the Cipher Office had its new building, heavily guarded and partly underground. For the next three hours, Major Maksymilian Ciezki—head of the German subsection and Rejewski’s boss—laid out how his men had broken Enigma. “I confess I was unable to understand completely,” wrote Denniston, who’d been cracking codes since Room 40 in the Great War. Only when “we were taken down to an underground room full of electronic equipment did I fully grasp the results of their reasoning.”
Dilly “accompanied us throughout but maintained a stony silence and was obviously extremely angry about something. It was only when we got back into a car that he suddenly let go and… raged and raged that they were lying to us,” Denniston wrote.
“The whole thing was a pinch”—a theft, Knox insisted. No one could have reasoned out the Enigma wiring. “They never worked it out,” he exploded. “They must have bought it or pinched it.”29
Other British codebreakers would have the same reaction.30 If you weren’t looking at the problem the way Rejewski had, it appeared unsolvable. And Dilly was about one-tenth right. Rejewski never saw a stolen machine, but the stolen papers did help him.
Denniston now had to clean up a diplomatic difficulty. Bertrand was in the cab, and even if he didn’t speak English, he could hear rage. In a note afterward, Denniston asked the French officer to “understand my big problem in the shape of Knox. He is a man of exceptional intelligence but he does not know the word cooperation. You surely must have noticed that off duty he is a pleasant chap loved by all. But… in Warsaw I had some deplorable experiences with him. He wants to do everything himself… He can’t stand it when someone knows more than him.
“Unfortunately, I cannot do without him,” Denniston wrote. Knox knew more about Enigma than anyone in Britain, he said.31
By the next day Dilly had sailed through his mind’s storm into calm waters. Zygalski and Rozycki explained their methods to him. Dilly decided that the Polish mathematicians were “charming young men,” and Rejewski happily saw that “Knox grasped everything very quickly, almost as quick as lightning.” Denniston and Knox promised that GC&CS would manufacture a full set of Zygalski’s sheets; the Poles promised to send their British and French friends each a working copy of the German military Enigma.32
This time, promises were kept. On August 16 Bertrand stepped off a train in Victoria Station in London. With him was a courier bearing a Polish-made working copy of a German Enigma. Stewart Menzies, the deputy of dying MI6 chief Hugh Sinclair, came to th
e station himself to receive it. The Rejewski team also sent technical drawings of their bombe, as it was called henceforth in English.33 The gifts arrived just as GC&CS was moving from London to Bletchley Park in readiness for oncoming war.
In September, during their flight from Warsaw, Rejewski and his colleagues had to destroy all their equipment—the bombes, the Zygalski sheets they’d dedicatedly cut by hand, the Enigma replicas.34 The meeting in the forest clearing at Pyry preserved their years of work. It made everything that Bletchley Park did possible.
HITLER FLEW TO Warsaw on October 5, 1939. With him came the commander of his security battalion, the man already known as the “Führer’s general,” Erwin Rommel.
In scattered corners of Poland, the country’s last troops were still fighting. Warsaw had fallen a week before, after the government fled to Romania, after German artillery ground the city down like a pestle grinding seeds in a mortar, after the German air force showed that Guernica had only been a training run. Most of Warsaw, a foreign correspondent reported, “was in ruins, buildings and streets battered to charred wrecks.” There were “huge craters in the middle of main thoroughfares and tram rails twisted into geographical patterns.” The reporter said nothing about seeing corpses. Perhaps they’d all been removed from the streets in honor of Hitler’s visit.35
Nor did the foreign correspondents see the daily reports of Reinhard Heydrich’s SS Einsatzgruppen as they followed the frontline troops into Poland. Heydrich ordered them to erase Poland’s elites. On an inspection tour early in the invasion, Heydrich especially urged his men to kill Jews or terrify them into fleeing to the Soviet Union. Speaking with Admiral Wilhelm Canaris, head of the Abwehr, German military intelligence, Heydrich described his mandate from Hitler: “The nobility, the Catholic clergy and the Jews must be killed.” This was only loosely a secret. The chief of the Army General Staff, General Franz Halder, told one of his officers that Hitler and his closest lieutenant, Hermann Göring, aimed “to destroy and exterminate the Polish people.” The officer wrote in his journal that Halder told him more but that it was too appalling to write down. No plan or bureaucracy of mass murder yet existed; Heydrich’s men improvised. By the time Hitler landed in Warsaw, they had executed between twelve and sixteen thousand people, about a third of them Jews.36
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