Authors: Hervie Haufler
Measures such as these enabled the Poles to narrow their unknowns to one: the single-letter substitutions resulting from the plugboard connections. But this test was simpler because in those early days, the German code clerks were instructed to connect only six cables, producing just twelve letter substitutions. Rejewski saw that he could run a test decipherment on one of the Enigma replicas now arriving from the electrical supplier and find passages where the underlying plaintext could be discerned. An example in English could be
which sharp eyes could translate into
REPORT ON TANK REPAIR,
with the plugboard substituting
Other passages revealed the remaining linkages and solved the plugboard in which the Germans placed so much faith.
By grinding through these burdensome procedures each day, the young Poles began reading the German messages and delivering to their superiors glimpses into the Reich military's plans. The process, however, used up most of the day. While that was a far cry from the endless millennia the Germans assured themselves it would take cryptanalysts to break into the Enigma, Rejewski and his mates recognized that their methods of decipherment were too slow to be of practical value. They had to improve the efficiency of the process and greatly increase its speed.
Machines to Defeat the Machine
Time was not the only problem they faced. Although the Germans were confident that their modified Enigmas were impervious to cryptanalysis, they also knew that prudent security precautions called for making changes in their systems periodically. The Poles could not rest on their laurels; they had to deal with a series of changes, each of which could make obsolete the process with which they had been succeeding.
The pressures on the young cryptanalysts were enormously increased by the ascendance of Adolf Hitler and his National Socialist Party. With their military strength on the rise, the Nazis raised an ever-mounting cry to regain their lost lands, particularly the corridor that gave Poland access to the seaport of Danzig, since it separated East Prussia from the rest of Germany.
Rejewski came up with a daring vision: to beat the machine, he and his team would create other machines that would help to find the Enigma keys far more swiftly than they could by their pencil-and-paper methods.
He turned first to that problem of developing a card index of all the 105,456 variations in the letter chains and their linkages. He conceived a device called the Cyclometer, which was designed to automate the process of identifying the characteristic cycles so that the results could be cataloged. It consisted of the interlinked elements of two Enigmas, with twenty-six glow lamps to cover the alphabet. Its function was to determine the length and number of chains in each set of characteristics for all six arrangements of the three rotors. The number of glow lamps that lit up indicated the length of a chain. When, after a year's work, the index was completed, the analysts had only to compare that day's notations on the number and lengths of the chains with the index to pin down the rotor order and settings. Developed in 1934 or '35, the Cyclometer together with the index sped the Poles' cryptanalysis efforts until November 1937.
All this work was then undone when the Germans changed the reversing reflector in the Enigma in a way that voided the Poles' catalog. Rejewski and his team started over, but it took them months to complete a new index and again succeed in decoding German communications.
In mid-September 1938 the Enigma output once more became an incomprehensible jumble. This time the Germans had made their first major change in procedures. Instead of all the operators setting their basic daily key uniformly, the code clerks were now told to choose a new basic setting for each message they transmitted. Their own three-letter choice would be sent unenciphered at the head of the message and was to be followed by a double encipherment of the message key. Since the Poles could no longer stack indicators to form letter chains, all that painstaking work in compiling cycle indexes went out the window.
The fact that the new procedure called for the German code clerks to continue enciphering their message keys twice was fortunate. The same relative positions of plaintext letters and. cipher letters were there, providing the opportunity to link the first enciphered letter and the fourth, the second and the fifth, and the third and the sixth. Rejewski's response was to invent a new and more complex Cyclometer the Poles named after the ice cream confection they were eating at the moment Rejewski came up with the idea. The desserts, in Polish, were called "bomby," but subsequently the machines became known as "bombes." When the machines were built and began ticking away like a time bomb working its way toward an explosion, the name seemed appropriate.
Each bombe, consisting of six Enigma machines connected together, was capable of swiftly running through the entire cycle of possible permutations, looking for those relatively rare places where, in an indicator, the plaintext letter would have the identical cipher letter, as in
Out of the thousands of combinations, however, the bombe would locate the identical letters in all three 1-4, 2-5, and 3-6 positions. When it found this lineup, it would stop, giving the analysts the information they needed to reconstruct the daily keys.
Yet the determination of the settings did not disclose the order in which the three rotors had been inserted in their Enigma slotsâan order which was now also changed daily. Rejewski proposed six bombes to try all six possible rotor sequences in one parallel process.
The machines were built. They worked well when the trios that included the identical letter were free of substitutions introduced by the plugboard. To produce results irrespective of the plugboard connections, Zygalski devised what became known as Zygalski Sheets, sheets of cardboard about two feet square. Each sheet was divided into a grid of small squares representing horizontal and vertical alphabets. For each of the six rotor orders, a set of 26 sheets was prepared, 156 in all. The complex procedure involved cutting holes in the grids where a repeat of plaintext letter and cipher letter from that day's indicators was possible. Zygalski and his coworkers used razor blades to cut out thousands of holes. By stacking the sheets over a light source and shifting them systematically, they found places where the light shone through, indicating a rotor order, and a few trial runs on a replica Enigma could determine whether or not this order was the right one. If it was not, gibberish appeared. If it was, the men read the German message. Together with the bombes, the Zygalski Sheets put the Poles back in the business of speedily deciphering the German traffic, which was now so voluminous that a new rank of Enigma-using technicians had to be added to the Cipher Bureau.
By the winter of 1937-38 the Poles were deciphering about seventy-five percent of the messages passed to them by their intercept stations. In all, during those five years after they first broke through in 1933, they read about one hundred thousand of Germany's military transmissions. They were able to inform Poland's government and military leaders about the German mobilization plans, the order of battle of the German forces and the output of their armaments industry.
Passing the Torch to the French and British
In December 1938, the Poles' entire operation again came to a screeching halt. Once more the Germans changed their system, and this time the change was, from the cryptanalysts' point of view, catastrophic.
The Germans put into service two additional rotors. As mentioned earlier, the Enigma had slots for only three code wheels at a time. To have the three chosen from the five available, however, multiplied the number of code wheel orders ten times, a huge additional cryptanalytic burden.
The Poles were confident they could solve the new system, but to do it they would need not 6 bombes but 60, and not 156 Zygalski Sheets but 1,560. These requirements simply outran the Cipher Bureau's resources. In addition, the Nazi drums of war were beating ever more loudly, and the Poles could see that time was running short.
Even though they could still read the messages of the Nazi Party's intelligence service, which continued to use the old keying system, the Poles came to a momentous decision. They would pass on to their allies, who now included the British, the knowledge they had acquired about the Enigma and the machines they had developed to break its output.
On July 24, 1939, the French delegates, including Bertrand and an aide, along with three top intelligence officials from Britain, arrived in Warsaw. They were taken to the building the Poles had created for their cryptanalysis crew outside the city. "At that meeting," as Rejewski depicted it, "we told everything that we knew and showed everything that we had." Ironically, the common language for their meeting was German.
The head of the Cipher Bureau, Lieutenant Colonel Gwido Langer, showed the visitors around the facility. Then he took them into a room in which several objects under covers rested on tables. Like an artist unveiling a new creation, Langer whisked off the covers. Under them were Polish clones of Enigmas. Everyone recognized what they were, yet they could not believe that the Poles had built the machines on their own. Bertrand called it
"un moment de stupeur"
"a moment of stupor."
Among the English party was Dillwyn "Dilly" Knox. Back in England, Knox had been wrestling fruitlessly with the Enigma, and he had a question for his Polish hosts: in what sequence had the Germans ordered their connections in the entry rotor? When told they were wired in alphabetical order, he blinked in disbelief. Something so obvious had never entered his mind. According to one report, later that evening when he was in a taxi with Bertrand returning to their hotel, he chanted happily,
"Nous avons le QWERTZU, nous marchons ensemble'1'':
"We have the QWERTZU, we march together."
Lariger led the way to another room. Here, lined up, were the six bombes. Langer switched on the machines and demonstrated how they worked. Rejewski answered questions. Zygalski explained his perforated sheets. Bertrand's moment of
Alastair Denniston, chief of the British delegation, wanted to telephone London right away to have technicians fly in to size up the specifications for the Enigmas and the bombes. That wouldn't be necessary, the Poles told him. They were ready to ship Enigmas by diplomatic pouch to Paris, where one could be forwarded to England, and to supply technical drawings of the bombes as well as samples of the Zygalski Sheets.
The conference ended, as Kahn has described it, "in an atmosphere of warmth, astonishment, gratitude, and anticipation."
The disclosure came none too soon. Just five weeks later, the German blitzkrieg overran the armies of Poland. The Cipher Bureau had to quickly destroy its files and smash its machinery. The cryptographic team also needed to escape, for in their heads was information the Gestapo might well extract by methods of torture.
Rejewski and his mates headed south and soon crossed the border into Romania. At the French consulate in Bucharest, the code name for Captain Bertrand brought a quick passage. Making their way through Yugoslavia and Italy, they joined up with Bertrand at his headquarters outside Paris. Soon, with British help that included providing new stacks of Zygalski Sheets, they were back to tackling the Enigma.
The Poles were not again to become the leaders in the attack on the Enigma, but before their flight they had shown what no one else, least of all the Germans, believed possible. The Enigma could be beaten and the secret contents of its messages divulged.
Britain Takes Over the Cryptologic War
Britain's cryptanalysts were busily making up for their years of halfhearted attempts to crack the machine. The Government Code and Cypher School (GC&CS)âa deliberately understated titleâwas moved out of crowded and bomb-vulnerable London. For its new home, the chief of the Secret Intelligence Service, eccentric millionaire Hugh Sinclair, had purchased the Bletchley Park estate in the town of Bletchley, a homely manufacturing and railway hub fifty miles to the northwest in Buckinghamshire. In August 1939, Alastair Denniston, picked by Sinclair to head up GC&CS, had investigated the accommodations at Bletchley Park. Even though the mansion was an architectural monstrosity, Denniston saw that the Park had other virtues. Chief among them was that it was located on a main rail line out of London and another line that connected Bletchley to both Oxford and Cambridge. Convinced, he made BP the GC&CS headquarters just before England was plunged into war against Germany.
British progress in cryptology owes much to Denniston. During the Great War, he was a bright young man in Britain's Room 40. He could have pursued a much more lucrative career elsewhere, but he stayed with the agency. With World War II approaching, he led the way in making changes that proved critical to BP's success. He realized, as the Poles had a decade earlier, that the new cryptology demanded different mind-sets, individuals with advanced mathematical skills, puzzle solvers, chess players, bridge addicts. He began tracking down such individuals, mainly in Cambridge and Oxford, and recruiting them. He launched a cryptography course to begin their training. Most important, he was a persuasive advocate for having most of Britain's cryptologic program centered in Bletchley. He knew that the kind of brains that excel in cryptanalysis are not common, and to have them joined in collaboration at one place was a distinct advantage. The Germans had bright analysts, but there were so many chiefs contending for Hitler's favor, with each zealously guarding his own turf, that the available brainpower was too fragmented ever to mount a coherent and consistent codebreaking program.