Authors: Hervie Haufler
Denniston was not particular about his recruits' backgrounds. He combed the military; he used his old-boy contacts among the universities; he brought in civilians; he tapped the Wrens (Women's Royal Navy Service) and Waafs (Women's Auxiliary Air Force) for legions of young women. BP became a melting pot of cryptologic expertise. When the war began, three of Britain's master chess players were attending an international Chess Olympiad in Buenos Aires. They promptly caught the blacked-out, unconvoyed
for home and joined Denniston's team at Bletchley.
GC&CS denizens formed a society ruled by meritocracy. Military rank didn't count. No saluting or other military hocus-pocus was tolerated. Everybody went by first names or nicknames. The only way to gain respect was by doing a superlative piece of work.
Most brilliant, and most eccentric, of the lot was Alan Turing. He had a strange and wonderful combination of talents: he was a mathematical and theoretical genius, yet he could descend from his visionary cloud to become the most practical mechanic. To look back at those times is to marvel at how fortuitous it was that this man became the pivotal figure in the conquest of the Enigma.
Turing's powerful and independent mind made him, as a schoolboy, intolerant of conventional classroom teaching. Frequently he neglected regular studies because his real attention was given to probing advanced mathematical theorems on his own. Adding to his drive to excel was his memory of an ardent friendship with a fellow student, Christopher Morcom. When Morcom died of tuberculosis while still in school, Turing resolved to achieve what he believed his friend would have achieved if he had lived.
Morcom had won a scholarship to Cambridge. Turing followed suit by attending Cambridge and being elected to a fellowship at the university's King's College when he was twenty-two. He was also sure Morcom would have sought stimulus by searching out the university's outstanding academic scions. Turing was strongly influenced, first, by David Hilbert, who raised the question, did there exist a definitive method which could, in principle, be applied to any mathematical assertion and produce a correct decision as to whether it was provable? Hilbert believed there was no such thing as an unsolvable mathematical problem.
The second Cambridge lecturer who most influenced Turing's thinking was Maxwell H. A. "Max" Newman, who asked if there wasn't a mechanical process that could put mathematical theorems to the test.
From this point on, Turingâin the words of his biographer Andrew Hodgesâ"dreamed of machines." In the early summer of 1935, when he was just twenty-three years old, he saw his answer. He created a theoretical "universal machine"âafterward known as the Turing machineâthat could, by using the binary system that later became the basis for digital computers, replicate logical human thought. The Turing machine could also write a verdict as to whether a specific assertion was or was not provable. This, together with his work on determining computable versus non-computable numbers, proved Hilbert wrong: there could be unsolvable problems.
The world of advanced mathematics was then centered in Princeton, New Jersey. There men such as Albert Einstein, Alonzo Church and Kurt Godel provided leadership in probing into mathematical unknowns. In 1936, Turing went to Princeton University and benefited from exchanging ideas with the older masters. While there he indulged both his theoretical and his mechanical bents in, as though by predestination, cryptology. He worked on a cryptographic system for which he needed an electrical multiplier. To build it he had to construct his own electrical relays.
Princeton Ph.D. in hand, and his multiplier in his luggage, Turing returned to Britain in July 1938 and soon afterward wound up at Bletchley Park. There, in the summer of 1939, spirits were animated by the knowledge that the Poles had broken the Enigma. Turing led BP's attack.
To him the German machine was a practical application of his theoretical machines. The Poles were right: to defeat the Enigma required counter-Enigmas. Yet the Poles were also wrong: their machines attacked the German machine through the message key indicators, and in his estimation, that was not the right way to go as indicators could be changed overnight, sending the codebreakers back to square one.
With astonishing speed Turing created an English bombe that took little from the Poles except the machine's name. Turing's bombe passed over the indicators; it sought to extract the key from the message itself.
Turing and Welchman Team Up
Brilliant as he was, to make his bombe effective, Turing had to have help from a colleague, Gordon Welchman. A lecturer in mathematics at Cambridge, Welchman had a frustrating time when he first came to Bletchley Park. Denniston assigned him to join Dilly Knox's small group at work in the BP building known as the Cottage. But Knox seemed to take a dislike to him and banished him to another building. There Welchman was told to study some German army messages and draw whatever information and patterns he could through an external examination. Welchman soon went beyond those parameters. On his own he realized the vulnerability of the double enciphering of the message key and independently evolved an equivalent of the Zygalski Sheets. When he reported his work to Knox, Welchman was dismayed to find that he had simply been duplicating the efforts of another BP associate and Cambridge alumnus, John Jeffreys, who had produced Bletchley's version of the Polish sheets.
Welchman's fortunes changed when he teamed up with Turing. Turing's approach to cracking the Enigma was to work with "cribs," or what Welchman called the "probable words" in a message. Since military parlance was highly standardized and repetitious, one could presume that certain words or phrases would appear in the text. The Poles had made rudimentary use of the technique by searching for messages that began with
Turing meant to use his bombes to carry the method much further by finding longer passages embedded in the message itself.
The British were aided, as the Poles had been, by German overconfidence in the security of their machine. The Germans could have made the use of cribs far more difficult if not impossible. All they needed to do was to add random bits of nonsense into their message beginnings and/or endings, or to insert
into long words, or to translate officers' titles into coded referencesâany such steps would have prevented accurate cribs from being applied. But they remained punctilious about spelling out honorifics and titles, and they continued to use repetitive phrases without any masking.
Turing's bombe, possessing the power of at least twelve Polish bombes, was designed to run an automatic test to determine whether a specific crib was contained in the message. He, however, had a limited view of what could be obtained even when his bombe succeeded. Essentially, he meant to look for the same sorts of closed letter loops that had been at the center of the Poles' technology. Turing's loops, however, had the great advantage of being drawn from cribs within the message rather than from its indicator. His bombe used the loops to detect incorrect positions and, by rejecting them, to arrive at the correct settings.
When it was built, though, this first bombe did not work well. To seek out merely small strings of letters did not produce enough rejections. There were many "Stops" that were found to be false only by hand testing. It was a slow and uncertain process.
Then Turing showed his plans to Welchman. In a flash of inspiration, Welchman saw that they didn't have to settle for closed loops. "By interconnecting the scramblers in a completely new way," he wrote in his memoir,
The Hut Six Story,
"one could increase the effectiveness of the automatic test by a very large number."
His new method involved adding to Turing's bombe the circuitry of what Welchman called a "diagonal board"âa matrix of terminals in a square in which the twenty-six letters of the alphabet were arranged horizontally, with another twenty-six vertically. His scheme capitalized on the reciprocal nature of the Enigma's plugboard connections. That is, if
is connected with Z and becomes Z in the encipherment, then the reverse is also true: Zbecomes
His change ruled out false stops that the plugboards could make in Turing's bombe. The insertion of the diagonal board, as Welchman described it, "greatly reduced the number of runs that would be needed to insure success in breaking an Enigma key by means of a crib."
Turing, Welchman wrote, was incredulous at first, "but when he had studied my diagram he agreed that the idea would work, and became as excited about it as I was."
Turing's earlier design had guided the British Tabulating Machine Company in producing the first BP bombe. Now an improved design incorporating Welchman's diagonal board was put into production. The conversion benefited from Turing's mechanical bent. To do their required switching jobs, the bombes needed fast-working electrical relays. Turing drew from his electric multiplier to suggest designs for the bombes.
Patricia Bing, a teletypist who worked for Turing, later recalled how fellow workers at BP quickly adjusted to the unconventional ways of the man they began referring to as "the Prof." They understood that Turing thought little of his appearance or the impression he made. His clothes were a mess; his chewed-up fingernails most often had crescents of dirt beneath them; he could show up at BP entirely unaware that he was wearing two odd shoes. To control his allergies in pollen season he donned a gas mask when riding his bike. The bike had a bad habit of periodically throwing its chain; instead of taking the time to fix it he would count off the number of revolutions and stop just in time to make an adjustment. Bing remembered seeing Turing arrive on his bike and then "scuttle past us giggling girls, eyes downcast, as though in fear he might have to speak to one of us before he disappeared into his office." The papers he wrote and the designs he produced were made almost unintelligible by scratch-outs and inkblots. When invasion threatened, he melted down a collection of silver coins into ingots, buried them and then, when the crisis had passed and it was time to dig them up, could not remember where they were buried.
In the hunt to unlock the Enigma, though, the Germans never dreamed they would be up against a man of Turing's genius. In those few months between the outbreak of the war and early 1940, he had analyzed the machine, discerned the chinks in its supposedly impenetrable armor and, with Welchman's help, devised the countermeasures that would defeat it.
Months must pass, however, before the redesigned bombes, with all their thousands of soldered connections, would be available. How were the codebreakers to achieve at least partial success in the meantime?
British patience and meticulous attention to detail came to the rescue. GC&CS analysts had been studying the habits of German Enigma operators and had found two subtle mistakes that could be exploited.
The first became known as "Herivel's tip," after John Herivel, a young mathematician recruited by Welchman. Much like Rejewski, Herivel tried to put himself into the shoes of a German code clerk and imagine what the operator might do incorrectly because of laziness or work pressure. Herivel had an insight. At the beginning of each new encoding day, the German operator had a boring series of steps he had to go through. Following instructions, he must choose the correct set of three rotors out of the five available, slide the rotors in proper sequence onto the axle, turn their alphabet rings to the required positions and link up the proper arrangement of the plugboard cables. Then he was supposed to select three random letters for his message key. It was all a big bother. Suppose, Herivel asked himself, the lazy or hurried operator didn't take that final step? Suppose he sent his first message of the day using the same three letters as his rotor ring settings? Herivel suggested collecting the new day's first messages. If there was more than one shortcutting operator, there would be repeatsâand the rotor settings could be surmised.
The second sloppy practice consisted of what BP labeled "cillis." The name may have been derived from the initials of one German clerk's girlfriend, which he used often instead of randomizing his three-letter selections. That was one type of cilliâthe repeated use of familiar sequences, such as
Another form was supplied by German operators who, instead of plucking their three letters out of the air, simply lifted them from their keyboards. A sequence down from the
One down from PFread
Although these practices were expressly for bidden in the Enigma operators' manuals, lazy or rushed code clerks did resort to them, and from these cillis BP's clever analysts could determine the wheel order for the day as well as the setting for these particular messages. "Unbelievable?" Welchman wrote. "Yet it actually happened, and it went on happening until the bombes came, many months later."
Using these and similar ingenious methods, the BP crew early in 1940 began deciphering the Luftwaffe messages known as Red because that was the color of the pencil Welchman used to demark it from other systems.
By then it had been decided that Welchman and Turing would divide the main Enigma decrypting responsibilities between them. Welchman had moved into Hut 6, one of the wooden structures hastily erected on the park's grounds, and took over its operation when the young John Jeffreys became terminally ill. Welchman's team concerned itself with breaking German air force and army traffic, then passed the decrypts on to Hut 3. There, another team translated them, judged their importance and urgency and determined where they should be disseminated. Turing was responsible for Hut 8, heading up work on the naval Enigma signals, with Hut 4 as his analysis center.
When the bombes arrived in August 1940, allowing cribs to be put to use, the Hut 6 team simply accelerated the breaking of the Red cipher. It was of particular value because it was used in army/air force coordination and disclosed information about both services.