Bletchley Park is a large ugly house in Buckinghamshire, about fifty miles from London. It was bought quietly in nineteen thirty eight by a man who paid for it out of his own pocket so that no government department would have to explain the purchase.
It was chosen for an unromantic reason. It sits beside the railway junction where the line from Oxford crosses the line from Cambridge, which meant that mathematicians and classicists from both universities could be brought in without anybody changing trains in London.
The house was not big enough, so wooden huts were thrown up in the garden. They were cold, they leaked, and they were referred to only by number, and the numbers stuck so hard that when the work later moved into proper brick buildings, people went on calling it Hut Eight anyway.
At the start of the war about a hundred and fifty people worked there. By nineteen forty five, nearly nine thousand did.
The Poles had given Britain a working Enigma and seven years of method, and within months that method stopped working.
The Polish approach depended on the German habit of sending each message key twice. In May nineteen forty the Germans stopped doing that. The crack was sealed.
So the problem had to be solved again from a standing start, and this time without any repeated letters to lever open. The man who did most to solve it was Alan Turing.
Turing was twenty-seven. Four years earlier he had written a paper that invented, on paper, the idea of a general purpose computing machine, at a time when no such thing existed and there was no obvious reason anybody would want one. He was not primarily a codebreaker. He was a mathematician who happened to think in machines.
Turing's insight was to stop trying to break the message and start trying to guess part of it.
German military messages were written by people following procedure, and procedure produces habits. A great many messages began with the same formalities. Weather stations sent a report at the same time every morning, in the same format, containing the word for weather. One outpost in North Africa reported, every single day, that there was nothing to report.
So if you can guess that a particular stretch of enciphered text says the German word for weather, you have something enormously valuable: a piece of plain text lined up against a piece of cipher text. That pairing was called a crib.
Now here is the clever part. Enigma had one absolute rule built into its wiring: no letter could ever encipher as itself. Press A and anything can light up except A.
That rule, meant as a tidy piece of engineering, is a gift. Slide your guessed word along the cipher text, and any position where a letter sits above itself is impossible and can be thrown away instantly. Most positions die in seconds, and you are left with a handful worth testing.
A crib plus that rule turns a search of a hundred and fifty million million million settings into something a machine could grind through in hours.
The machine was called a bombe, keeping the Polish name, though it worked on a different principle. Turing designed it, and a mathematician called Gordon Welchman then added one piece to it, the diagonal board, which made it several times faster. Both names belong on it.
It was about seven feet wide and six feet tall, the shape of a large wardrobe, with over a hundred rotating drums on the front, each one imitating a rotor of an Enigma machine. It weighed a ton. There were eventually more than two hundred of them.
It did not decode messages. It answered one narrow question extremely fast: given this crib, which rotor orders and starting positions are not immediately self-contradictory? Everything it could rule out, it ruled out, and it stopped when it found something that survived. Then a human being took the surviving settings and tried them.
The noise was famous. A room of bombes sounded, everyone said, like an enormous room full of knitting needles.
They were operated almost entirely by young women of the Women's Royal Naval Service, who set up the drums by hand from a menu of wiring instructions, watched for a stop, telephoned the result through, reset the machine and started again. It was heavy, oily, deafening work in eight-hour shifts, and the operators were told nothing at all about what the machine was for.
Turing was, by every account, extremely odd, and the oddness has been told so often that it sometimes crowds out the work.
He chained his tea mug to a radiator to stop anybody borrowing it. He cycled to work in a gas mask because it helped with hay fever. He ran extraordinary distances for relaxation, and was good enough that he once ran a marathon in a time not far off Olympic standard. When he became worried about invasion he converted his savings into silver ingots and buried them in a wood, and afterwards could not remember precisely where.
He was also, and this matters more, generous with ideas in a way that clever people often are not. Colleagues describe him explaining things patiently, repeatedly, to anybody who asked, and being entirely uninterested in who got credit.
In nineteen forty one, with the naval Enigma still resisting and resources being handed elsewhere, Turing and three colleagues did something almost unheard of. They wrote directly to Churchill, over the heads of everybody above them, and asked for staff and machines.
Churchill read it and sent a note to his chief of staff the same day. The note said: Action this day. Make sure they have all they want on extreme priority and report to me that this has been done.
The intelligence produced at Bletchley was given the codename Ultra, and the rule about using it was iron: never act on Ultra in a way that could only be explained by having read the enemy's mail.
So if a convoy was to be diverted around a wolf pack, a reconnaissance aircraft was sent to the right area first, and allowed to be seen. If a supply ship was to be sunk, it was found by a submarine that appeared to have stumbled on it. Every use had to be given a cover story.
This was maddening for the people doing it, and occasionally worse than maddening, because it meant knowing things and not always being able to act on them.
Estimates of what it was all worth vary, and anybody who gives you a confident figure is guessing. The most commonly quoted judgement is that the work shortened the war in Europe by around two years. Even the cautious estimates are measured in months, and months at that scale mean a very large number of people who did not die.
The Atlantic is the clearest case. Reading the orders that sent the wolf packs out is one of the several things that turned that battle around in nineteen forty three.
Turing's later life was not kind, and the series will come back to this properly at the end. In nineteen fifty two he was prosecuted for being a gay man, which was then a crime in Britain. He was stripped of his security clearance and forced to undergo hormone treatment, and he died two years later, at forty one. In two thousand and nine the British government formally apologised, and in two thousand and thirteen he was granted a posthumous pardon. He is on the fifty pound note.
None of this was known to the public for a very long time. The Bletchley story stayed secret until the nineteen seventies. Turing died having been publicly disgraced for who he was, by a country that was not allowed to know what he had done for it.
The huts are still there and you can walk through them. Hut Eight has been restored to look as it did, and it is much smaller and shabbier than anybody expects.
Here is something to think about tonight. The bombe did not find the answer. It found the answers that were impossible and threw them away, and left the rest to a person. Is ruling things out a different kind of thinking from working things out, or the same kind wearing a different coat?