On your first day at Bletchley Park you were shown into a room, handed a document, and asked to sign it.
It was the Official Secrets Act, and the officer who watched you sign it usually said the same thing, in one form or another: that you were now bound by it for life, that the penalty for breaking it was severe, and that if you ever spoke about this work to anybody at all, you would be responsible for the deaths of the people it protected.
Then you were taken to a hut and given a job, and in most cases you were not told what the job was for.
By the end of the war about nine thousand people had signed that form. Roughly three quarters of them were women, most in their late teens and early twenties.
Not one of them talked. Not during the war, and not for thirty years afterwards.
The place was deliberately built so that almost nobody could see the whole picture.
You worked in one hut. You did not go into other huts. You did not discuss your work with the person you shared a billet with, even though she also worked at the Park, because she was in a different hut and it was none of her business.
The women running the bombes knew they were setting drums according to a menu and reporting when the machine stopped. Many of them did not learn until decades later that the machines had anything to do with Enigma.
A girl in Hut Six might spend a year handling German army traffic without ever being told what happened to it afterwards. Another might index names and place names onto cards, thousands of them, with no idea that the index was one of the most valuable objects in Britain.
This was not unkindness. It was the design. A secret known in fragments cannot be given away whole, even by accident, even under pressure.
The cover stories were thin and everybody used them anyway.
You said you were a clerk. You said you did filing for the Foreign Office. You said it was boring and changed the subject.
This produced situations that people found genuinely painful. Young women were asked by neighbours and relatives why they were not doing something useful for the war, when other girls were in factories or on the land or in uniform. Some were handed white feathers, the old symbol for cowardice. Fathers who had fought in the previous war expressed disappointment. And the answer, every time, was to say nothing.
One woman recalled her mother telling her, years later and with real bitterness, that the family had been ashamed of her. She had been decrypting German naval signals at the time.
There is a well-known story, and it is true, of a woman whose brother was killed at sea, and who had handled signals relating to the action he died in, and who sat through the family funeral unable to say a word about any of it. Several versions of this happened to several people.
Others could not explain, when the war ended, why they had no job to go back to and no skill they could name on an application form. They had spent five years doing something they were not allowed to put on a curriculum vitae.
The war ended and the secret did not.
The machinery was broken up. Most of the bombes were dismantled and the drums scrapped, and the people who had operated them were told to go home and never mention it. Some of them watched their own machines being taken apart.
So they went home and had ordinary lives. They married people they could not tell. They raised children who grew up believing their mother had been a filing clerk. Couples who had actually met at Bletchley sometimes did not discuss it with each other, because the rule had been that you did not discuss it, and thirty years of not discussing something is a very hard habit to break.
Several of them said afterwards that the strangest part was the histories. Books about the war came out through the fifties and sixties explaining how a battle had been won by luck, or by a brilliant guess, and they read them, and knew, and said nothing.
There is a practical reason the secret held after nineteen forty five, and it is not a noble one.
Britain had a large stock of captured Enigma machines, and it distributed them. They went to other governments, some of them in countries Britain had recently governed, on the recommendation that they were an excellent and thoroughly secure way of protecting official communications.
Which they were, unless you happened to be the country that had spent six years learning to read them.
So the silence was maintained partly because the machines were still in use, and telling the world that Enigma was breakable would have shut off a source that was still running.
The wall came down in nineteen seventy four, when a former officer published a book about it, and the government decided not to stop him. Slowly, over the following years, veterans were told they could speak.
Even then it was partial. The existence of Colossus, the electronic machine built at Bletchley to attack a different and much harder German cipher, stayed classified for another two decades on top of that, which is why the men who designed it watched other countries be credited with inventing the electronic computer and were not permitted to say otherwise.
A great many of them found that they could not, or would not. Interviewers in the nineteen nineties met people in their seventies who still said, politely, that they had signed something and would rather not.
The popular image is of eccentric professors, and there were some.
But recruitment cast a much wider net than that, partly out of desperation. The Daily Telegraph was persuaded to print a fiendish crossword and to invite readers who could complete it in under twelve minutes to write in. Those who did received a mysterious letter inviting them to an interview.
Chess players were recruited, including several of the strongest in the country. So were linguists, papyrologists, a champion bridge player, and the man who ran the crossword club at a large department store.
And in overwhelming numbers, the place was staffed by young women who had answered an advertisement, been interviewed about nothing in particular, and found themselves on a train to a station they had never heard of.
One of them described arriving at midnight, being met by a driver who would not say where they were going, and being shown into a wooden hut where somebody handed her a stack of paper covered in five-letter groups and told her to get on with it.
The best-known single test of the silence came in nineteen seventy four, when the story broke and journalists went looking for veterans. What they mostly found was people who wanted to check first whether it was really allowed.
There is a memorial at Bletchley now, and there was eventually a commemorative badge, issued in two thousand and nine, sixty-four years late. Many of the people it was meant for had died. Families of veterans have described opening the envelope and finally understanding what a grandmother had done.
It is worth being precise about the scale of what was kept quiet. Nine thousand people, most of them young, most of them not career civil servants, for thirty years, with no supervision after nineteen forty five and no realistic way of catching anybody who talked. There is no other example quite like it.
Here is something to think about tonight. Every one of those people could have told one person, just one, and almost certainly got away with it. Nearly none of them did. What do you think actually stopped them, once the war was over and nobody was watching?