---
title: 'How Do You Keep 8,995 Codebreakers Secret?'
source: 'https://youtube.com/watch?v=tDLbO9KeddY'
video_id: 'tDLbO9KeddY'
date: 2026-08-24
duration_sec: 1584
channel: 'Tom Scott'
---

# How Do You Keep 8,995 Codebreakers Secret?

> Source: [How Do You Keep 8,995 Codebreakers Secret?](https://youtube.com/watch?v=tDLbO9KeddY)

## Summary

This video explores the incredible story of Bletchley Park, the top-secret British codebreaking facility during World War II. It explains how thousands of people worked there in complete secrecy, the technical challenges of cracking the Enigma code, and the cultural, legal, and personal reasons that kept the secret for decades.

### Key Points

- **The Scale of the Problem** [00:46] — Bletchley Park intercepted Axis communications and employed thousands of codebreakers. The challenge was immense due to the complexity of the Enigma machine and daily-changing settings.
- **How the Enigma Machine Works** [01:15] — The Enigma is an electromechanical machine, not a computer. It uses rotors and a plugboard to create a polyalphabetic cipher. Each key press changes the rotor positions, so the same letter encrypts differently each time.
- **The Enigma's Weaknesses** [03:30] — The plugboard added 150 trillion combinations, but a key weakness was that no letter could be enciphered as itself. Codebreakers used 'cribs' (known plaintext) to find potential locations in messages.
- **The Bombe and Human Support** [05:40] — The Bombe machines mechanized the brute-force work, but they required many human operators. At peak in January 1945, 8,995 people worked at Bletchley Park, with about 7,500 on site.
- **Secrecy and Billeting** [06:20] — Workers were billeted in local homes, hotels, and pubs. The government requisitioned spare rooms, giving extra ration coupons to hosts. Every spare bedroom in Bletchley had a codebreaker.
- **The Need-to-Know Rule** [08:18] — Workers couldn't talk about their work outside their immediate team, even in the canteen. The codebreaking process was broken into a production line of basic tasks, so most people didn't understand the big picture.
- **The Boredom of the Work** [10:13] — Despite the importance, most work was repetitive and boring. The organisation monitored absenteeism and sickness levels, interpreting them as morale issues. They even played 'Music While You Work' to help with repetitive tasks.
- **Punch Cards and Early Computers** [12:05] — Bletchley used punch cards, storing 80 characters per card, as precursors to computers. The process was a 'mechanical and woman-powered computer' programmed by chalk on the floor, processing data in bulk.
- **The Industrial Phase and Building Design** [15:13] — By 1942, Bletchley moved into larger, robust buildings. Floors were colour-coded: red for circulation spaces, grey for private areas. This helped enforce the need-to-know rule.
- **Intelligence Output and the Vatican** [17:27] — Intelligence was sent to military commanders via teleprinter. They intercepted and decrypted traffic from multiple countries, including neutral traffic. The Vatican was a key source of information, including reports about the Holocaust.
- **The Myth of Acting on Intelligence** [18:37] — A common myth is that Bletchley always acted on decrypted intelligence. In reality, acting on it would reveal that codes were broken. They often used cover stories, like a 'lucky' spy plane finding a convoy, to hide the source.
- **Three Reasons for Secrecy** [19:49] — 1) Cultural: 1930s society accepted government secrecy. 2) Bletchley Park didn't seem special—many country houses had similar wartime roles. 3) The Official Secrets Act and threats of prosecution kept people quiet.
- **The Secret Comes Out** [22:24] — The secret was revealed through books published in America and France. The government started releasing documents in 1977 and formally acknowledged the work in 2009. Many veterans still chose not to talk about it.
- **The Emotional Appeal** [23:31] — Posters and documents used emotional language to reinforce secrecy, warning that indiscretion could endanger loved ones. The message was that their safety depended on their ignorance of the work.
- **Personal Incentives for Secrecy** [25:09] — Many workers had personal reasons to keep secrets, such as family members in the military. The fear of being the one to give the game away was a powerful motivator.

### Conclusion

Bletchley Park's secrecy was maintained through a combination of cultural norms, the unremarkable appearance of the site, strict legal enforcement, and powerful personal incentives. The story highlights the immense human effort behind the codebreaking achievements and the lasting impact on those who served there.

## Transcript

You may know it already, though, the tale of the code breakers that cracked the Enigma code in World War II that I couldn’t add anything there.
of a few boffins working into the night, isn’t anywhere near the whole story. and then there’s many, many other wartime buildings next to it,
There were thousands of people working there, how on earth did they keep it secret? I met a couple of the team at Bletchley Park.
the painstakingly recreated offices. that I’m very thankful for: Let’s set up the scale of Bletchley Park’s problem first:
the Allies could intercept some Axis communications, that were passed on to the code breakers. Lily, the learning manager at Bletchley Park showed me
It’s an electromechanical machine. It’s not a computer. So, it does run on a battery. So, we’re going to turn it on to “hell”, which is bright.
And would you like to press a key and see what happens? On an original Enigma machine! That’s it. So “T” has lit up.
Absolutely. So, one rotor, the one on the right, always turns. turn sporadically based on how the machine has been set up.
So, if you now press X again, (clunk) So, we’re seeing this is a polyalphabetic cipher machine.
No integrated circuit boards, no chips, just wires and connections. Some of them unchanging in the machine,
There was a plugboard at the bottom with interchangeable connections -Can we see inside?  -We can, yep. There we go.  -(Tom gasps)
(clunk)  There are a lot of wires So, each time it’s going through a rotor, it’s following a wire from one contact to the other.
because at least one rotor is moving, hence why a different letter lights up on the lamp board. Enigma was originally intended for use in banks and private enterprise,
It wasn’t secret Bletchley Park had Enigma machines during the war, The trouble was that the cipher was so complicated,
and the settings were changed every day. so, originally there was only the three rotors in the machine They added the two additional ones for this type of machine,
The plug board adds 150 trillion sort of options to the combinations. If you don’t know the settings for the day and they change every 24 hours,
you can set up this machine...  -Right. 'cos you change the plug board,  -Yeah. Yep.
That was the scale of the problem. the mathematicians recruited through old-boy networks and recommendations, and famously, once a newspaper ad looking for good crossword solvers.
It was to figure out the weaknesses in the encryption, for whatever encryption systems Germany, Italy or Japan were using. is that no letter can be enciphered as itself,
and C is never going to appear on the lamp board. The thing about encryption is that it feels like such a tiny weakness. You know, C is never going to be C...  -Yeah.
if they have an idea of what might appear in a message, or a station that was always sending nothing to report every day But if you’ve got that word, of course in German,
you’re looking for places where your "crib", as we call it, because you know then you’ve found a potential location.
It was just, figure it out. But eventually you just have to brute force everything until words come out. Sometimes those clues accidentally came from the people who were using Enigma.
Sometimes it was a hunch or even a lucky guess. But a huge amount of work was basically brute-force, lining up cribs, some by early electronic systems at Bletchley Park, the Bombe machines.
But that mechanisation required a lot of humans to support it, and those humans had to stay quiet about it. there were only about 180 of them,
And at peak, which is in January 1945, there were 8,995 people working in the organisation, so...
and of those, about 7,500 are on this site. So, every day, three times a day, 'cos they’re on coming into the site and going off the site again.
and one of the most qualified people and the incredible secrecy required. Initially everyone was billeted, as the term went, in Bletchley.
So, billeted is basically the government comes along and says, Sometimes it’s in hotels and pubs,
“Do you have a spare room? Can you take somebody?” And then they get an extra ration coupon, they'd get a little bit of money, so... So, basically, every spare bedroom in Bletchley had a code breaker in it.
the Army and Air Force built camps behind the park, and the Navy requisitioned nearby country houses like Woburn Abbey which became dormitories for the Women’s Royal Naval Service, the Wrens.
We have a lot of testimony from Wrens who were billeted in Woburn Abbey for example, but the Duke of Bedford basically hid everything of any value in the cellar, -Right!  -And then the Navy...
Which I’m laughing about, but that’s not a pleasant place to be. The house is actually a good mile and a half, two miles from Woburn Village. And so, every Sunday morning they had to form up in fours and march to church
It sounds very glamorous and luxurious, but actually it wasn’t. No, it really isn’t. Like a lot of things, it’s not a nice place to be, that, So, how do the armed forces get thousands of people
Driven, or they cycled, walked. Oh, right. Well, they had buses, they had cars, they had trucks.
because they’re a Foreign Office organisation, from wherever they could get it. it would be close to the top of the list of bombing targets.
did they know what the big picture was? The rule was that you weren’t allowed to talk to anybody about what you did outside your immediate team,
Even if you went to lunch in the canteen, which was over there, you couldn’t talk to people from other parts of the organisation about work. And also, a lot of the code-breaking processes
that had been developed to attack Enigma or Japanese codes or whatever it was, but that process was broken down into a production line of fairly basic tasks.
would be brought in and they’d be kind of put in a room, and pieces of paper with mysterious stuff on them would come in and then they’d pass those papers out again.
and never getting it marked. (laughs)  -Oh! yes, they knew what they were doing, but what they were doing in the big sense.  -Yeah.
that it was to do with signals intelligence, you’re not going to know anything about Japanese. Right, because you’re just getting characters in,
Yes. Here’s some numbers, add these numbers to these numbers and then put them in this tray, you know? And by the way, this is vital to the war effort and you must do it for eight hours.
-Exactly.  -Wow! you must plug up and unplug this machine every ten minutes. it must have been so wonderful to work at Bletchley.
No, it was probably really boring for most people. I mean, we have paperwork that show that the organisation were very careful in monitoring absentee levels, sickness levels and that kind of thing.
It being the 1940s, they interpret absence in the context of morale. that’s not because they’re sick, it’s because they’re miserable. -(laughs) Okay!  -So, an example of this was...
There’s gonna be tourists, there’s gonna be someone with a lawnmower One wonderful example is the building we’re coming up to, so, he introduced Music While You Work.
I mean, Music While You Work was literally a programme They specially selected music that they thought had rhythms ...with repetitive processes.  (laughter)
so, they opened the windows. who ran the Naval Section in the building that’s right next door, and all this racket coming from he building next door was unacceptable,
Bletchley Park isn’t really associated with doing repetitive work, the “thing-ummy-bob that’s going to win the war”, Most of the people there were on a production line,
Block C, which is now our entrance to the café, These are sort of the precursors to computers.
And basically you have a card about that size And you can store 80 characters of information on a card.
Oh, this is the reason why to this day It comes from the old punch cards. You would put your initial lump of data on cards into one machine,
and you put it in another machine And Freddie Freeborn who ran the section for a particularly complicated Italian job that they were doing,
from the outputs to the inputs, (gasping, laughter) Literally it’s a mechanical and woman-powered computer...
...programmed by chalk on the floor.  -Wow! What this site does is it sucks in data at one end, It processes that data in all kinds of different ways,
decoding it, translating it, indexing it, cataloguing it, you know, whatever, actionable intelligence for government and army and everybody else. And it does that very, very quickly and for the period, massive bulk.
So, how do you keep that secret with so many people involved? and also, after this break. is an incredible thing for today’s sponsor to ask.
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You can join their mailing list and get their free career guide, So, into the Blocks, a museum now, but back then, it was where the codebreakers were working.
so I swapped cameras, I did my best. David is very tall. during what we refer to as the industrial phase of the site
-Industrial?  -Yes. And... But that’s really only ‘39, ‘40, ‘41. By 1942, they’re starting to move into these much bigger buildings
which means if a bomb lands outside They’re not completely bomb-proof, but they are pretty robust. While it’s a little difficult to see, the floor changed from red to grey.
Red meant a circulation space, people were free to move about there. Grey meant private: if they don’t work there, they must not enter. These are the original floors, they’re built into the building, yes.
So, all the corridors are red and all the rooms are grey, basically. And this was all constructed to order...  -Yes. -Right.  -We’ve actually knocked these holes
All of these would have been individual compartments, basically, There used to be buildings like that all across the country, about the hastily-constructed wartime stuff.
The Bletchley Park Blocks became a teacher training college, Alan Turing’s office, which is now painstakingly reconstructed
that was a dormitory for a while, because Bletchley Park was still secret for decades. again, now a museum with interactive exhibits,
but this is where maps of the war would have been managed in real time, could be a code-breaking clue, you can figure out whether it’s low-priority
and you need to tell the armed forces about it. you say, did people know what they were doing? The women in here would have known exactly what was going on
in that respect because most people, So those women are unusual in that they get an insight on the war.
The intelligence factory’s output was sent to the military commanders by teleprinter. Their codes are being attacked. The Italians, the Japanese, the Germans, the Hungarians, the Romanians.
And they’re also intercepting and decrypting neutral traffic. It’s a very important place, the Vatican, because it’s kind of a crossroads of knowledge and information.
-Right.  -Pencil and paper. Code books. The Vatican was neutral, Rome was occupied but the Vatican never was.
the Pope is in contact with Mussolini, But also more specifically, there are Catholic reporters providing information about the Holocaust.
know some of the things they know about what’s happening. Well, they do. No, they didn’t.
one that appears in fiction about Bletchley Park over and over again: because acting on it would reveal that the codes were broken.
That is a much, much rarer occurrence, if at all, than the myth suggests. that doesn’t reveal that it comes from message traffic. A lucky spy…a “lucky” spy plane that happens to come across a convoy.
they’re breaking Italian codes on a daily basis So, the British are attacking the convoys across the Mediterranean. But what it means they can do
to a grid square that just happens to have a convoy in it. and the RAF reconnaissance squadron don’t know that, So, the Italians just thought we had many more reconnaissance aircraft
On the wall in the next room was the answer to my first question: to the point where a lot of the historic site was abandoned How did Bletchley Park stay secret?
Three reasons. The first is cultural. In the 1930s, the culture that we have now of “the right to know” If the government didn’t tell you something,
I mean, it was a class-riddled society as well, you know. and people didn’t bother about it. Secondly, as soon as you have a war going on,
and they’re much more aware of not asking questions. we could all be taken over and forced to speak German. So the threat is real.
The second reason is that at the time, Bletchley Park did not seem special. is by no means unique in the UK.
will have "Trolley Park at war" as part of their display, And some of it was stores, some of it was training, So almost every country house in the country
So, Bletchley Park wouldn’t have really stood out from that point of view. except that it had these slightly peculiar people coming in and out. is that when someone came to Bletchley Park,
the Official Secrets Act, the laws that set out the penalties for leaking government information. 'cos they think by signing it, you’re bound by it.
If I told you a government secret and you then blabbed it to someone, just by the nature of the information. particularly if you’re not working for the government.
and know that blabbing it would harm the country. it was made clear that leaking anything was a very serious offence. You also hear stories, although pinning them down exactly is difficult.
and threats of prosecution for treason and all sorts of things being made, take it to your grave," etc, etc.
They may have taken it to their grave, but the government… The secret came out because books were published in America and in France And so, the government decided in 1977 to start releasing documents.
formally by the government in 2009. And a lot of them, even despite that, choose not to talk about it. but they won’t talk about the work.
And there was a definite sense among some of the veterans of betrayal, And they feel a bit like, you know, So, I kept the secret even from my husband,
So, it’s definitely a mixed feeling about that. But also they have the problem that when the war ends Especially the men.
If their children asked them, “Dad, what did you do in the war?” You might have decrypted the intelligence but you can’t tell anyone that.
“We Also Served”. so, the folks in charge thought of something more…emotional.
And one of those documents is on this poster. And this distils what the Official Secrets Act demands that the ordinary person would understand, you know.
Do not talk in your billet. This one has a phrase in it which I particularly love. It says, “If you are indiscreet and tell your own folks,
They are not in a position to know the consequences Moreover, if one day invasion came, as it perfectly well may,
Nazi brutality might stop at nothing to wring from those that you care for the secrets that you would give anything then to have saved them from knowing.” “Their only safety will lie in their utter ignorance of your work.”
don’t tell her what your job is. Someone in a government department in the 1940s and it clearly paid off.  -Yeah, and it’s not an exaggeration.
but it’s serious. It rams home the point. however slightly, the great trust placed in me,” and then you sign it.
And if you’re an 18-year-old Wren who last week was in high school, you know, And for some, it wasn’t that threat or that knowledge of the law.
There are certain examples of veterans who… Their concern wasn’t necessarily prosecution or anything else. You don’t want to be the one that gives the game away.
And if you think many of them had family members, you know, if your boyfriend’s a pilot, you’ve got a very strong incentive to keep secrets secret.
And so it’s one of those questions I get asked all the time, that actually you have to put your mind in the frame of somebody who is in the 1940s
And we’re very lucky that most of our later generations But I’m sure if we were again, Next time: high tech wheels that creak and groan
across the surface of another planet.