Bletchley Park

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It is March 1941.

A woman called Mavis Lever leans over her desk in an outbuilding known as the Cottage at the Bletchley Park Estate in Buckinghamshire, 50 miles northwest of London.

Rain taps at the windows, and a cold wind shakes the panes.

Mavis's eyes flick across a seemingly random string of letters she's working on.

She's just 19 years old, a linguist turned codebreaker, but she's already familiar with these strange patterns produced by the Enigma machines.

Today, she is working on a test message for a new Italian naval cipher.

She taps her pencil lightly against the desk as she concentrates.

In the cold room, The steady tick of a clock on a wall behind her is a reminder that time is always against them.

The Enigma key settings will shift again at midnight.

As she stares at the letters, a nagging anomaly tugs at her attention.

Then, suddenly, it swims into focus.

Mavis blinks, then checks, and double-checks.

There it is.

There is not one single letter L in the entire sequence.

The Enigma machine can never encode a letter as itself, which means an an operator could never press L and get the same letter back.

But here is a relatively long message that avoids L entirely, which sets alarms ringing in Mavis's head.

Often enemy operators send decoy messages to delay allied codebreakers, and rather than making up a message, they just hold down a single letter to fill the space.

The way the board is wired, repeatedly pressing the same results in a string of different letters in the cipher, but never the letter actually being depressed by the German operative.

What if this operator has simply been hammering the L key to fill out his message?

If he has, he has effectively given away the encryption sequence for the day.

Galvanized, Mavis swiftly fetches a colleague from Hut 6 nearby, and he's at her desk in minutes.

With the clue they have and plenty of what passes for coffee, they dive into the machine settings, testing plugboard swaps and rotor positions.

It's painstaking work, but it helps them to break another message that now arrives.

Today's the day, minus three,

it reads.

Something big is coming.

No one in the cottage sleeps much over the next three days as the team waits for the next piece of news.

Mugs grow cold beside half-eaten sandwiches.

Each day brings new settings to be broken again, but with the clues gleaned from the previous day's repetitions, the locks are easier to pick now.

Then, at last, on the third night, the message they've been waiting for arrives.

Mavis watches as the intercept unspools before them, her breath held.

They are detailed plans for an attack on a Royal Navy convoy carrying supplies from Cairo to Greece, complete with a full timing schedule.

The intelligence is swiftly forwarded to the Admiralty in London, which in turn relays it to Admiral Cunningham, the commander of the Royal Navy's Mediterranean Fleet.

Armed with this crucial insight, Cunningham seizes the opportunity to turn the table on the enemy, saving the lives of the British sailors on board and winning what becomes known as the Battle of Cape Matapan.

It is claimed by the Prime Minister Winston Churchill as the greatest victory since the Battle of Trafalgar.

And it all began with an absent L.

Once just another grand English country estate, in the Second World War, Bletchley Park became a crucial top-secret headquarters that changed the course of the conflict.

The heart of the government code and cipher school, it hosted some of the UK's top minds as they worked to crack enemy communications.

Most famously, the German Enigma cipher.

It is estimated that the breakthroughs at Bletchley shortened the conflict by at least two years, saving over 14 million lives.

But why was Bletchley Park chosen?

And what was life like there during the war?

Who were the brilliant scientists working tirelessly behind its walls?

And when did the veil finally lift on the shadowy world of wartime intelligence?

I'm John Hopkins.

From the Noiser Network, this is a short history of Bletchley Park.

In 1883, Sir Herbert Leon, a financier and Liberal Party politician, is looking for the perfect country retreat for his family.

Eventually he finds just the spot, a red brick mansion in Buckinghamshire called Bletchley Park.

Sir Herbert becomes the MP for Buckingham.

He holds summer fetes and parties in the gardens of the Grand House.

The sports pavilion is used by the local cricket and hockey club and as the headquarters of the Liberal Party during elections.

During his time at Bletchley, Sir Herbert expands it, adding a grand ballroom and billiard room and a turret to the roof.

But when the estate passes to his children, they decide to sell it for development.

Much of the land is converted into smaller plots, but the main mansion, with its small lake at the front, remains unoccupied until 1938.

Then, in September that year, a small group of around 20 people arrive.

They have the air of friends enjoying a relaxed weekend at a country house and tell locals they're here for the pheasants as members of their friend Captain Ridley's shooting party.

They've even brought one of the best chefs from the Savoy Hotel to cook their food.

But it's just a cover story.

The group, in fact, includes members of the Government Code and Cipher School, or GC and CS, a secret team of codebreakers.

Founded in 1919 after the First World War, the GC and CS is Britain's central code-breaking agency, a joint initiative between the Admiralty and Army's experts in the field.

The mansion in Bletchley has been acquired by Admiral Hugh Sinclair, head of the Secret Intelligence Service.

With his eye on events over in Germany, where Hitler is escalating his territorial demands and focusing now on Czechoslovakia, Sinclair knows that war is on the horizon.

As such, he has been seeking a secure base for operations outside of London, far enough to evade the anticipated air raids on the capital, yet close enough to remain within strategic reach.

Bletchley Park fits the bill perfectly.

It is also well placed for visitors from both Oxford and Cambridge.

An important point given that academics will be among the most sought-after code breakers.

But though Sinclair is sold, persuading the government to buy it wasn't without its problems.

British historian, writer, and broadcaster Tessa Dunlop is the author of The Bletchley Girls.

Government bureaucracy then was much like government bureaucracy today, very, very slow.

And the head of the secret intelligence service, the SIS, which would nowadays be called MI6, Admiral Hugh Sinclair, realized that they weren't going to get their hands on this piece of prime real estate with accommodation unless he did something about it.

So he actually coughed up his own money, some £6,000, to clinch it for the GCCS.

The staff who start to arrive in September 1938 comprise the bulk of the GC and CS and a number of sections of the Secret Intelligence Service, better known today as MI6.

But as time goes on and the numbers swell to around 70, the shooting party cover story proves unsustainable.

It is quietly replaced by a more plausible claim that the estate has been requisitioned for air defense operations.

After just three weeks though, the project is abruptly paused.

The so-called Czechoslovakia crisis appears to have passed.

Hitler is allowed to annex part of that country, and war appears to be averted.

Prime Minister Neville Chamberlain returns from Munich, confident that he has brokered what he calls, peace for our time.

For now, this means a respite for the codebreakers, and many return to their homes.

In the meantime, the GCNCS begins discreetly enlisting mathematicians, linguists, and classicists as emergency recruits if war breaks out.

Veteran cryptanalysts from the First World War form a core part of the team.

But Alastair Alastair Denniston, head of the GCNCS, is also keen for new blood.

Alastair Dennison was a creative man.

He was somebody who really I think we can give credit to for recruiting the university cohort of Bothins, of code breakers.

So something of a visionary.

If you think Bletchley Park, this was a blueprint.

It was a prototype code breaking nexus, the like of which hadn't really been done before.

And he was an original thinker, Dennison.

In creating his list of potential recruits, Denniston is looking for what he calls men of the professor type from Britain's top universities, as well as chess champions, top bridge players, and crossword solvers.

A few of the names on that emergency list have stood the test of time.

such as J.R.R.

Tolkien, though the famous writer never ultimately makes it to Bletchley.

Then there are the mathematicians, some of the most notable being Gordon Welshman and of course, Alan Turing, known by his colleagues as the prof.

I think we generally think of Alan Turing as the most famous of Bletchley Park's great minds.

He was indeed an unprecedented individual, both personally and professionally.

I've talked to women who recall him quiet, modest, eccentric, highly intelligent, although although most of them actually never really spoke to him, let's be honest, or understood the magnitude of what he was doing and what he would subsequently achieve.

But this was an individual who was directly recruited from Cambridge because of his mind.

He was something of a showman in his own way, but he was someone who was able to attract attention and I think therefore must have quite liked some of that attention.

He went round deliberately with a gas mask on to ease his hay fever.

He had a chain that always fell off his bicycle unless at a specific moment in time he cycled backwards.

You know, these were very much the prof idiosyncrasies, which became quite quickly famous.

He was also a wonderful marathon runner, so a very fit man.

And alongside all of this, a great thinker who also was able to share his thoughts.

Before the war breaks out, Turing and the other emergency recruits begin their preliminary training in codes and ciphers.

Some are even told to keep a suitcase packed, ready to be summoned at a moment's notice.

That training proves vital.

When on September 3rd, 1939, Britain and France declare war on Germany in response to Hitler's invasion of Poland.

All the preparations have now been made for the opening of a hush-hush military intelligence center at Bletchley Park, known as Station X.

The very next day after war is declared, 180 people make their way to the Buckinghamshire base.

Station X is open for business.

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Initially, the naval, military, and air sections are based on the ground floor of the mansion, together with the telephone exchange, kitchen, and dining room.

There is also a teleprinter room, where messages are sent and received using the machines that can transmit text-based communications over wires similar to a typewriter connected to a phone line.

The top floor is allocated to the secret intelligence service better known as MI6.

But one of the most important early code-breaking teams at Bletchley Park is housed in a cluster of modest outbuildings known as the cottages in the stable yard.

It's here that Alfred Dilwyn Knox, a classical scholar and veteran cryptanalyst, sets up his base of operations.

The research section he leads includes Alan Turing, who Knox soon reports is producing a steady stream of helpful ideas.

With his high forehead and round glasses, Knox's scholarly demeanor is a little at odds with his boyish nickname, Dilly.

He was the man who established this research department early on to try and work out ways into Enigma.

He actually preferred to work with women.

He found working with code-breaking females preferable to working with code-breaking men.

And they were, and this rather speaks to the times, known as Dillies-Phillies.

The female code-breakers, such as Mavis Lever, Margaret Rock, and Joan Clark, play a vital role in the early efforts at Bletchley.

Many of these women are recruited from universities or through wartime intelligence networks for their skill in mathematics, languages, or logic.

Equipped with just pencils and paper to begin with, the team in the cottages works in utmost secrecy.

In fact, everyone who works at Bletchley is bound by the Official Secrets Act, which means that they cannot breathe a word of what they are doing to another living soul, including others at the park, under threat of prosecution.

Knowledge was imparted to Bletchley's employees on a need-to-know basis.

This was one of the great ways in which they maintained their level of secrecy, because if you don't know what you're doing, you can't really be indiscreet about it.

And of course, the other way was through intimidation, forcing all the individuals or obliging all the individuals who worked there to sign the so-called Official Secrets Act.

Now, different employees have different recollections of what that involved.

Some remember signing something, some remember saying something, some remember reading something, but all of them remember the importance, the gravity of the situation as it was explained.

Other individuals remember saying imprisonment would follow if they were indiscreet.

Others recall a gun on the table.

So I think the one uniform aspect of the park, it was a very, very different experience for an Alan Turing than, for example, it was for an 18-year-old Wren.

But the one commonality was the secrecy.

With everyone in place, the work can begin.

The first step is to access the coded messages they want to decipher.

British listening posts, called Y stations, often situated on the coast, pick up the Germans' encoded messages, which are broadcast over radio in Morse code.

Once intercepted, the messages, usually comprising long strings of seemingly random letters, are written down and sent to Bletchley Park for decoding.

Bletchley doesn't have its own WISE station because of fears that large radio antennae might draw attention from the enemy.

Early in the war, physical delivery is the most reliable method.

Young, armed drivers carry the paper intercepts, punched tape or recordings in containers or sealed pouches to Bletchley Park, where work immediately begins on decoding them.

It is an evening in October 1939.

In a soundproofed hut on the grounds of Bow Manor Hall in Leicestershire, England, a young wireless operator sits upright, pencil in hand.

She is listening to the thin stream of Morse code that is coming through her headphones.

As the signal pulses, she scribbles furiously, capturing the clipped bursts coming from a German naval station somewhere deep in the Atlantic.

She finishes the message, rips the paper from her pad, and holds it up high for her shift supervisor to take away.

The message is locked, coded with today's intercept number, sealed in a brown envelope and sent on its way.

The clock is ticking.

Outside, a dispatch rider waits in the October drizzle, the engine of his motorcycle idling.

The moment the envelope is handed over, he slips it inside his oilskin pouch, zips it shut, and guns his bike into motion.

It is just over 60 miles to Blesky Park, but the path is drilled into his mind.

The motorcycle growls beneath him as he tears down the narrow country lanes.

The trees and hedgerows whip past him in a blur, and rain spits sideways, streaking his goggles.

The road is slick, but he leans into the curves with practiced ease.

He passes through a red brick village, its windows darkened and the pub boarded up.

His headlamps are partially blacked out too, but he's learnt to navigate by whatever light is available along the way.

As he exits the village, suddenly the flash of a torch cuts across the lane.

He breaks hard, tires screeching as the bike skids sideways on the damp road.

The rider slams one boot down to steady himself.

Two shadowy figures step out from the verge, hands raised in warning.

Instinctively, The rider's gloved hand flies to the oilskin pouch strapped to his chest, then to the holstered revolver at his hip.

All dispatch riders are armed and trained to defend the intelligence they carry with their lives.

But as the men approach, he recognizes the uniform of the home guard.

He identifies himself and the pair wave him on.

It's not an enemy ambush after all.

Making up for lost time, he moves more quickly now.

passing through more quiet market towns as he heads southwards.

At last, the roof of Bletchley Park begins to rise in dark silhouette beyond the line of trees that surrounds the property.

The rider pulls up outside the gate near the garage block.

A uniformed guard checks his pass, then, once vetted, the rider slips through the gate and hands the envelope to a runner, a young woman from the Wrens.

She disappears towards the mansion, where the registration room will log its contents and move it onto the breakers.

With barely time to catch his breath, the dispatch rider swings his leg over the bike and turns it around for the return journey to the Y station.

Especially in the early years, much of the code breaking in Pletchley Park hinges on the most famous code system of all, the Enigma cipher.

Invented by a German engineer, Arthur Schabius, shortly after the First World War for securing commercial communications, the Enigma machine was later adopted by the German armed forces in the 1920s.

By the Second World War, Enigma isn't a single device, but an evolving family of machines.

And solving Enigma swiftly becomes the biggest code-breaking challenge of the 1930s.

The Enigma machine resembles an old-fashioned typewriter with a keyboard, above which is a lamp board with a series of lights, one for each letter of the alphabet.

Each time a letter is pressed on the keyboard, the act of depressing the key sends an electrical current through the machine, which lights up a different letter on the lamp board.

For example, pressing an A, the W might light up.

Crucially, Enigma does not encrypt the same letter the same way twice, even within a single word.

This is because rotors inside it advance with each keystroke, constantly changing the internal wiring.

So for example, the word book once encrypted would emerge with the two O's being entirely different letters.

The many parts of the Enigma machine, including rotors, rings, a reflector, and a plugboard, contribute to the encryption.

Each different combination of settings for these parts produces a different encryption.

The problem is the total possible variations number up to 159 million, million, million.

Even with 10 decoders working 10-hour shifts, checking the settings at a rate of one per second, it would take 1.2 trillion years.

Something like 100 times the age of the universe.

A particular combination of settings used for a message is known as a key, and Enigma messages can only be deciphered using this key.

At first, the messages are painstakingly decrypted by hand.

The first successful breaks on German Army and Air Force Enigma traffic occur in late 1939, though the messages of the German Navy prove much harder to crack.

Dilly Knox and his team rely on what are called cribs, essentially educated guesses about what parts of a message might say, which they then test against.

Although the Enigma machine is incredibly secure in theory, human errors on the part of the German operators often introduce mistakes.

For example, some start every message with Heil Hitler, or repeat routine weather reports.

These repetitions give the British valuable clues to crack the code.

Knox also develops a technique called rodding.

A set of cardboard strips or rods representing the wiring of Enigma's rotors are lined up against each other in different configurations to test commonly used words and reveal possible settings.

Finding a combination that encrypts one of these simple test words can then be used to decode the rest of that day's messages.

But despite some success in the early days of the war, It soon becomes clear that a much quicker method is needed.

Not least because the Germans change the settings for Enigma every night when their clocks strike midnight.

There is not much point in reading your enemy's hand if what you're reading is no longer relevant to what's happening in the field.

So you need to be able to read your enemy's hand quickly so that it is operationally useful.

In this challenge, the British codebreakers have a little help from their friends.

Back in July 1939, knowing Poland would soon be invaded, Polish cryptographers revealed to the British and French that they had succeeded in breaking Enigma.

In this task, the Polish codebreakers were helped by a machine known as the Bomber.

Though their methods are now out of date and the way the Enigma messages are encoded keeps changing, the Polish bomber machine inspires Alan Turing and his colleague Gordon Welchman to create a machine they call the bomb.

These electromechanical devices help to find the daily key settings of the Enigma cipher machine.

They are Turing's most famous achievement at Bletchley Park and a complete game changer in the war.

Alan Turing's invention of the bomb, the precursor of which was the Polish-born bomber machine, the first bomb arriving in Bletchley Park in March 1940, would have ultimately a transformative effect in terms of the way in which we were able to harvest Enigma encrypted communications on a macro scale.

So, for example, about 18,000 messages a day come 1944 and the D-Day landings.

Think of a sort of black bookshelf, a giant throbbing black bookshelf full of soldered connections and wheels, an intimidatingly enormous contraption that had to be plugged and deplugged every time it was used.

And instrumental in making it effective was this diagonal board, which Welchman came up with.

And that meant that the Enigma codes and ways into finding the keys for that day, the settings for that day, could be reached within a matter of hours rather than within a matter of days, which, as you can see, is crucial, especially as the Germans are changing their settings at midnight every day.

Named Victory, the first bomb becomes operational on March the 18th, 1940.

Later that year, Turing authors a report detailing the methods he and his colleagues employed to break into Enigma.

At Bletchley Park, this reference manual becomes known as the Prof's Book.

The bomb machines are primarily operated by members of the Women's Royal Naval Service, known as Wrens, with other women being recruited from universities.

Indeed, in many areas, women become a vital part of the Bletchley workforce, eventually accounting for 75% of the staff.

Members of the Women's Auxiliary Air Force and Auxiliary Territorial Service are employed here in roles ranging from dispatch riders to translators.

You will recall the film The Imitation Game with famously Benedict Cumberbatch playing Alan Turing.

In one of the promotional posters, there's a picture of him and I think four other men and Kieran Knightley playing Joan Clarke, this woman who was sort of romantically connected briefly to Alan Turing.

And it's somewhat ironic because actually the numbers in terms of the gender balance at the park was the precise opposite of that poster.

So certainly by 1942-43, you had four women to one man approximately operating at the park.

And I suppose there was a key difference in their roles.

There certainly were women in the code-breaking teams, but it was a predominantly male affair.

And that speaks to the times when it's predominantly men at university, it's predominantly men who are physics and maths professors.

However, we know that there's a chronic manpower shortage across all services by really, very early on.

And we've got a recruitment crisis.

And the answer is to enlist more women.

And if you can't get them to enrol, then you can force them them to enroll because it's a war.

So you have female conscription.

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From April 1940, a shift pattern is introduced to keep the code-breaking efforts running 24 hours a day.

Hardly anyone lives on site, so a steady stream of buses and other transport brings staff from nearby billets and requisition properties, including Woburn Abbey, a nearby stately home where many wrens are housed.

When off-duty, personnel can take advantage of the wide range of clubs and activities, as well as dances and performances at Wilton Hall, a purpose-built venue near the main entrance.

There is a library, a drama group, music and choral societies, and clubs for bridge, chess, fencing, and Scottish dancing, as well as sports like tennis and cricket.

In the early 1940s, a series of numbered wooden huts is built across the grounds to accommodate the expanding operation.

Hut 11 contains the bomb machines used to determine Enigma's daily rotor settings.

Over in Hut 6 is the team working on breaking Army and Air Force Enigma messages.

Once they have completed that task, they place their decrypts into a basket and use a broom handle to push it through a specially built chute to their linguist neighbors in Hut 3 for translation into English.

Crucially, the personnel in one hut are not fully informed about the work of the other, maintaining strict security.

With the bomb machines up and running, the team at Bletchley Park is now regularly cracking Enigma-encrypted communications from the German army and Luftwaffe.

This intelligence, known as ULTRA, plays a role in Britain's defense during the Battle of Britain from July to October 1940, allowing the RAF to anticipate some German operations before they unfold.

But nearby, in Hutt 8, Alan Turing and his colleagues face an even greater challenge.

Despite the progress breaking the Enigma messages of the Army and Air Force, the messages of the German Navy prove more challenging.

In the ongoing Battle of the Atlantic, the German Navy deploys submarines in coordinated groups known as Wolfpacks, which attack Allied convoys at sea, sinking ships faster than they can be replaced.

Frustrated by delays and limited resources, in October 1941, Bletchley Park's senior intelligence officials draft a letter to the very top, a position now occupied by Winston Churchill since Chamberlain's resignation around 18 months ago.

Upon receiving the letter requesting more staff and equipment to tackle the critical naval codes, the Prime Minister immediately responds with the directive, Action This Day.

Once again, resources and personnel pour into Bletchley Park, which is fast developing into a code-breaking factory.

When the United States enters the war in December 1941 after the Japanese attack on Pearl Harbor, the scale of the Atlantic campaign expands dramatically.

Cracking naval Enigma becomes even more vital.

to protect American troops as they move across the ocean.

The German Navy divides its Enigma communications into different settings or keys depending on the type of operation.

Two of the most important keys used during the war are known as dolphin and shark.

Bletschli is able to break the dolphin key used by German surface ships and naval shore stations without too much trouble.

But in contrast, the shark key used by German U-boats operating in the Atlantic, proves much more difficult.

In February 1942, the German Navy adds a fourth rotor to its Enigma machine, and with it, the number of possible settings skyrockets.

Before, the code breakers at Bletchley Park were already facing 150 quintillion potential combinations.

But now, with the upgraded machine, that number jumps to over 180 septillion.

A similar number to the grains of sand on Earth, 130,000 times over.

And that staggering increase in complexity occurs just as the war in the Atlantic reaches a critical point.

Somewhere between 60 and 110 vessels were sunk every month for eight months in 1942.

It's real carnage and there's grave concern.

And this is the one place where Bletchley Park isn't able to even deliver a chink of light, blocked out.

And you can imagine the stress of that within Hut 8 and the likes of Alan Turing Turing having to operate under very stressful conditions.

What breaks the deadlock is a so-called pinch, a risky operation in which Allied forces board an enemy vessel to capture intelligence.

On October the 30th, 1942, the HMS Petard carries out such a mission on U-559, a German submarine.

And before it sinks, they get on and they manage to get out both an Enigma machine and crucially a code book which will show them the way into the so-called Shark Key, this fourth rotor.

And that's transformative.

So by the end, I think it's November 1942, you're back in action, full speed ahead in terms of being able to predict where your wolf packs are going to be hunting your allied convoys.

Because until that point, we're losing about 500,000 tonnage a month.

But even as progress is made with the Shark Key, Bletchley Park is already facing another, even more complex enemy cipher used for the most secret communications of Hitler himself.

In the early 1940s, the British WISE stations began picking up strange radio signals that sound like unreadable gibberish.

These turn out to be messages encrypted by the Lorentz cipher machine.

used for the highest level communications between Hitler and his high command.

Far more complex than Enigma, Lorenz is a much larger and more sophisticated cipher system.

And breaking it becomes one of Bletchley Park's greatest challenges.

It's interesting because the Lorenz machine, if you think your standard Enigma either has three wheels or four, the Lorenz machine had 12.

It was far more sophisticated and it was the cipher-encoded teleprinter communications.

They were much longer.

They went between the high command and all the way back to Berlin and ultimately to Hitler.

So you're able basically to know what the Fuhrer is thinking and the extent to which he has a control over what his commands are doing on the ground.

In the summer of 1941, a German operator makes a crucial mistake in resending a Lorentz encrypted message using the same key setting for both messages.

Gletchley Park's chief cryptanalyst, John Tiltman, seizes the opportunity and manages the first break into a Lorentz message.

Taking up the baton, Bill Tutt, a Cambridge mathematics graduate, begins to study the patterns in intercepted Lorentz messages.

For weeks, he works alone in quiet concentration, prompting curiosity among his colleagues about what he is up to.

But at last, his silence pays off.

Using mathematical analysis, Tutt deduces the structure of the Lorentz machine without ever having seen one.

From mid-1942, intercepted Lorentz cipher messages are punched onto paper teleprinted tape and sent to a special news section at Bletchley, where cryptanalysts manually work to turn streams of gibberish into intelligible German.

However, by 1943, the German operators have adopted more complex encryption practices, making it almost impossible to break Lorentz using manual methods alone.

The code breakers now need a machine capable of handling vast volumes at speed.

The General Post Office, or GPO, is enlisted to help develop an electromechanical solution.

The result is a cumbersome but ingenious device that automates part of the process.

Nicknamed the Heath Robinson, after the eponymous British cartoonist known for his drawings of overly complicated machines, it proves unreliable at high speeds.

To meet the challenge of improving it, engineer Tommy Flowers, working at the GPO's Dolles Hill Research Station, designs and builds a more advanced solution.

Some of Alan Turing's thinking was involved, but ultimately, it is built by an individual called Thomas or Tommy Flowers, who is a humble general post office engineer, and he builds it at personal cost.

He takes out an overdraft to help him fund this invention, this creation.

It's built in Dollis Hill in North London in one of the post office's buildings.

And he's very understated.

He's your original working-class lad, done good.

The result, which is known as Colossus, is the world's first programmable electronic digital computer.

It would be fair to say that the Colossus is really your prototype first computer because it's programmable.

What you have are thousands of vacuum tubes.

They're actually valves.

The first one, I think, had 1,600 of these vacuum tubes.

And latterly, by the time of Child D-Day, the second one arrives on the scene and it's got about 2,400 tubes.

And it's about five times faster than the first Colossus that's operational.

And effectively, those 2,400 valves replicate the pattern of an encrypted Lorenz machine as electrical signals.

And to give you some idea of the scale of this, the Colossus machine could read 5,000 characters a second.

Colossus arrives at Bletchley Park in January 1944 and helps the code breakers to decipher the Lorenz messages in near real time.

To give you an idea of just how understated Tommy Flowers was, the day his machine, his Colossus, achieves its first scientific miracle, he actually writes in his diary: Colossus worked, car broke down on the way home.

So the thing is that

it's almost like this irony is the understated way in which it all takes place in the time, under the cover of darkness, shrouded in secrecy.

The first Colossus is soon joined by others.

A new section is set up to house these computers and the teams of Wrens operating them.

By the last months of the war, the workforce at Bletchley and its outstations numbers around 9,000.

The detailed knowledge gained by human and machine of enemy troop movements is particularly helpful on the lead-up to D-Day.

the Allied invasion of German-occupied France in June 1944.

You have two things that are game-changing deliverables for the summer of 1944 and one is a thorough awareness of the German armed forces and what they represented in the Normandy area in June 1944.

Now information that we had was hugely detailed thanks to information that was harvested by the code-breaking nexus.

Now that includes numbers of troops, vehicles, tanks, as well as maintenance and serviceable information, all sorts of information about where your enemy's at, where he's positioned.

The other really valuable part of what Bletchley Park delivered prior to the D-Day landings was a clear confirmation that the German enemy high command had swallowed the deception structured into Operation Fortitude.

So Operation Fortitude is this ruse that the Germans believe we're going to land in Padukele, which is where they put the majority of their defensive forces to repel our invasion.

Padukele is the very obvious place to land, of course, because it's the shortest distance between Britain and France.

So when you're armed with not only a knowledge of what's happening on the ground in Normandy, but also the sure certainty that the Germans don't expect you to land in Normandy, that encourages you, gives you confidence.

And remember, victory in war is partly born of good or high morale.

Thanks to the misdirection of Operation Fortitude, Key German forces are kept away from the real landing sites in Normandy.

The successful D-Day landings on June the 6th, 1944 mark the start of the Allied push to liberate Western Europe from Nazi control.

Over the following months, Allied forces advance across France and into Germany, leading to the German surrender in the spring of 1945.

And the news is heard at Bletchley Park before the rest of the world.

It is a night in late April 1945.

Deep inside Block H at Bletchley Park, a team of wrens tend to the giant glowing bulk of a colossus machine.

The room pulses with noise.

A high-pitched whirr comes from the reader as it feeds a looping ribbon of punched tape through optical sensors at an astonishing astonishing speed.

The machine hums and clicks, its blinking bulbs lighting the faces of the women who monitor it.

A young wren leans close to the machine, straining to make an adjustment, then shouts across the room to be heard over the din.

It's always loud in here, always hot.

But tonight, something feels different.

The sense of pressure that has filled these rooms for so long is beginning to ease.

It's in the way people glance at each other, the way they smile more readily.

Even the newspapers, always a step behind what they know here, have begun to hint at the truth.

Germany is collapsing.

The end is coming.

Then the door swings open.

A Wren rushes in, breathless, her uniform slightly askew, a small slip of paper clutched in her hand.

It's over, she shouts.

The war is over.

For a moment, no one moves.

Then, one Wren, lets out a loud whoop and reaches for her colleague's hand, and suddenly the whole night shift spills outside.

They hurry across the gravel paths in their sensible shoes, skipping and shouting, hugging each other in celebration.

Someone rushes to the supply closet and returns triumphantly, arms piled high with rolls of toilet tissue.

In minutes, a tree on the driveway is festooned with white streamers, and the women dance around it as if it were a maypole.

Then a voice cuts through the jubilation.

A man demanding to know what on earth is going on.

Everyone freezes as the senior officer, face furious, storms over.

He reminds them that this is a classified sight, and they still have work to do.

The wrens swallow their giggles.

One of them slowly unwraps a wreath of lu-roll from her head and they make their way soberly back to Block H and the whirring colossus.

The celebrations have ended for now, but there will be far more to come.

After the Second World War, Bletchley Park's vital role in breaking enemy codes endures as a closely guarded secret for decades.

In the spring of 1946, GC and CS officially leaves Bletchley Park and moves to Eastcote, northwest London.

Four years later, the organization renamed as Government Communications Headquarters, or GCHQ, relocates again to its current site in Cheltenham.

At Bletchley, many of the wartime huts are left to deteriorate, while the thousands of men and women who worked there during the war remain sworn to secrecy.

It wasn't cast as a women's war in our nation's imagination, so women therefore weren't asked questions.

All the girls or women who I worked with who operated at Bletchley Park and were part of the code-breaking nexus, they don't remember being asked questions about their war because people didn't associate women with war, even though women are being conscripted for the first time ever.

And that helps, of course.

And it also helps that you don't really know what you were doing.

You were checking off crosses on a sheet of paper, were you?

Or maybe you were actually using your mask to do something a bit more exciting, but you don't really have a macro scale idea of what you achieved.

And you didn't have a reference.

You had some kind of vague job description that you worked for the Foreign Office, for example, if you'd been in civilian clothes, or that you were a REN doing some administrative work in Buckinghamshire.

So that doesn't put much pressure on people, does it?

If you think nobody's asking you and you don't really know, so you're not really going to say anything.

For decades, the public learns little of Bletchley Park's extraordinary contribution.

Winston Churchill famously refers to the code-breaking teams there as the geese that laid the golden eggs and never cackled.

In fact, in the early days after the war, the biggest leak threatens to be Churchill himself.

Ironically, given that there was this sort of idea that young women might be garrulous, it's the men and the men much higher up who are more likely to endanger the secrecy of what we achieve at Bletchley Park.

And that goes for Churchill as well as anyone else.

He found it very difficult to write his war memoirs without mentioning a staggering achievement, which he had becked wholeheartedly.

He was compulsorily forced to edit much of his World War II volumes because, of course, they crowed about the achievements of Bletchley Park.

For one of Bletchley's stars, Alan Turing, life proves much harder.

In 1952, Turing is prosecuted for gross indecency related to his homosexuality, a criminal offence in the UK at the time, and undergoes chemical castration as part of his sentence.

He dies two years later on June the 7th, 1954, at the age of 41 from cyanide poisoning.

His death is ruled as suicide.

It isn't until decades later that the government issues a formal apology for his treatment, and in 2013, he receives a royal pardon.

Turing is today recognized as one of the most brilliant scientific figures of the 20th century.

Eventually, in the 1970s, a veil of secrecy begins to lift.

A government-approved book, titled The Ultra Secret, is published in 1974 by former intelligence officer F.W.

Winterbottom.

The first publication to reveal the secret code-breaking work carried out at Bletchley Park, it brings public attention to the remarkable achievements of the code-breakers who begin to have the chance to tell their own stories.

Efforts to preserve the site as a museum and heritage center begin to gather momentum.

And in 1994, the chief patron of the Bletchley Park Trust, the Duke of Kent, opens the site to the public as a museum.

Today, Bletchley Park draws a quarter of a million visitors a year.

Some of them even pop by for refreshments at a local pub called Captain Ridley's Shooting Party.

But Bletchley Park is not only a museum of code breaking and computing history, it is also a lasting tribute to the quiet heroes whose work helped shorten the war and shape the modern world of intelligence and cybersecurity.

We won the war partly thanks to our ability to read our enemy's hand.

I don't think it became decisive in terms of helping victory really until about 42, 43.

Certainly we are reading Enigma codes, encrypted codes from 1940, but that sort of pinch point where it helps us win the war was really from 1942 onwards.

But we can't be complacent going forward about where our genius then is going to take us now.

Because we all know that at any one time the enemy is looking to outsmart us.

And I think that's probably the takeaway of Bletchley Park, that war is not just bullets and beaches and dead men and nowadays dead women.

It's actually something that's often invisible and unpredictable and arguably even more dangerous.

Next time on Short History of, we'll bring you part one of A Short History of the Victorians.

I think the appeal of the Victorians is they are the germ of the ideas that we live with now.

And a lot of the debates that we have now, you can find an earlier incarnation in the 19th century.

But they're also just different enough to be fascinating with their corsets and their funny gloves and hats and veils and rules about everything and anything.

So I think a lot of people see them as modern enough to be relatable, but also strange enough to have a nostalgia for.

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