Original subtitles

- [Narrator] The race for the ultimate weapon.

- Literally the future of the world

was hanging in the balance.

- [Narrator] To stay ahead of the Soviets,

American codebreakers must do the impossible.

- All the computers working for a billion years

would not be able to break that cipher.

- [Narrator] What they achieve reveals a shocking truth.

Soviet spies infiltrating America's most top secret project.

- The Soviets have gained enough information

to build their own atom bomb.

- [Narrator] The US government.

- There is a leak at the very highest level.

- [Narrator] And even the upper echelons

of the British establishment.

- Were you, in fact, the third man?

- It's a story that's stranger than any spy thriller

and the stakes couldn't possibly be higher.

- [Announcer] Two, one.

[bomb explodes]

- [Narrator] These are the codes that changed our world.

Bizarre markings,

random letters,

and numbers,

words that make no sense,

but cracking them unlocks military secrets,

decodes ancient civilizations,

and reveals enemies in our midst.

Now we uncover how they were decoded,

the genius minds that broke them,

and the secrets they reveal.

1943.

In World War II, the tide has turned.

The Nazis are heading for defeat.

In November,

the Allies meet to discuss the conflict's final stages,

but theirs is an uneasy alliance.

- The Soviet Union and America were allied in World War II,

but Stalin is known for his brutality,

his mercilessness,

and the Americans have no illusions about

what's gonna happen when the war is over,

that it's all going to be tra-la-la,

and rainbows, and happiness.

- [Narrator] Fearing the Soviet Union may emerge

from World War II as a superpower to rival America,

the US needs to know what Stalin is planning

and stay one step ahead.

- This is just like a game of poker.

If you know your opponent's cards that he's holding,

well, it certainly gives you a real advantage.

- [Narrator] But the Soviets are keeping their cards

close to their chest.

So US intelligence chiefs have little way of finding out

what they know.

In desperation,

they turn to an ever growing pile

of encrypted Soviet telegrams

being collected in a back room

of the Army's Signal Intelligence Service HQ

in Arlington, Virginia.

Thousands upon thousands of pieces of paper

covered in strings of letters

arranged in seemingly meaningless groups of five.

- So after Pearl Harbor,

as part of a massive intelligence effort,

the US government requires telegraph companies

to turn over to them

every single telegraphic message

sent by a foreign power either to

or from the United States.

And one of the results is they end up with this stockpile

of encrypted diplomatic messages

passing between Soviet diplomats in America

and their counterparts in Russia.

- [Narrator] In the hope these diplomatic telegrams

contain information about Stalin's post-war ambitions,

US intelligence sets up a project to investigate them,

a project called Venona.

The operation is top secret,

but US intelligence is playing a dangerous game.

If the Soviets discover the US is spying on them

before Germany is defeated,

it could jeopardize their alliance

and risk turning the war in the Nazis' favor.

So no one can know that Venona Project exists.

- Even the president doesn't know about it.

Just the whole goal is that

you don't want to tip your hand.

You don't want the Soviets who are your ostensible allies

to know that you're spying on them.

- [Narrator] But the pile of encrypted messages is growing

by at least 4,000 new telegrams each week.

The Venona Project urgently needs manpower,

but America's men are away fighting the war.

So the Army turns to the nation's women.

They'll make up 90% of the Venona Project staff,

and almost all of them are new to the art of code breaking.

- So they came to Arlington Hall,

the messages keep on pouring in,

and they would look at these and try to make sense of it,

but it was overwhelming.

- [Narrator] What secrets do these telegrams conceal

and what can they reveal about Stalin's ambitions?

The first step is easy.

The letters are actually numbers

and the codebreakers know this

because the messages only ever feature 10 letters

rather than the full alphabet.

This is because the telegrams have been sent as Morse code

via regular commercial cable companies,

which require numbers to be converted into letters.

Using a standard and widely available conversion table,

the letters can be quickly converted

into their original form.

Then comes the hard part.

The novice codebreakers must then scour

the strings of numbers for a clue, however small,

that will help them crack the Soviet code.

- They're looking for the merest hint of a repetition,

the merest hint of a shape or structure

that will allow them to unravel the whole message.

- [Narrator] But there's nothing,

because the Soviets have hidden their secrets

behind not one,

but two layers of encryption.

- What the Russians have done is, in effect,

locked the door once,

and they lock it again,

and then they throw away the key.

So you can never break it.

- [Narrator] First, the Soviets encode the messages

with a standard issue intelligence service code book.

- A code is something that involves meaning,

you take one word and you substitute another.

- [Narrator] In the code book,

every word is assigned a unique four digit number.

So for example,

the unique code word or number "Moscow"

might be 3-2-1-4.

But then the Soviets make it yet more complex

by adding a second layer of security,

a cipher.

Unlike a code in which entire words are replaced

by alternative words or numbers,

with a cipher, individual characters,

whether they're letters or numbers,

are changed.

In a commonly used cipher,

one might be substituted for three,

three for five,

six for eight.

Each time the numbers have been moved along two digits.

- We're looking a recipe,

an algorithm, a system,

and every recipe in every system leads to a pattern.

And that's what codebreakers are looking for.

- [Narrator] But the Soviet messages

show no signs of a pattern

because they've been encrypted

using a far more sophisticated cipher,

known as a one-time pad.

- The one-time pad is the greatest encryption system

in the world ever,

forever,

because it doesn't give any patterns in the resulting text.

- [Narrator] The Soviet one-time pads

consist of sheets filled with five digit numbers.

These have been randomly generated,

meaning they contain no trace of a pattern or system.

The random numbers are added to

the already encoded number groups.

- See, if I'm adding something that has no pattern,

then the result will have no pattern.

And if the result has no pattern,

there's no way I can decipher it.

- [Narrator] The only way to decrypt each message

is with a duplicate of the same page

from an identical one-time pad.

- The strength of this code relies entirely

on the fact that you and I are the only two people

that have that sequence of numbers.

And it relies on the fact that

we only ever use that sequence once.

- [Narrator] To make the message even more secure,

the page from the one-time pad is destroyed

as soon as the message has been encrypted.

And the only duplicate page

at the receiving end is destroyed

as soon as the message has been decrypted.

- And that's it.

If you don't have those pieces of information,

you really can't break the code.

- [Narrator] Faced with a seemingly uncrackable code,

US intelligence has no way of knowing what secrets,

if any, these messages contain.

But the fact they've been encrypted with a one-time pad

strongly suggests espionage,

because the one-time pad was a key component

of Soviet spycraft.

- [Film Narrator] Perhaps there is no better proof

of the steady intensity of the Soviet offensive

against the security of the United States.

- [Narrator] In 1953,

a similar single use Soviet code surfaced

in the unlikely setting of a Brooklyn sidewalk.

- [Film Narrator] A young news boy had just finished

the weekly collections from his paper route,

a five cent piece slipped out of his hand

to the pavement.

He was astonished at what he saw.

The nickel had split into two equal segments.

- [Narrator] On closer examination,

the FBI discovered a minuscule hole

in the R of the word "trust,"

drilled so a fine needle could be used

to force the nickel open.

- Somebody obviously went to a lot of work

to engineer this nickel.

- [Narrator] Inside the coin was a tiny piece of microfilm

covered with microscopic columns

of neatly typed five digit numbers.

- This looks strikingly similar

to the one-time pad code

that was used by the Soviets in the 1940s.

The evidence certainly pointed to Soviet espionage.

- [Narrator] But with nothing to compare the numbers with

the message proved impossible to decode.

It would take another four years

to confirm the FBI's suspicions,

when a former Soviet spy, called Reino Hayhanen,

defected to the west.

- Apparently, Hayhanen had been given this coin

when he was first deployed in New York,

but he showed himself to be something less than a master spy

because he accidentally spent it.

And so this nickel with microfilm inside of it

went into the general circulation

and was passed from hand to hand for who knows how long

before that news boy happened to break it open.

- [Narrator] Heyhanen revealed to US intelligence

how the coded message was deciphered.

- The irony is the message wasn't really that important.

It was basically a welcome message

with a few basic instructions for how to get started.

- [Narrator] A decade before

the news boy found the hollow nickel,

the Venona codebreakers aren't so lucky.

They have the coded messages, thousands of them,

but no Soviet defector to help unlock them.

And without the specific page used to encrypt each message,

there's no math and no technology in existence

that can unlock the value

of these randomly generated numbers.

The only way for the codebreakers to crack the secret code

is if someone on the Soviet side makes a mistake.

- A one-time pad can be broken

if it becomes a two time pad.

If you reuse one,

then things are no longer random.

- [Narrator] The Venona team has already analyzed

thousands of messages.

They haven't spotted a single mistake,

but they have to keep looking

because it's becoming clear there's far more at stake

than the balance of power in postwar Europe.

The future of the world itself

may depend on unlocking these numbers.

[bomb explodes]

A year before the start of World War II,

German scientists Otto Han and Fritz Strassmann

set in motion a chain of events

that will change the world forever.

- They do something that is going to

eventually make history.

They split the atom.

- [Narrator] The splitting of one atom

produces around 200 million times more energy

than a traditional fuel source, such as coal.

In theory, this vast new source of power

could be harnessed to produce a weapon unlike any other.

It's an idea that soon attracts the attention

of both superpowers,

America and the Soviet Union.

- Everyone knew this was the technology

that held power in the post-war world.

And so the nuclear race was on.

- [Narrator] Eight months after the attack on Pearl Harbor,

President Roosevelt tasks his most brilliant scientists

with creating an atomic bomb for America.

The Manhattan Project.

- Roosevelt authorizes the single largest,

most expensive,

and most complex secret scientific military project

in history.

- [Narrator] America and Russia are still allies

in the fight against Hitler and Japan.

But the US government knows that an atom bomb

will give America the upper hand in the post-war world.

And they know Stalin will be thinking the same.

- The American government considers it

a matter of highest urgency

to find out just how far the Soviets have progressed,

just how much knowledge they have,

and what their goals are.

- [Narrator] Knowing what the Soviets know

will allow America to stay one step ahead of Stalin

and potentially save the world from nuclear war.

- It's hard to think of anything,

any conflict where the stakes have been higher.

Literally the future of the world

was hanging in the balance.

- [Narrator] While the top secret

Manhattan Project scientists conduct tests

in the New Mexico desert,

pressure is mounting on the equally top secret Venona team

to find out whether there's a similar Soviet atomic project.

A clue comes not from Moscow or Washington,

but from the distant past.

Two and a half thousand years ago

in the ancient city of Persepolis in modern day Iran,

royal scribes developed a fiendishly complex

written language known as old Persian cuneiform.

Archaeologist, and now Venona codebreaker,

Richard Hallock,

has spent years decoding these ancient inscriptions,

searching for patterns and repetitions.

This gives him an eye for detail

and an idea.

- The diplomatic military messages that were conveyed

during the Persian empire,

it had a particular form,

there was a salutation at the beginning of the message

and a sign off signaling the end of the message.

- [Narrator] Hallock wonders if the Soviets,

just like the ancient Persians,

are using a standard phrase to start their messages.

- If that was true, that would give them a big leg up.

- [Narrator] In an age before digital computing,

comparing the opening numbers of all the messages

in search of repetitions will take years,

perhaps even decades,

with no guarantee of finding anything.

So the team turned to one of the few pieces of technology

at their disposal.

- They put all the pages, thousands of pages,

into a primitive IBM computer.

- [Narrator] These mechanical punch card data processors

are widely used in government and business

to collate and sift information.

Each machine can process at least 250 cards per minute,

which makes them perfect for testing Hallock's hunch.

If the one-time pad protocol has been followed correctly,

then no five digit number will occur in the same position

in two messages.

But that's not what the punch card machines reveal.

The machines sift through the cards,

finding messages with the same initial digit,

then the same two digits,

three digits,

until pairs of messages remain,

each pair beginning with the exact same five digit number.

- They crunch the numbers on this

and they decide that it's something like

a billion to one chance that this is just random.

They're onto something here.

- [Narrator] Hallock's hunch is correct.

The Soviets begin each message

with the first five digit number from a one-time pad page.

This tells the recipient who has an identical pad,

which page to use to decode the message.

Because it comes from a unique one-time pad,

which is destroyed after use,

this number should never appear again.

But the Venona team has discovered

the same number appears more than once.

There's only one explanation.

The same one-time pad page has been used

to encrypt two different messages.

Someone on the Soviet side has broken protocol

and made a mistake.

- The factory in the Soviet Union

who made the one-time pads,

for whatever reason,

made a whole set with the same groups of numbers.

So instead of just having two duplicate pads being used,

a number of messages come using the same sets of numbers.

- Often it's little mistakes that give codebreakers

that chink in the armor that gets them in,

that gets them started.

- [Narrator] The Venona team runs more cards

through the machines and gets more hits.

A lot more hits.

In total, more than 35,000 messages have been created

with duplicate one-time pads.

It's the human error the Venona team has been hoping for.

- An impossible mission has suddenly become possible.

We can crack this code.

We can get rid of the encipherment.

- [Narrator] Now the Venona team can try

one of the most reliable techniques

in the codebreaking arsenal,

a brute force attack.

- The brute force attack means you just try everything.

So imagine you've got a bicycle padlock with three wheels

and each wheel runs from zero to nine.

You could create all the numbers from 0-0-0 to 9-9-9.

So you've got a thousand combinations there.

Sooner or later you'll find the right combination.

So a brute force attack would be trying out

every single one of those thousand combinations.

- [Narrator] Find the correct combination

and the bicycle lock will open.

But if the Venona codebreakers do stumble across

the correct numbers,

how will they know they found them?

- A codebreaker is faced with gobbledegook

and apparently random series of letters

or symbols or numbers.

And what they're looking for

is some clue to the underlying structure

or some clue to the underlying meaning.

- [Narrator] The different messages

enciphered with the same pad could contain that clue.

If the original messages contain some of the same words,

for example, "Moscow,"

then a brute force attack on the cipher layer

should reveal the matching code number for "Moscow."

The codebreakers won't know these numbers

represent that specific word,

but the more often these numbers appear

in a brute force attack,

the greater the chance

they're the correct Soviet code numbers.

- They can start finding repetitions, patterns,

and start narrowing down the range of options in the code.

- You can take the encipherment apart brick by brick.

It's no longer an impossible wall to scale.

- [Narrator] Through trial, error,

and educated guesswork,

the codebreakers remove fragments

of the one-time pad cipher,

only to find another wall behind it.

- They've gotten to the underlying code,

but that's still a code they need to break.

They know those numbers represent words,

but to figure out what those words are,

they really need a code book.

- [Narrator] Without a Soviet code book

all the Venona team's hard work will be for nothing

and Stalin's atomic research will remain a mystery,

which is a major problem

because America is about to raise the stakes.

On August 6th, 1945,

America detonates an atomic bomb

in the skies above the Japanese city of Hiroshima.

- The first bomb kills 140,000 people.

Many of them vaporized instantly,

others dying lingering deaths from radiation sickness.

- [Narrator] In a split second of devastation,

America has proved it's capable of making an atomic bomb.

- The dropping of the atom bomb

showed that America was going to be

the world dominating power.

It had this weapon that made them

immediately the most powerful nation on earth.

- [Narrator] The war is over,

but without a common cause to bind the superpowers together,

the former Allies are rapidly becoming enemies,

opponents in a new Cold War.

Now more than ever,

the IS needs to know the Soviets' game plan.

And the Venona team believes it may have

the final piece of the puzzle

to crack Stalin's secret code.

It's come from the enemy both countries were fighting

just months earlier.

- The Americans hear rumors that

there is a castle in Germany

that had been used by Nazi intelligence,

where they've been listening in to Soviet messages.

- [Narrator] Knowing this could be a treasure trove

of intelligence,

in the waning months of the war,

America sent a top secret team to investigate.

- The Germans lead them down into the cellar

and they see boxes and boxes and boxes of information

the Germans have gathered during the war

in trying to break Russian codes.

- It's a treasure trove,

including one of the things that they wanted most of all.

- [Narrator] There among the chaos is the holy grail.

The missing piece of the puzzle

that the Venona team has been hoping for,

a Soviet code book.

But when the book arrives at Arlington hall,

it's been badly burnt and worse,

it's out of date.

- So what does Venona have at this point?

They have a code book,

but they're really still at the base of the mountain

they have to climb.

- It doesn't give any real answers.

It gives a sense of how the Russian code book

would be assembled.

- [Narrator] After two and a half years,

the Venona Project, now 600 strong,

is still nowhere near decoding

the hundreds of thousands of Soviet messages.

They have the charred code book,

but they're out of ideas.

- Once you've got rid of the enciphering,

you still have to get rid of the encoding.

And that's where a man of true genius steps in.

His name is Meredith Gardner.

- [Narrator] In early 1946,

linguist Meredith Gardner joins the Venona Project

and changes everything.

- Meredith Gardner was a loner.

He had grown up in a boarding house in Texas

and started reading foreign newspapers

the travelers would bring into the boarding house.

He then goes on to the University of Wisconsin

and gets his doctorate in German.

By the time he's 23,

he's talking German, Russian,

Yiddish, Spanish,

he teaches himself Japanese,

he knows Sanskrit,

and he's recruited to Arlington hall.

- [Narrator] Gardner proposes something

seemingly impossible,

to reconstruct the current Soviet code book

based on the charred, out of date version.

- It's as if you had a copy of Moby Dick

and you say, well, this is what a novel looks like.

Now go write a novel.

- [Narrator] The talented linguist

decides to start from scratch

and begins to work through the partially deciphered messages

to see if something has been missed.

Within weeks, he spots something

none of the Venona team has noticed before.

- Gardner recognizes that in a lot of these messages,

a five digit number will appear in the text,

and then the same five digit number will appear

a little bit later.

And this happens over and over again.

What do these numbers represent?

- [Narrator] He suspects the five digit numbers

are a signal to the reader to do something different

when decoding the message.

- Since these messages involve the United States,

there might be English words

that don't have an easy Russian equivalent,

maybe names and places and things like that.

Is it possible that this five digit repetition

is some bit of code that says

we're switching to English now,

or, you know, switching out of English now?

That's his hunch.

- [Narrator] As a linguist,

Gardner realizes the Russians won't have an equivalent

for many English names.

So the chances are they won't have a code number

for those names either.

- So they have to break down English words

into something simpler,

some system of substituting certain numbers

for certain letters

in order to spell out the words

that aren't in their code book.

So that's a chink in the armor.

That's a way to get into the code

without having the code book in hand.

- [Narrator] Letter by painstaking letter,

the Venona team concentrates on identifying

the English words spelled out within the messages.

- And if they could find a familiar word

or maybe a noun followed by a verb,

they could start making educated guesses

as to what those things might be.

The odds are if it's a diplomatic cable,

you might have a word like Washington DC,

or President Truman,

or other phrases that you would think

the Soviets would be concerned about.

They're probably not talking about the latest Broadway play.

- [Narrator] It's a process of guesswork,

trial, and error.

- It's an incredibly complex kind of

linguistic crossword puzzle

that requires a deep understanding of language

and how it works

as well as the principles of encryption.

- [Narrator] Three years after the Venona Project began,

Meredith Gardner is finally able to decode

fragments of a Soviet telegram.

Against all the odds,

the Venona Project has achieved

its seemingly impossible goal.

And what the code breaking team uncovers

from this first telegram is disturbing.

The telegram had been sent two years earlier

from New York to Moscow.

It contained thoughts on the potential success

of President Roosevelt in the 1944 presidential election.

- What's shocking to Gardner is

this message isn't about trade,

it's about American politics,

and it seems to have been sent by an English speaker.

Is there a spy or spies working in the American government

and using this Soviet code system

to send messages back to Moscow?

- [Narrator] The first telegram is just the beginning,

as Gardner and the team slowly tease out words

from one telegram after another,

their findings are explosive.

- By focusing on the English components of these messages,

Gardner is coming up with lists of English names and phrases

that are appearing multiple times in these messages.

- [Narrator] Many of these names belong to the scientists

working on America's top secret atomic bomb program,

the Manhattan Project.

- Their identities were classified.

They were all closely watched and supervised.

They did not communicate anything about their project

to the outside world.

They didn't even tell their own families

what they were working on.

- [Narrator] There's only one way the Soviets could know

the scientists' names.

They must have been leaked

from within the Manhattan Project.

- The United States government realized

that the Soviets have spies working in America

and these spies have not only discovered

the existence of the Manhattan Project,

this program to manufacture an atomic weapon,

but they've penetrated it,

and they've stolen secrets.

- [Narrator] Stalin is the last person on the planet

who should have access to information

from inside the top secret Manhattan Project.

But if Gardner's decoding is correct,

he's been receiving reports

about America's atomic bomb program since at least 1942.

- It becomes clear that the Soviets have gained

enough information to build their own atom bomb.

[bomb explodes]

- [Narrator] But what kind of atom bomb?

- The Americans were hard at work at nuclear weapon 2.0,

the big boom.

The hydrogen fusion bomb,

an exponential scale up in the power of these weapons.

- If Stalin were able to get his hands on a hydrogen bomb

before the US,

it would be a complete game changer.

Even if the weapon was never used,

if Stalin was able to threaten the west

with the potential of a hydrogen bomb,

the Cold War would effectively be over

and the Soviet Union would've won.

- [Narrator] The only way to discover what the Russians know

is to identify the spies who leaked the information

from inside the Manhattan Project.

- As Gardner begins to unbutton the messages,

he enters into a whole new world of intrigue.

He sees references to various Soviet agents.

Their names are not revealed,

but their code names are.

He uncovers names like "Rest,"

or "Goose,"

or "Antenna,"

or "Liberal,"

and then Gardner has to try to identify who is Rest,

who is Liberal,

who is Antenna.

- [Narrator] No longer just a fact finding mission,

the Venona Project becomes a man hunt.

- Now, the FBI working with the codebreakers

at Arlington Hall,

have to form a team to track down these Russian agents

already embedded and hiding in America.

- [Narrator] By 1949,

it becomes clear that one Soviet agent in particular

has given critically important secrets to the Soviets,

a spy known as Rest,

and in later messages, as Charles.

- This code name will start appearing

again and again in these transmissions.

And many of them associated with the research

that's going on in Los Alamos.

- [Narrator] But these code names only appear

in messages sent up to 1945.

After that, the Soviets stopped using

the duplicate one-time pads

and the messages become unreadable once again.

In the intervening four years,

the spy could have continued to share

America's most closely guarded secrets,

including research into the next generation

of atomic weapons.

The race is on to find Rest and stop him,

if it's not already too late.

- Gardner's team decrypts a partial telegram

that reveals that he is a resident of the island,

which is the code for Britain.

So that narrows the field considerably.

- [Narrator] Rest, AKA Charles,

must be one of the 15 British scientists

employed in key positions within the Manhattan Project.

- Bit by bit,

they're able to figure out from other messages

that Rest had a sister

who was here in America, in Massachusetts.

So now they know it's a British scientist

who has a sister in Massachusetts.

- [Narrator] Only one of the Manhattan Project scientists

ticks both these boxes,

senior theoretical physicist Klaus Fuchs.

- It turns out that Fuchs has been feeding the Soviets

all kinds of information

from how much plutonium they'll need for the project,

to various design specs,

technical information.

A lot of the essential pieces of the puzzle

they'll need to assemble their own bomb.

- [Narrator] Confronted with the accusation,

Fuchs confesses and spends nine years in prison.

But he wasn't acting alone.

Letter by letter,

message by message,

the Venona codebreakers untangle an entire web of spies

centered on the Manhattan Project.

Caliber is David Greenglass,

a machinist at Los Alamos.

Liberal and Antenna are code names

for Greenglass' brother-in-law, Julius Rosenberg.

Goose is chemist Harry Gold.

MLAD, physicist Theodore Hall.

Many of the spies are motivated

not by an ideological attachment to communism,

but by a belief that the world will be safer

if both superpowers have an atomic bomb.

And most of them never face justice.

- Because Venona was still so secret,

they have no evidence they can offer in court.

They couldn't afford to let the Russians know

that they were unraveling all their secrets.

That would've been immensely compromising

for ongoing intelligence.

- [Narrator] Around 200 other spies

mentioned in the Soviet cables

remain unidentified to this day.

The only spies to face the most severe punishment

are perhaps two of the least significant,

Julius Rosenberg and his wife, Ethel.

- Julius was a recruiter and also a courier for information.

His wife, Ethel,

she was nowhere near as involved

and probably really only typed up information.

But regardless,

they were both convicted of treason and executed.

- [Narrator] After six years of painstaking work

against overwhelming odds,

Venona's codebreakers have fixed the leak

in the Manhattan Project.

Only to find a deeper, more dangerous leak.

And this one comes direct from the heart of power

in Washington, DC.

- Among the messages that Venona uncovers

is a verbatim record of a telegram

that's gone directly from Churchill

to the American president.

- [Narrator] The telegram had been sent in September, 1944,

at a time when communication between the wartime leaders

was more closely guarded than ever.

And yet here, deep inside the corridors of power,

a Soviet agent saw and copied the message.

- What it shows is that there is a leak

at the very highest levels

in either the British or the American establishment,

or in both.

- [Narrator] That leak was an agent code named Homer,

and he was a prolific and committed spy.

The Venona codebreakers discovered

that between 1944 and 1945,

he sent at least 12 messages to Moscow,

all revealing highly classified,

politically sensitive information.

- If this spy is still active,

information might still be leaked

about the highest level of activity

in the scientific and military establishment to the Soviets.

And worst of all,

specific technical information

about this devastating new weapon,

the hydrogen bomb,

might fall into Soviet hands.

- [Narrator] As the Venona team decodes

more of Homer's messages,

it becomes clear that when the telegrams were sent

five years earlier,

the spy was working inside the British embassy

in Washington, DC.

- The FBI narrowed down the possible people

who might have seen this telegram

to about 6,000 people.

Then they start to think

who might have communist sympathies,

who might be the likeliest suspects

to have had a motive

to want to send this top secret document back to Moscow?

- [Narrator] Enter the man trusted with finding the spy.

In September, 1949,

high ranking British diplomat, Kim Philby,

arrives in Washington.

To identify Homer,

Philby gains the confidence of his colleagues

in the British embassy,

the Venona Project,

and the FBI.

- Philby is very charming, very well dressed,

a marvelous storyteller.

Everyone liked him.

Everyone trusted him.

- [Narrator] And that is a disaster

for British and American intelligence,

because unknown to his employers and colleagues

on either side of the Atlantic,

Kim Philby is the Soviet agent.

- He is actually protecting the very people

he's meant to be exposing in the British establishment.

- [Narrator] In Washington's British embassy,

Philby finds himself walking a fine line.

On the one hand,

he has to be seen to be hunting for the spy.

On the other,

he's working to protect that same spy's identity.

Philby is above suspicion.

He has unrestricted access to Washington's political elite,

to the FBI investigation,

and to the Venona Project.

- He's walking their halls.

He's sitting in meetings with Meredith Gardner.

He's learning everything about what they're discovering.

And so, in a sense,

the fox is really in the henhouse,

but no one knows he's a Fox.

- [Narrator] But this fox is about to run out of luck.

In the spring of 1951,

cryptological genius, Meredith Gardner,

uncovers the clue that finally unmasks Homer.

- The words he decodes are "Homer will be visiting tyre,"

which was the code for New York,

"monthly to visit his pregnant wife."

- [Narrator] The authorities narrow their list of suspects

to embassy employees who had a pregnant wife

in New York in 1945.

- The list is a very short one.

Donald Maclean,

who had been the first secretary to the British ambassador.

- Donald Maclean is the high flying diplomat

of his generation.

When he goes to Washington, he's only 31 years old,

and yet he's a first secretary in the Washington embassy,

the biggest embassy of all.

So he's very much going to the top.

- [Narrator] Maclean operates

at the highest level of security clearance.

- He is the British representative

on the most secret committee of all,

which is the Atomic Energy Authorities Committee.

- [Narrator] But Philby already knew all of this.

In fact, he knew Homer's identity

from the moment he was assigned to unmask him.

- Kim Philby recruited Maclean in 1933

when they both left Cambridge.

- [Narrator] At Cambridge University,

both men had developed a deep seated belief

in the communist cause.

- So as soon as Philby is shown the paperwork

that says Homer is one of these spies,

it doesn't take him long to realize

that Maclean is the spy they're looking for.

Kim Philby is the instigator of everything

that's happened to Maclean.

- [Narrator] Before Maclean can be interrogated,

Philby tips him off.

Maclean defects to Moscow

along with another British diplomat,

his friend from Cambridge and fellow Soviet spy,

Guy Burgess.

- It becomes front page news.

The diplomats are missing and all hell breaks loose.

- Were you in fact, the third man?

- No, I was not.

- [Narrator] The finger points at Philby,

but because he wasn't stationed in the US

when the Venona messages were sent,

there's no evidence against him.

But his close association with Maclean and Burgess

leaves him disgraced.

He's forced to resign from the British foreign office

and no longer able to spy for the Soviets.

The Venona Project codebreakers have succeeded

in neutralizing perhaps the most dangerous spies

ever to threaten the United States.

Without their painstaking work,

Maclean, Philby, and their associates,

together with the atomic spies of the Manhattan Project,

would have continued to share America's

most closely guarded secrets

and Stalin may have won the Cold War.

Instead, on November 1st, 1952,

on a remote Pacific atoll,

America detonates the world's first hydrogen bomb.

[bomb explodes]

Stalin loses the race to develop the hydrogen bomb.

He dies four months later

and under his successor, Nikita Khrushchev,

US-Soviet relations begin to thaw.

- A lot of US atomic secrets did leak to the Soviets,

but the Venona Project helped throw a wrench

in that process

and prevented Stalin from getting his hands

on a hydrogen bomb before the US had developed its own.

- If Stalin had had the hydrogen bomb,

would it have made it more war likely?

Who knows.

But the world would've been a less stable place,

that's for sure.

- [Narrator] The Venona Project continues until 1980,

when US intelligence officials decide

there's nothing more to be gleaned

from the Soviets wartime cables.

- The files were all packed away and stored

at the National Security Agency's headquarters in Maryland.

And everyone involved maintained this vow of secrecy

that they'd been working under their whole lives

at that point.

- [Narrator] It's not until 1995

that a handful of documents are declassified.

- This information was so important,

deadly important,

that it's only today we are beginning

to piece together this story.

- [Narrator] The Venona Project reshaped the post-war world.

Its codebreakers, mathematicians, linguists,

and administrators, most of them women,

most of them unknown and unsung,

made the world a safer place.

- In the wars of the 20th century,

and especially the Cold War,

some of the most important battles were fought

behind the scenes by these codebreakers

and much of their work,

work that, in some ways, was pivotal in saving civilization,

was never acknowledged in their time.

These secrets were so deep that they were kept for decades.

- [Narrator] Even today,

nearly 80 years after the first novice codebreakers

began work at Arlington Hall,

the true significance of their work

is still shrouded in mystery

because much of the Venona Projects work

remains classified.

[exciting music]