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In this day and age, we have a lot of secrets.
And we're constantly having to give them away to the internet.
Like, Amazon has my credit card number.
I typed it in voluntarily.
So I could order a BSG pendant and some ferrofluid.
But I don't even know where my information is kept.
How can I trust that Amazon doesn't accidentally
give it away?
This challenge of secret keeping is an important problem
for companies and governments.
That's why encryption, or translating information
into a code only the right people can read, is
so important-- so important that it was put on the United States
Munitions List, along with flamethrowers and bombs,
as a weapon regulated for national security.
That was until a student took the United States
government to court and encryption
was ruled free speech.
Now most of the focus is on improving encryption,
because as computers get smarter and faster,
these codes become easier to unscramble.
That's why we need to turn to cutting-edge physics
to improve encryption.
But let's find out how.
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Before I go physics on you, let's talk about codes.
A super simple type of code but one
that was used by the Spartan army and probably Scooby-Doo
is simple substitution, where you just
swap out each letter in the alphabet
for a different letter, like so.
For example, DIANNAROCKS would become
EJBOOBSPDLT. This code is easy to generate
and almost as easy to crack.
You could even use extra information,
like guessing that the six-letter word
with the double O's was my name, helping
you to crack the code faster.
So cryptographers turned to math to develop harder codes.
They thought a good code should be easy to create
but difficult to decode, like a mess.
In math terms, they sought processes
called one-way functions-- easy to compute
one way but hard in reverse, like multiplying prime numbers.
If I told you to multiply 79 by 73,
a few punches on your calculator would give you 5767.
But if I told you to factor 5767,
well, you'd have to start by saying,
does that I divide by 2 and then 3
and then 5 up through the prime numbers
until you finally found 79 times 73.
That would take some key punching.
The largest prime number known today
is 2 to the 74,207,281 minus 1, a number
that I couldn't even write on the screen if I wanted,
because that's more digits than pixels in my HD video.
Now, imagine multiplying that by the second largest known
prime number and giving that to your little cousin
to factor on a rainy day.
That's just cruel.
But it's great for encryption.
Imagine I want to send the message DIANNA
ROCKS to another secret agent, Sophia.
I'm going to use this math.
First, I substitute the letters in my message for bits,
like we did in our easy Scooby-Doo code.
If we were using binary, DIANNA ROCKS would look like this.
But we'll just use 1, 2, 3, 4, 4, 3, 5, 6, 7, 8, 9, 10.
The 5 is for the space, and the conversion key
from numbers back to letters is publicly known.
No surprises yet.
But then, I multiply each of the numbers by two prime numbers--
say, 11 and 13-- for a total multiplication factor of 143.
DIANNA ROCKS becomes 143, 286, 429, and so forth.
I've told Sofia ahead of time that the key is
to divide by 143 and she can decode the message.
Now, say someone intercepts the encrypted message.
They have to figure out what to divide by
to decode the message.
This example wouldn't be so difficult.
But what if we used prime numbers that
are thousands of digits long?
An eavesdropper might be crunching numbers
for longer than they are alive.
This is the idea behind using prime numbers for encryption,
though real-life techniques like the RSA algorithm
are a bit more complicated.
Now, the limitation with using prime numbers
is that we keep building smarter and faster computers
that factor out ever more quickly.
Experts warn that with new technologies like quantum
computing, codes that currently take hundreds of years to crack
could be solved within minutes.
And this is where the physics comes in.
Enter quantum cryptography, a technology
that hides information in photons
or the particles of light.
Here's how it works.
Say you want to enlighten-- heh, heh-- a chosen stranger
on our mantra, or this.
Instead of deciding upfront what the secret multiplication
factor or key is, you use quantum mechanics
to make one randomly and send it to your recipients.
Here's how the random key is made.
Alice, the message sender, sends photons
that are polarized or vibrating in four different directions--
horizontal, vertical, diagonal, left, or right.
Bob, the recipient, measures which direction
they're polarized.
Note by using two differently polarized detectors
for each photon one at a time back and forth guessing
which detector to use randomly, the detectors translate
the photons into bits.
Like, a horizontal measurement could
register as a 1 and a vertical as a 0 on this detector.
And eventually, Bob will get the multiplication key
from this set of bits.
So now, Bob has a measurement of 1, 0, 0, 1,
et cetera, for each photon, keeping in mind
that he measured them on two different detectors randomly.
Now he compares with Alice.
And for each photon, he'll tell her which detector he used.
And she'll tell him, wrong, right, wrong, right,
based on which filter she used to send the photons.
Because, see, Alice sent either vertical horizontal photons
or diagonal photons.
And if Bob uses a diagonal detector
on a horizontal or vertical photon,
according to quantum mechanics, he
will have a 50% chance of measuring
a 0 and 50% chance of a 1.
That's why Bob's detectors need to match Alice's filters.
After they go through this public check
of the order the detectors were used,
they throw out each result from when Bob guessed incorrectly.
And now they have a sequence of identically polarized
and measured photons.
That sequence is the key.
Alice can now send the actual encrypted message
through a traditional channel and use the quantum key
to decrypt it.
And mathematicians have proven that if you
make a truly random numerical key,
you can theoretically make a code called a one-time pad that
is unbreakable.
So why can the order of polarized detectors used by Bob
be public?
Well, it's not the ones and zeros
that he obtains that are being shared.
It's just the order of detectors.
You would still need to send the polarized photons
in through those detectors in the correct order
to figure out the key.
But those photons were polarized randomly,
so the eavesdropper is outta luck.
And things on this scale-- 1,000th of the width of a human
hair-- get weird, as they say, because of quantum mechanics.
If an eavesdropper hacks into the system
and tries to copy some of the photons using
the wrong order of detectors, they'll change the key.
Bob and Alice will know, because they
can check for errors in a subset of the bits in the key,
and they can try again.
Of course, getting quantum cryptography
to work in the real world is not so easy.
Small disturbances can change photon polarization.
And when creating photons, if you get them off
by even just a degree, those errors will add up.
Physicists have only been able to send quantum keys
over 200 kilometers.
You can even sabotage quantum detectors
by shining a bright light on them.
And even if quantum encryption does become commercially
viable, much of the internet's infrastructure
would have to be rebuilt.
But still, think of how powerful this technology would be.
An eavesdropper has to measure in order to get the key.
But when measured, the key changes.
You can know if you've been hacked even before you
send the message because of this fundamental aspect of nature.
The universe is based on probabilities.
Quantum cryptography hides information
not by besting a computer but by stowing it
within the unknowability of nature itself.
Thank you so much for watching.
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