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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