How do you know your information is safe?
Every time you buy something online or
send a private Twitter message how do
you know no one can see it? You don't, not
for sure anyway. But you trust in the
encryptions protecting it. But just how
trustworthy are these encryptions? We'll
talk about why all your secrets may have
an expiry date in just a bit
but first let's talk about how it works.
A very common type of encryption is
based on secret key cryptography. This is
where a message is scrambled using a
key known only to the sender and
receiver. Anyone can see the message but
they can't read it without the key. This
is what Queen Mary of Scots used to try
to assassinate queen Elizabeth. It's
also what some websites used to hide
movie spoilers. Say you find this message,
it looks like garbage, but someone tells
you the key is that every letter is
shifted forward one in the alphabet so
to decrypt this message you just shift
every letter back one. The keys used to
guard your bank details are obviously a
lot harder to break but they may not be
the much longer. What makes these codes
practically unbreakable is the
tremendous amount of time and computing
power they take to break. If all the
world's personal computers were working
nonstop to try and break the code that
keeps your email safe it would take them
around 12 million times the age of the
universe. But the modern computer may
soon be replaced by the quantum computer
and what takes a modern computer
billions of years could be
done by a quantum computer in days or
even hours. No secret would be safe. But
researchers are ready for this. They've
come up with a new type of cryptography.
One that's not just hard to break but
impossible to break.
It's called quantum cryptography and
what makes it so powerful is that
instead of math it relies on the laws of
physics. One law in particular: the
Heisenberg uncertainty principle. It says
that you can't know absolutely
everything about the state of a quantum
particle. It's not because you're not
smart enough or your equipment isn't
good enough, it's just because nature
keeps some things hidden. Take this
scenario. You want to send Bob a private
message. You encrypt the message using a
key made up of ones and zeros. You send
the key to Bob by a secure line. This
could be email or wire,  and then mail
the message to Bob. Remember it doesn't
matter if anyone else reads the message,
they wouldn't be able to understand it,
what's important is that no one sees the
key. But an eavesdropper Eve has tapped
into the secure line. Once she has the
key she can read the message and your
secret will be publicly known. This would
never happen if you used quantum
cryptography. With quantum cryptography
the key is a stream of photons or light
particles. Photons have a property called
spin which can be changed when it passes
through a filter. You have four filters:
vertical, horizontal, and two diagonal.
We'll lump these into two groups: the
diagonal scheme and the rectilinear
scheme. To translate photon spin into a
key a photon with vertical or this
diagonal spin means one, and horizontal
or this diagonal spin means zero. If you
wanted to send one one zero zero you
could send with this sequence, or this
one. You start sending Bob photons,
switching between filters at random.
Now here's where the Heisenberg
uncertainty principle becomes important.
The only way Eve can measure a photon's
spin is by passing it through a filter.
If Eve measures this photon with a
rectilinear filter she'll guess correctly
then it has vertical spin and note down
a 1, but if she uses the diagonal filter
the photon spin will be altered as it
passes through. She now incorrectly reads
a 0. The same will happen if she measures
a diagonal photon with the rectilinear
filter.
Unless Eve knows beforehand which filter
to use she runs a pretty big risk of
changing the spin, and because you're
switching between filters at random
she'll get it wrong about half the time.
The laws of quantum physics prevent her
from knowing the key! Now you may have
spotted a pretty big problem, Bob is in
the same boat as Eve, he doesn't know
which filter to use either. This problem
actually took a long time to solve. After
all, what's the point of a message so
secure even the receiver can't read
it? But there is a way around it. After
you've sent the key you call Bob and
tell him which scheme you used for what
photon. You don't tell him the spin of
the photon or whether it was a 1 or a 0,
just whether it was rectilinear or
diagonal. If he used the right filter
you both keep the digit, if he chose the
wrong filter you get rid of it.
Probability tells us that he got about
half wrong so you keep the half he got
right and that's your key!
If Eve is listening in on your
conversation it won't help her. Say for
the first photon you both used a
rectilinear filter so you keep the
digit. Eve used the wrong filter and she
can't tell if it's a 1 or 0 from just
which scheme you used. For the second
photon you used a diagonal filter, so did
Eve, but Bob used the rectilinear filter
so you get rid of this answer. So now you
and Bob have a totally secure key and
Eve has a bunch of useless 1's and
0's. When the first quantum
cryptographic system was tried in 1989
a key was sent 36 centimeters from a
computer called Alice to a computer
called Bob.
Now newer models have reached 150
kilometers. There's still a long way to
go and some people think this is a whole
new era for privacy. Quantum cryptography
is different from any encryption system
before it so I'm curious as to how the
code breakers go about breaking this one. Thanks so much for watching guys and a
big thank you to mr. Jarnould for
suggesting this idea for a video. I
really loved learning about this topic
and I hope you guys liked it too. See you in
the next video. Bye!
