[APPLAUSE]
– If you'd like to stand there. And what's your name?
– Olivia
– Olivia, all right. So Olivia is going to be the internet
and again Andy and I going to try to exchange keys,
but this time in the electronic world.
We're going to send all our information through you
and you're going to try as hard as you can to steal our secret key.
Alright so keys in the electronic world are really numbers,
but we're going to represent numbers by colours.
So each colour represents a different number.
And we're going to start with the color blue.
Now blue is not a secret colour, everybody knows blue.
So I've got some blue, Andy's got some blue,
and Olivia's got some blue.
The next thing I'm going to do
is I'm going to choose a secret colour,
and my secret color is going to be yellow.
Now I can't just send yellow across the internet to Andy,
because it will be stolen on the way.
So to keep it secret, what I'm going to do
is to mix it with the blue.
If I mix yellow and blue, I get green.
And that's another example of a one-way process.
Just took a few seconds to mix the yellow and the blue to make green,
but if you had to take the green and separate all the molecules out,
to get the blue and the yellow again
it would take millions of years, it would be very hard.
Okay so I'm going to take this green and I'm going to send it over the internet,
but remember you're trying to steal all our secrets.
So what I want you to do is to make a copy of this colour.
Okay, so what I want you to do
is to pour half of the liquid into this container.
If you just pour it up to that line, that's good.
And keep going a bit more...
And I'd stop about there, that's excellent.
Okay and you can pass the rest over to Andy.
Now Andy's going to do something similar,
he's going to dream up a secret colour,
in his case the secret color is red.
Again, he can't send the red to me through the internet
or it will be copied, so he's going to use this one-way process
he's going to mix it with the blue.
So he gets a sort of purpley colour.
And now he's going to send that mixture to me over the internet.
And again, you're trying to steal our information,
yeah you've got the hang of it, so you pour half of it
into that container, just up as far as the line.
A little bit more... bit more...
Excellent thank you very much, and you give the rest to me.
Okay so here I have a mixture of red and blue.
What I'm going to do is to add my secret colour which is yellow.
So I have a mixture of red, yellow and blue.
Andy's got a mixture of yellow and blue
and he's going to add his secret colour which is red.
Ao Andy now also has a mixture of red yellow and blue.
So we've managed to agree on the same secret colour.
And if we just put these together in the middle...
I hope you'll be able to see that those are indeed the same colour.
Alright, what about our internet thief, what information do you have?
Can you make this same colour? Let's have a look.
Well this colour is a mixture of red and blue
and that's definitely a bit different.
You've got some yellow in here, mixed with some blue
so you can't unmix the colours, remember you can only mix things.
So why don't you try adding this to that and see if you can copy our secret colour.
And what we'll see is that – that's it just pour it all in, that's good.
Now this is a different color it's a different color because
this is a mixture of red plus blue, plus yellow plus blue,
so it's got twice as much blue in it.
And the only other color you know about is blue,
so if you pour that in as well...
That's actually just going to make things even worse,
there's even too much blue now.
You can stop there that's fine.
So we've seen how Andy and I
have been able to agree on a secret colour,
and in the electronic world that means we've agreed on a secret number.
So next time you see that little yellow padlock on your web browser,
you'll know that your computer and the website computer
have done a little exchange of secret numbers just like this one.
And that means it's now okay to put your credit card information
or whatever it is into that website, because it'll be scrambled
and other people won't be able to read it.
Okay thank you very much.
[APPLAUSE]