Hi, folks, in this video, we're going to take some time and

look at the way public key cryptography can be used to send a secret message.

So we're going to start playing with the concepts of Diffie and Hellman.

And you recall in previous video, we said that Alice and Bob, or

whose sender or receiver, are in some sense doing something locally to

generate their key pair, there's no key distribution center.

And the key pairs are set up such that encrypt with secret,

decrypt with public and so on.

So let's assume, and we'll pop up a little chart here that shows Alice and

Bob and is their pre-condition, we'll list out what they have,

what they know, or what they've been setup with.

And the setting up is called infrastructure and for public key it's

called public key infrastructure, PKI, which you may have heard of.

Tends to be a lot of work to set up PKI for a typical setting.

But for now let's assume Alice is going to have both a public and secret key pair

which Alice generated locally, pushed the button, ran the RSA algorithm, boom!

Pair pops up, PA, SA.

But she also has the public key of Bob because we're going to

make the assumption that everybody knows everybody else's public key.

Is that fair enough?

Similarly Bob, what does Bob do?

Runs an algorithm locally, generates PB, SB, has that key pair.

He didn't have to go to a key distribution center, for he did it locally.

But also has PA, has Alice's public key.

Everybody's got everybody's public key.

And we can throw Eve in there too, Eve is the eavesdropper.

I don't care what Eve's public key pair is.

Yeah, she generates her own key pair, but who cares?

But she does know PA and she does know PB, fair enough?

That's the precondition.

Now, what I'm going to do is Alice is going to encrypt a message.

And what are the things she can encrypt with?

Well, there's PA, SA, PB.