So, I wanted to start off this demonstration to understand that we're using two key pairs. So, the first key pair will be generated by myself: Mark's public and private key. Remember, Mark's private key is like my personal wallet. I do not give that to anybody. The public key is available to anyone, and I've also, for the purpose of this demonstration, also generated Bob's public and private key. Bob would obviously keep his private key to himself, and his public key will be available to me. So, in front of us we have two boxes and I want to send a message to Bob. My message is 'top secret', so I'm gonna send the message to Bob and I'm gonna put the message inside my carrier, and then I'm going to lock the box using encryption, using the recipient's public key, and that'll be Bob's public key. You always use the recipient's public key if you're gonna send a message for confidentiality purposes. So, that gives the message, this confidential message, I send the message to Bob. Bob receives the message, and because Bob has his private key, he can unlock the message and read the message inside, which he does. He unlocks the message, and I'm acting on this-, on behalf-, as Bob, and there's Bob can read my message. It says 'top secret'. But on this occasion, Bob's going, 'I'm not too happy at-, I'm not 100% certain that it's you, Mark. I've got confidentiality but I want non-repudiation. I wanna to prove it's you.'  

So, on this occasion, I use the same methodology before. I'm gonna send a message to Bob and I-, and I use the recipient's public key, which is Bob's public key, so I'm gonna use Bob's public key to encrypt the message again. I'm gonna put it into the small box and that, obviously, proves the confidentiality aspect, but I'm gonna put this small box into the bigger box, which is the second carrier I'm going to use, then I'm gonna lock it with my private key, and this is to prove it is me. So, I'm gonna lock it with my private key, which proves it's me. It's like a digital signature. It's proving something as a one-way function. So, the message is then given to Bob. Bob receives the message, and the message is obviously locked with my private key, so the only way he can unlock it is to use my public key to unlock it, which proves, straight away, it is me that's sending the message to him. That's all it does, it proves that it's me, and inside the box, you can actually see the message inside it. I take the message out, and this is, obviously-, now it's Bob's receiving the message, and Bob will unlock that using his private key to unlock the message. So, he unlocks it with his private key and that gets the message that's inside, which is the 'top secret' message.  

Now, I'm gonna introduce a third person now, Eve or Eavesdropper, so I going to go through the same methodology again. I'm gonna send the message to Bob, just gonna put my message, top secret message, into the box and I'm going to lock the message using the recipient's public key. And, obviously, in this occasion, this is Bob's public key I'm gonna lock it with, which proves confidentiality. I'm gonna to put the box into these-, small box into the big box and I'm gonna lock the message using my private key, which is proving it's me sending it to him. I'm sending the message to him, and unfortunately, on this occasion, the message is intercepted by Eve. Eve is the eavesdropper. Now, she can access only public keys and she's got access to my public key, because I locked it with my private key, only my private key and public key can unlock each other. So, my public key can unlock the outer box but, unfortunately for Eve, the message which is inside the box, she cannot open 'cause she does not have Bob's private key.