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So far we learned how to use PDP to encrypt messages so that only the receiver can read the content

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of the message.

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To do that the sender David in our example uses the public key of the receiver so the receiver makes

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their public key public as the name suggests.

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That's fine because the public key cannot be used to decrypt messages.

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It can only be used to encrypt messages.

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So David encrypts the message with the receivers.

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John's public key.

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The message is sent and won't John receives it.

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He uses his own private key that he never shares with anybody to decrypt the message.

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Therefore David can send the message to John in any way he wants.

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He can send it as a text message as an instant message or even use in an insecure service.

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That's fine because if anybody reads the message they will see gibberish.

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They want to be able to see the content unless they have the private key.

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And as long as John doesn't share the private key there is no need for him to share it.

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Then nobody will be able to read this message except for John.

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So that's really really good.

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The only problem here is that there is no way for John to verify that the message that they received

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has actually been sent from David.

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So like I said for this to work John needs to make their public key public.

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So getting the public key of John is easy.

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Therefore John could have it in his signature in his email.

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He can have it in his signature in a forum.

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He might have it publicly shared because he wants people to send him encrypted messages.

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So there is nothing wrong with making your public key public.

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The only problem is someone can come in pretend to be David.

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Use John's public key to encrypt a message and send it to John.

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And that way John has no way of knowing whether this message did actually come from David or not.

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To solve this problem David will have to sign the message.

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This can actually be done with PDP.

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So let me show you how this will work.

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With this example.

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So again we have David.

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He wants to send a secret message to John.

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And as we learned before the first thing that David will do he will use John's public key in order to

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encrypt the message.

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The message will change into gibberish.

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Now at this stage in the previous lecture we sent the message.

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But this time David is going to sign the message with his own private key so he still hasn't sent the

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private key.

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The message is still at David's end and what he is going to do is he's going to create a signature for

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this message.

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This signature corresponds to this message.

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And if anything gets modified within the message.

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If one letter gets modified the signature will change.

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Therefore this signature can be used to verify that the message has not been modified since it got signed

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by David's private key.

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Now keep in mind David is still keeping his own private key.

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He did not send it through any method of communication.

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So now we have a message with an encrypted content and with a signature that corresponds to David private

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key.

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Then the message is sent.

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Using any method of communication like we said you can even use an insecure method of communication.

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John is going to receive the message along with its signature and before decrypt in this message with

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his own private key what he is going to do is he is going to use David's public key in order to verify

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the signature.

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So like I said the signature was created with David's private key.

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And then John doesn't have David's private key.

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But he is going to use David's public key to verify the signature if the message was not modified.

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The verification will be successful.

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And this way John will know that this message was actually sent by David and was not modified by anybody

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because like I said if one letter gets modified the public key David's Public Key will not verify the

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signature.

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So when the signature is verified we know that David was the actual sender of the message and the message

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was not modified as it was sent.

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Whether it was sent over the Internet as a text message or using any other method of communication.

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The next step is very similar to what happened in the previous lecture.

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John will use his own private key in order to decrypt the message and read its content which is just

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secret message.

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So as you can see as a result of this each party still kept their own private key.

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Nobody sent their private key to the other party so the sender encrypts the message with the receiver's

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public key and signs the message with his own private key.

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The message is sent the receiver verifies the signature with the sender is public key and decrypted

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with his own private key this way he can verify that this message came from the sender.

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He can verify that the message did not get modified as it was sent and the message is encrypted and

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the only person that can read it is the receiver because he kept his own private key private and it

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was not shared with anybody.

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Now all of this should become clearer in the next lecture as I'm going to show you how to encrypt messages

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and sign them as a sender and I'm also gonna show you how to verify the signature and decrypt the messages

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as a receiver.