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In this section and in the next lectures I'm going to show you how to use PDP to encrypt and decrypt

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text files emails and we're also going to use it to sign and verify the integrity of files emails and

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so on.

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This way we'll be able to communicate safely because everything will send and receive will be encrypted

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and we'll be able to verify the integrity of the data being sent.

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So we'll be able to sign whatever we send and the receiver will be able to verify that this data has

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actually been sent from us and has not been tampered with.

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Now the reason why we're going to use BGP for encryption and to verify integrity because this is a very

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strong encryption that stood the test of time and has not been broken yet even based on leaked reports

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it seems like even government agencies are not able to break it yet.

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So it's a very very strong encryption.

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Even though PDP stands for pretty good encryption.

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This statement is actually sarcastic because this is a very strong and powerful encryption.

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Now in this lecture I want to explain to you how PDP works in general and then in the next lectures

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we'll see how we can use it to encrypt all sorts of data and how to use it to sign and verify integrity.

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Now PDP is a public key or an asymmetric encryption.

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And to understand how that works let's first have a look on the other traditional type of encryption

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symmetric encryption.

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So let's have an example where we have two people David and John were going to forget about how they're

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going to communicate.

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We're just going to assume that David wants to send a message to John.

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And the content of this message is secret message.

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Now if David wants to protect his message from anyone who might intercept this message or read it he

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can use an encryption key in order to encrypt this message and this will transform the message into

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gibberish then he can go ahead and send this message to John using any method by sending it as an email

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or by post or as a text message.

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It doesn't really matter.

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And if this message gets intercepted the contents of the message is going to be gibberish.

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So we want to be useful to the person intercepting this.

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John will receive the message.

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He will open the message.

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The message will still be gibberish.

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And then John is going to use the same encryption key to decrypt this message and reveal its content

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which was secret message.

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So very very simple.

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Basically David uses a key to encrypt the message.

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John uses the same key to decrypt it.

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And this way they're both able to read the message.

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But anybody who does not have the key will not be able to read the message.

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So the same key is used by David and by John.

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Therefore this is known as symmetric encryption.

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Now you probably guessed it by now.

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This key needs to be private.

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That's why it's known as a secret key because anybody who manages to get his hands on this key they

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will be able to decrypt any message that David sends to John and vice versa.

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So based on everything that we said so far we can see that the secret key can be used to decrypt the

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messages.

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Because of that it should be kept secret.

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But David somehow has to share it with John and anyone else.

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David wants to communicate with.

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Therefore this is a major flaw with symmetric encryption because the key has to be secret.

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But at the same time has to be shared.

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And sharing it with more people increases the attack surface.

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Not to mention the problem of sharing the actual key.

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How are we going to share it.

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Are we going to send it in a separate message.

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What if that message gets intercepted.

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What if we're sending stuff over the Internet and we know how many hubs our data could pass by.

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This could be intercepted read.

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And then the rest of our communication will be decrypted.

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This was the main incentive to come up with a more secure encryption.

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And this is where asymmetric or public key encryption comes.

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So let's go back to David wanting to send a message to John.

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And the content of the message is secret message and asymmetric encryption one key is used to encrypt

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the message.

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The message is sent in the air and then another key is used to decrypt the message.

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So as you can see in this encryption two different keys are used and hence the name asymmetric encryption.

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Now these two keys are referred to as a key pair.

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They are mathematically related.

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One is used for encrypting the message and the other is used for decryption.

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Therefore the decryption key is never shared.

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And that's why it's more secure.

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Now you're thinking if the decryption key is never shared how is this going to work.

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Well let's go back and have a closer look on how this is going to work.

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So again David wants to send a message to John the content of the message a secret message but before

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sending this message and before encrypting it John is going to create a key keeper a public key and

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a private key.

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Like I said These keys are mathematically length.

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And John is going to send the public key to David to the person that will send the message.

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So the public key can be shared with anybody.

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You can actually make it public on the Internet on key directories.

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It doesn't really matter because it cannot be used to determine the private key.

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Therefore it's completely safe to share the public key.

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So David received the public key.

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He uses the public key to encrypt the message.

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The message is sent using any method doesn't really matter.

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And even if it's intercepted It's going to be gibberish.

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John will receive the message.

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He will open the message the message will still be gibberish but he will use the private key to decrypt

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this message this way he will get the content of the secret message.

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And as you can see the private key was never shared.

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John created the private key.

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And John kept the private key.

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The only thing that is shared is the public key which cannot be used to determine the private key and

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cannot be used to decrypt the message.

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It can only be used to encrypt the message.

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So by the end of the communication David is going to have John's public key.

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And John is going to keep their private key.

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Therefore now David can always send messages to John and John will always be able to decrypt them using

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his own private key.

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Not only that but when David encrypt something with John's public key he can be rest assured that nobody

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can decrypt this message except for John.

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Because John is the only one that has the private key and his private key should never be shared.

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If John knows what he is doing because there is no point of sharing it even if he wants 100 people to

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send him messages all he has to do is share his public key and not his private key.

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Now if John want to send something to David the same can be done in an opposite direction.

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So David would create a keeper.

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He would send John his public key and then John would use David's public key to encrypt messages.

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When David receives them he will use his own private key to decrypt them.

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So the idea is very very simple.

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You share your public key has the name public.

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That is completely safe because it can't be used to determine the private key.

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And it can't be used to decrypt the messages.

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So you share the public key.

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And anyone who wants to send you a message they will encrypt that message with your own public key.

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And this way you will be the only one that can decrypt this message.

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Same goes when you want to send another message.

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For example if you wanted to send a message to me all you have to do is encrypt that message with my

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public key and this way because I don't share my private key with anyone.

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I will be the only one that can read the message.

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So you can put this message anywhere you can share it publicly and it will still be safe because no

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one will be able to read that message.

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But me.