WEBVTT 00:00.550 --> 00:07.810 In this section and in the next lectures, I'm going to show you how to use Pjp to encrypt and decrypt 00:07.840 --> 00:17.230 text files, emails, and we're also going to use it to sign and verify the integrity of files, emails 00:17.230 --> 00:18.250 and so on. 00:18.730 --> 00:25.060 This way we'll be able to communicate safely because everything will send and receive will be encrypted, 00:25.060 --> 00:29.890 and we'll be able to verify the integrity of the data being sent. 00:29.890 --> 00:36.730 So we'll be able to sign whatever we send and the receiver will be able to verify that this data has 00:36.730 --> 00:40.690 actually been sent from us and has not been tampered with. 00:41.640 --> 00:48.270 Now the reason why we're going to use BGP for encryption and to verify integrity, because this is a 00:48.270 --> 00:54.240 very strong encryption that stood the test of time and has not been broken yet. 00:54.330 --> 01:01.530 Even based on leaked reports, it seems like even government agencies are not able to break it yet. 01:01.530 --> 01:07.360 So it's a very, very strong encryption, even though PJP stands for pretty good encryption. 01:07.380 --> 01:13.500 This statement is actually sarcastic because this is a very strong and powerful encryption. 01:14.070 --> 01:19.110 Now, in this lecture, I want to explain to you how Pjp works in general. 01:19.110 --> 01:24.810 And then in the next lectures, we'll see how we can use it to encrypt all sorts of data and how to 01:24.810 --> 01:27.420 use it to sign and verify integrity. 01:28.490 --> 01:33.800 Now Pjp is a public key or an asymmetric encryption. 01:33.800 --> 01:40.070 And to understand how that works, let's first have a look on the other traditional type of encryption, 01:40.070 --> 01:41.720 symmetric encryption. 01:42.620 --> 01:47.850 So let's have an example where we have two people, David and John. 01:47.870 --> 01:50.630 We're going to forget about how they're going to communicate. 01:50.630 --> 01:55.580 We're just going to assume that David wants to send a message to John. 01:55.580 --> 01:59.090 And the content of this message is secret message. 01:59.720 --> 02:05.780 Now, if David wants to protect his message from anyone who might intercept this message or read it, 02:05.810 --> 02:12.860 he can use an encryption key in order to encrypt this message, and this will transform the message 02:12.860 --> 02:14.300 into gibberish. 02:15.080 --> 02:22.670 Then he can go ahead and send this message to John using any method by sending it as an email or by 02:22.670 --> 02:24.620 post or as a text message. 02:24.620 --> 02:26.060 It doesn't really matter. 02:26.150 --> 02:31.850 And if this message gets intercepted, the contents of the message is going to be gibberish. 02:31.850 --> 02:35.390 So it won't be useful to the person intercepting this. 02:35.780 --> 02:37.610 John will receive the message. 02:37.610 --> 02:39.020 He will open the message. 02:39.020 --> 02:41.090 The message will still be gibberish. 02:41.090 --> 02:48.950 And then John is going to use the same encryption key to decrypt this message and reveal its content, 02:48.950 --> 02:51.050 which was secret message. 02:51.530 --> 02:53.450 So very, very simple. 02:53.450 --> 02:56.740 Basically, David uses a key to encrypt the message. 02:56.750 --> 03:02.300 John uses the same key to decrypt it, and this way they're both able to read the message. 03:02.300 --> 03:07.340 But anybody who does not have the key will not be able to read the message. 03:07.880 --> 03:12.020 So the same key is used by David and by John. 03:12.020 --> 03:15.740 Therefore, this is known as symmetric encryption. 03:16.130 --> 03:18.350 Now, you probably guessed it by now. 03:18.350 --> 03:20.530 This key needs to be private. 03:20.540 --> 03:28.520 That's why it's known as a secret key, because anybody who manages to get his hands on this key, they 03:28.520 --> 03:34.010 will be able to decrypt any message that David sends to John and vice versa. 03:35.220 --> 03:42.510 So based on everything that we said so far, we can see that the secret key can be used to decrypt the 03:42.510 --> 03:43.290 messages. 03:43.590 --> 03:46.360 Because of that, it should be kept a secret. 03:46.380 --> 03:53.580 But David somehow has to share it with John and anyone else David wants to communicate with. 03:54.030 --> 04:00.930 Therefore, this is a major flaw with symmetric encryption because the key has to be secret, but at 04:00.930 --> 04:06.720 the same time has to be shared and sharing it with more people increases the attack surface, not to 04:06.720 --> 04:10.020 mention the problem of sharing the actual key. 04:10.050 --> 04:11.300 How are we going to share it? 04:11.310 --> 04:13.410 Are we going to send it in a separate message? 04:13.410 --> 04:15.720 What if that message gets intercepted? 04:15.750 --> 04:21.810 What if we're sending stuff over the Internet and we know how many hops our data could pass by? 04:21.840 --> 04:27.600 This could be intercepted, read, and then the rest of our communication will be decrypted. 04:28.080 --> 04:35.400 This was the main incentive to come up with a more secure encryption, and this is where asymmetric 04:35.400 --> 04:37.470 or public key encryption comes. 04:38.190 --> 04:42.540 So let's go back to David wanting to send a message to John. 04:42.540 --> 04:47.640 And the content of the message is secret message and asymmetric encryption. 04:47.640 --> 04:50.460 One key is used to encrypt the message. 04:50.490 --> 04:57.480 The message is sent in the air, and then another key is used to decrypt the message. 04:57.960 --> 05:06.090 So as you can see in this encryption, two different keys are used and hence the name asymmetric encryption. 05:06.810 --> 05:10.700 Now these two keys are referred to as a key pair. 05:10.710 --> 05:13.020 They are mathematically related. 05:13.020 --> 05:18.180 One is used for encrypting the message and the other is used for decryption. 05:18.180 --> 05:23.700 Therefore, the decryption key is never shared, and that's why it's more secure. 05:24.120 --> 05:28.680 Now you're thinking if the decryption key is never shared, how is this going to work? 05:29.040 --> 05:33.120 Well, let's go back and have a closer look on how this is going to work. 05:33.210 --> 05:36.690 So, again, David wants to send a message to John. 05:36.690 --> 05:39.630 The content of the message is secret message. 05:39.630 --> 05:47.040 But before sending this message and before encrypting it, John is going to create a key pair, a public 05:47.040 --> 05:48.930 key and a private key. 05:49.560 --> 05:56.790 Like I said, these keys are mathematically linked and John is going to send the public key to David, 05:56.790 --> 05:59.460 to the person that will send the message. 05:59.820 --> 06:02.970 So the public key can be shared with anybody. 06:02.970 --> 06:07.470 You can actually make it public on the Internet, on key directories. 06:07.500 --> 06:13.030 It doesn't really matter because it cannot be used to determine the private key. 06:13.050 --> 06:16.830 Therefore, it's completely safe to share the public key. 06:17.310 --> 06:19.920 So David receives the public key. 06:19.950 --> 06:23.100 He uses the public key to encrypt the message. 06:23.100 --> 06:25.860 The message is sent using any method. 06:25.860 --> 06:27.120 It doesn't really matter. 06:27.120 --> 06:30.320 And even if it's intercepted, it's going to be gibberish. 06:30.330 --> 06:32.430 John will receive the message. 06:32.430 --> 06:33.840 He will open the message. 06:33.840 --> 06:40.410 The message will still be gibberish, but he will use the private key to decrypt this message. 06:40.860 --> 06:44.310 This way he will get the content of the secret message. 06:44.310 --> 06:48.200 And as you can see, the private key was never shared. 06:48.210 --> 06:52.210 John created the private key and John kept the private key. 06:52.230 --> 06:58.470 The only thing that is shared is the public key, which cannot be used to determine the private key 06:58.470 --> 07:01.280 and cannot be used to decrypt the message. 07:01.290 --> 07:04.530 It can only be used to encrypt the message. 07:05.420 --> 07:12.200 So by the end of the communication, David is going to have John's public key and John is going to keep 07:12.200 --> 07:13.330 their private key. 07:13.340 --> 07:19.610 Therefore, now David can always send messages to John, and John will always be able to decrypt them 07:19.610 --> 07:21.800 using his own private key. 07:21.890 --> 07:29.060 Not only that, but when David encrypt something with John's public key, he can be rest assured that 07:29.060 --> 07:35.420 nobody can decrypt this message except for John, because John is the only one that has the private 07:35.450 --> 07:38.990 key, and his private key should never be shared. 07:39.020 --> 07:44.660 If John knows what he's doing because there is no point of sharing it, even if he wants 100 people 07:44.660 --> 07:50.420 to send him messages, all he has to do is share his public key and not his private key. 07:51.140 --> 07:56.780 Now, if John wanted to send something to David, the same can be done in an opposite direction. 07:56.780 --> 07:59.100 So David would create a key pair. 07:59.120 --> 08:06.000 He would send John his public key, and then John would use David's public key to encrypt messages. 08:06.020 --> 08:10.760 When David receives them, he will use his own private key to decrypt them. 08:10.910 --> 08:13.100 So the idea is very, very simple. 08:13.100 --> 08:20.030 You share your public key, hence the name public that is completely safe because it can't be used to 08:20.030 --> 08:24.830 determine the private key and it can't be used to decrypt the messages. 08:24.830 --> 08:31.100 So you share the public key and anyone who wants to send you a message, they will encrypt that message 08:31.100 --> 08:37.160 with your own public key, and this way you will be the only one that can decrypt this message. 08:37.340 --> 08:39.740 Same goes when you want to send another message. 08:39.740 --> 08:45.770 For example, if you wanted to send a message to me, all you have to do is encrypt that message with 08:45.770 --> 08:47.180 my public key. 08:47.180 --> 08:52.790 And this way, because I don't share my private key with anyone, I will be the only one that can read 08:52.790 --> 08:53.390 the message. 08:53.390 --> 08:55.370 So you can put this message anywhere. 08:55.370 --> 09:01.430 You can share it publicly, and it will still be safe because no one will be able to read that message 09:01.430 --> 09:02.360 but me.