WEBVTT 1 00:00:00.000 --> 00:00:01.230 In this lesson, 2 00:00:01.230 --> 00:00:04.020 we will learn about digital signatures. 3 00:00:04.020 --> 00:00:07.410 A digital signature is a cryptographic technique 4 00:00:07.410 --> 00:00:09.420 that ensures the authenticity 5 00:00:09.420 --> 00:00:12.630 and integrity of a message or document. 6 00:00:12.630 --> 00:00:17.370 It uses a pair of keys, a private key to sign the data 7 00:00:17.370 --> 00:00:21.480 and a corresponding public key to verify the signature. 8 00:00:21.480 --> 00:00:23.190 With a digital signature 9 00:00:23.190 --> 00:00:25.320 and the sender's public key, 10 00:00:25.320 --> 00:00:28.440 the signature recipient can confirm the identity 11 00:00:28.440 --> 00:00:29.640 of the sender 12 00:00:29.640 --> 00:00:31.230 and ensure the content 13 00:00:31.230 --> 00:00:34.860 of the digitally signed message has not been altered. 14 00:00:34.860 --> 00:00:37.380 Let's learn more about digital signatures 15 00:00:37.380 --> 00:00:38.970 and non-repudiation. 16 00:00:38.970 --> 00:00:42.420 Then let's do a walkthrough of a digital signature. 17 00:00:42.420 --> 00:00:44.940 First, we have digital signatures. 18 00:00:44.940 --> 00:00:47.970 Digital signatures ensure the authenticity 19 00:00:47.970 --> 00:00:52.290 and integrity of a message, document, or piece of data. 20 00:00:52.290 --> 00:00:55.830 They work by using a pair of cryptographic keys, 21 00:00:55.830 --> 00:00:59.490 a private key that is used to create the signature 22 00:00:59.490 --> 00:01:03.480 and a public key that is used to verify the signature. 23 00:01:03.480 --> 00:01:06.390 Let's walk through the digital signature process 24 00:01:06.390 --> 00:01:08.190 to better understand it. 25 00:01:08.190 --> 00:01:11.190 In our scenario, let's assume Alice wants 26 00:01:11.190 --> 00:01:14.310 to send Bob a digitally signed email. 27 00:01:14.310 --> 00:01:16.680 It's important to note that both Alice 28 00:01:16.680 --> 00:01:20.610 and Bob have their own public and private key pairs. 29 00:01:20.610 --> 00:01:24.270 So to create a digital signature for her email, 30 00:01:24.270 --> 00:01:28.170 Alice's email client will take her message for Bob 31 00:01:28.170 --> 00:01:33.170 and run it through a hashing algorithm such as SHA 256. 32 00:01:33.300 --> 00:01:36.480 This creates a unique hash digest 33 00:01:36.480 --> 00:01:39.480 that is based on Alice's message to Bob. 34 00:01:39.480 --> 00:01:43.410 Next Alice's email Client will use her private key 35 00:01:43.410 --> 00:01:47.580 to encrypt the hash digest, which again is a hash 36 00:01:47.580 --> 00:01:49.350 of her message to Bob. 37 00:01:49.350 --> 00:01:53.490 This encrypted hash digest is appended to her email 38 00:01:53.490 --> 00:01:56.340 and is known as a digital signature. 39 00:01:56.340 --> 00:02:01.320 Note, Alice did not encrypt her actual email message to Bob. 40 00:02:01.320 --> 00:02:05.100 That's because digital signatures focus on authenticity 41 00:02:05.100 --> 00:02:08.520 and integrity, not confidentiality. 42 00:02:08.520 --> 00:02:09.930 Back to our scenario. 43 00:02:09.930 --> 00:02:13.170 Bob's email client now receives the email 44 00:02:13.170 --> 00:02:16.800 and digital signature from Alice's email client. 45 00:02:16.800 --> 00:02:17.970 At this point, 46 00:02:17.970 --> 00:02:22.080 Bob's email client will decrypt Alice's digital signature, 47 00:02:22.080 --> 00:02:25.950 which remember, is an encrypted hash of her email. 48 00:02:25.950 --> 00:02:29.250 Bob's email client does this signature decryption 49 00:02:29.250 --> 00:02:33.030 with Alice's public key, which is publicly available. 50 00:02:33.030 --> 00:02:37.650 So Bob's email client decrypts Alice's digital signature, 51 00:02:37.650 --> 00:02:42.030 and the result is the hash digest of Alice's message 52 00:02:42.030 --> 00:02:44.760 that Alice's email client created. 53 00:02:44.760 --> 00:02:49.050 Now, Bob's email client takes Alice's email message 54 00:02:49.050 --> 00:02:51.870 and using the same hashing algorithm 55 00:02:51.870 --> 00:02:56.310 that Alice's email client used, he hashes Alice's email. 56 00:02:56.310 --> 00:02:58.800 If the calculated hash digest 57 00:02:58.800 --> 00:03:01.350 matches the received hash digest, 58 00:03:01.350 --> 00:03:05.070 then the email message was not modified in transit 59 00:03:05.070 --> 00:03:08.940 and the message integrity has not been compromised. 60 00:03:08.940 --> 00:03:10.980 But what about authenticity? 61 00:03:10.980 --> 00:03:14.190 How do we know that it was really Alice's email client 62 00:03:14.190 --> 00:03:15.720 that's signed the message? 63 00:03:15.720 --> 00:03:18.390 That's where non-repudiation comes in. 64 00:03:18.390 --> 00:03:22.020 And one of the key benefits of digital signatures is 65 00:03:22.020 --> 00:03:24.750 that they not only provide integrity, 66 00:03:24.750 --> 00:03:28.140 they also provide non-repudiation. 67 00:03:28.140 --> 00:03:31.740 So second, we have non-repudiation. 68 00:03:31.740 --> 00:03:33.330 Non-repudiation means 69 00:03:33.330 --> 00:03:36.360 that once a digital signature is created, 70 00:03:36.360 --> 00:03:41.190 the signer cannot later deny having signed the document. 71 00:03:41.190 --> 00:03:43.920 This is because the signature is created 72 00:03:43.920 --> 00:03:46.530 using the signer's private key, 73 00:03:46.530 --> 00:03:49.470 which only the signer has access to. 74 00:03:49.470 --> 00:03:54.420 After all, in our digital signature example, the only person 75 00:03:54.420 --> 00:03:56.970 who could have encrypted Alice's email 76 00:03:56.970 --> 00:03:59.550 with her private key was Alice. 77 00:03:59.550 --> 00:04:03.300 This is because Alice keeps her private key private. 78 00:04:03.300 --> 00:04:04.950 It is never shared. 79 00:04:04.950 --> 00:04:09.750 So at email receipt, Bob can determine that Alice had 80 00:04:09.750 --> 00:04:12.960 to be the one to encrypt the digital signature 81 00:04:12.960 --> 00:04:15.810 because he can use Alice's public key 82 00:04:15.810 --> 00:04:18.540 to decrypt the digital signature. 83 00:04:18.540 --> 00:04:21.120 And in asymmetric encryption, 84 00:04:21.120 --> 00:04:25.140 a public key can only decrypt something that was encrypted 85 00:04:25.140 --> 00:04:27.540 by the associated private key. 86 00:04:27.540 --> 00:04:31.140 So Bob decrypts Alice's digital signature 87 00:04:31.140 --> 00:04:33.000 using her public key, 88 00:04:33.000 --> 00:04:37.020 providing both integrity and non-repudiation. 89 00:04:37.020 --> 00:04:40.950 But if Alice also wants to provide confidentiality, 90 00:04:40.950 --> 00:04:43.800 she will need to encrypt the message itself. 91 00:04:43.800 --> 00:04:47.370 And this can be done using Bob's public key. 92 00:04:47.370 --> 00:04:49.830 This works because if something is encrypted 93 00:04:49.830 --> 00:04:52.530 with Bob's public key, then only Bob 94 00:04:52.530 --> 00:04:54.810 with his private key can decrypt it. 95 00:04:54.810 --> 00:04:57.720 And since Bob keeps his private key private, 96 00:04:57.720 --> 00:05:00.810 the confidentiality of the message from Alice 97 00:05:00.810 --> 00:05:02.760 to Bob is assured. 98 00:05:02.760 --> 00:05:05.520 This is where people get a bit confused. 99 00:05:05.520 --> 00:05:09.180 So you may need to remember that to provide privacy 100 00:05:09.180 --> 00:05:12.750 and confidentiality, the receiver's public key, 101 00:05:12.750 --> 00:05:15.120 that's Bob's public key, is used 102 00:05:15.120 --> 00:05:16.920 to encrypt the message. 103 00:05:16.920 --> 00:05:19.410 To ensure the integrity of the message, 104 00:05:19.410 --> 00:05:23.520 Alice needs to hash it to create a hash digest. 105 00:05:23.520 --> 00:05:26.220 And to ensure authenticity of the message 106 00:05:26.220 --> 00:05:27.840 through non-repudiation, 107 00:05:27.840 --> 00:05:31.500 Alice needs to encrypt the hash digest of her message 108 00:05:31.500 --> 00:05:33.180 with her private key. 109 00:05:33.180 --> 00:05:36.570 If all three actions are taken, we call this encrypting 110 00:05:36.570 --> 00:05:39.030 and digitally signing the message. 111 00:05:39.030 --> 00:05:42.840 Most email clients can be configured to support this 112 00:05:42.840 --> 00:05:44.610 as long as you have public 113 00:05:44.610 --> 00:05:48.630 and private key pairs assigned to all your users. 114 00:05:48.630 --> 00:05:53.220 Now using OpenSSL, let's walk through a demonstration 115 00:05:53.220 --> 00:05:55.230 of a digital signature. 116 00:05:55.230 --> 00:05:58.860 In the real world, this process would be done for you, 117 00:05:58.860 --> 00:06:02.730 but it's important to understand academically how each 118 00:06:02.730 --> 00:06:04.350 of these steps would occur. 119 00:06:04.350 --> 00:06:07.440 So we'll walk through the process manually. 120 00:06:07.440 --> 00:06:09.060 Again, our scenario 121 00:06:09.060 --> 00:06:12.960 will be Alice sending a digital signature to Bob. 122 00:06:12.960 --> 00:06:14.790 In the real world to start off, 123 00:06:14.790 --> 00:06:17.790 both Alice and Bob would already have public 124 00:06:17.790 --> 00:06:19.590 and private key pairs. 125 00:06:19.590 --> 00:06:21.510 In this case, they don't. 126 00:06:21.510 --> 00:06:24.330 But for a digital signature, we're only going 127 00:06:24.330 --> 00:06:27.570 to need Alice's public and private key pair. 128 00:06:27.570 --> 00:06:29.430 So let's create those. 129 00:06:29.430 --> 00:06:33.150 We'll start by creating Alice's private key 130 00:06:33.150 --> 00:06:36.843 using this OpenSSL command. 131 00:06:39.810 --> 00:06:41.280 And it's created. 132 00:06:41.280 --> 00:06:44.400 Then we'll create Alice's public key, 133 00:06:44.400 --> 00:06:49.400 alice_public.key from Alice's private key, 134 00:06:49.740 --> 00:06:51.963 which is called alice_private.key. 135 00:06:57.300 --> 00:06:58.890 There we go. 136 00:06:58.890 --> 00:07:02.880 Now, the first step in the digital signature process 137 00:07:02.880 --> 00:07:06.690 is Alice creating a message to send to Bob. 138 00:07:06.690 --> 00:07:09.210 We'll do this using the echo command 139 00:07:09.210 --> 00:07:11.920 and create a file called alice_message.txt 140 00:07:14.580 --> 00:07:18.063 that just says, this is Alice's message to Bob. 141 00:07:19.650 --> 00:07:23.880 Next, we need to create Alice's digital signature. 142 00:07:23.880 --> 00:07:28.350 This will require hashing Alice's alice_message.txt file, 143 00:07:29.790 --> 00:07:30.810 and signing it 144 00:07:30.810 --> 00:07:34.507 with Alice's private key called Alice_private.key. 145 00:07:36.630 --> 00:07:38.940 This is done in one step, 146 00:07:38.940 --> 00:07:43.407 and the result will be a file called alice_signature.bin 147 00:07:45.870 --> 00:07:49.860 where bin stands for binary, which is the format 148 00:07:49.860 --> 00:07:52.173 that the signature file will be in. 149 00:07:55.140 --> 00:07:56.820 And it's created. 150 00:07:56.820 --> 00:08:01.327 Now Alice would send her alice_message.txt file 151 00:08:03.360 --> 00:08:05.670 along with her digital signature, 152 00:08:05.670 --> 00:08:09.840 the alice_signature.bin file 153 00:08:09.840 --> 00:08:14.840 and her public key, alice_public.key to Bob. 154 00:08:15.390 --> 00:08:17.010 With all of that in hand, 155 00:08:17.010 --> 00:08:21.960 Bob will decrypt the alice_signature.bin file 156 00:08:21.960 --> 00:08:25.950 with the alice_public.key file. 157 00:08:25.950 --> 00:08:28.530 The result will be the hash digest 158 00:08:28.530 --> 00:08:33.530 of the alice_message.txt file that Alice created. 159 00:08:34.110 --> 00:08:38.610 Then Bob will take the alice_message.txt file 160 00:08:38.610 --> 00:08:40.170 that Alice sent him, 161 00:08:40.170 --> 00:08:43.140 and he will hash it with the same hashing algorithm 162 00:08:43.140 --> 00:08:46.713 that Alice used, in this case, SHA 256. 163 00:08:47.610 --> 00:08:50.580 At this point, Bob has the hash 164 00:08:50.580 --> 00:08:54.120 of the alice_message.txt file 165 00:08:54.120 --> 00:08:57.990 that Alice created and the one that he created. 166 00:08:57.990 --> 00:08:59.820 So he compares them. 167 00:08:59.820 --> 00:09:04.260 If they are the same, the digital signature is verified OK 168 00:09:04.260 --> 00:09:08.010 and the integrity of the message is validated. 169 00:09:08.010 --> 00:09:12.123 This will all happen in one OpenSSL command. 170 00:09:15.360 --> 00:09:20.173 As you can see, the digital signature has been verified OK. 171 00:09:21.360 --> 00:09:24.600 This is the end of our demonstration. 172 00:09:24.600 --> 00:09:26.610 So remember, 173 00:09:26.610 --> 00:09:30.480 digital signatures are a cryptographic method used 174 00:09:30.480 --> 00:09:34.350 to ensure authenticity and integrity of data. 175 00:09:34.350 --> 00:09:37.770 They use a pair of keys, a private key 176 00:09:37.770 --> 00:09:39.780 to create the signature 177 00:09:39.780 --> 00:09:42.810 and a public key to verify it, confirming 178 00:09:42.810 --> 00:09:45.630 that the data has not been altered. 179 00:09:45.630 --> 00:09:49.740 Digital signatures also provide non-repudiation, 180 00:09:49.740 --> 00:09:53.610 meaning the signer cannot deny having signed the data 181 00:09:53.610 --> 00:09:56.910 because the signature was created with their private key, 182 00:09:56.910 --> 00:09:59.280 which only they possess. 183 00:09:59.280 --> 00:10:02.310 While digital signatures ensure authenticity 184 00:10:02.310 --> 00:10:06.300 and integrity, they do not provide confidentiality. 185 00:10:06.300 --> 00:10:07.530 To achieve that, 186 00:10:07.530 --> 00:10:10.050 the message itself must be encrypted 187 00:10:10.050 --> 00:10:12.840 using the recipient's public key. 188 00:10:12.840 --> 00:10:16.290 Combining these techniques, digital signatures 189 00:10:16.290 --> 00:10:18.450 for authenticity and integrity 190 00:10:18.450 --> 00:10:22.020 and encryption for confidentiality, creates a secure 191 00:10:22.020 --> 00:10:25.143 and trustworthy communication process.