WEBVTT 0 00:01.700 --> 00:07.460 In this lecture we'll talk about the applications of hash algorithms. 1 00:07.460 --> 00:11.640 You'll find hashes in many areas of computer science. 2 00:11.860 --> 00:13.210 Hashes are everywhere. 3 00:13.230 --> 00:20.930 They're a critical part of password systems, digital signatures, security certificates, blockchain technology, 4 00:21.260 --> 00:26.950 or even how the ATM of your bank verifies the PIN of your credit card 5 00:27.080 --> 00:36.140 when you want to withdraw some cash. Let's see the most useful use-cases! Cryptographic hash functions 6 00:36.380 --> 00:41.750 are very commonly used in password verification. For security reasons 7 00:41.770 --> 00:50.240 password are not stored in clear text. If an attacker was to break into the system and steal the file or 8 00:50.240 --> 00:53.450 the database where passwords are stored, 9 00:53.450 --> 01:01.850 the attacker could then access each user account. A more secure way to store a password is to transform 10 01:01.850 --> 01:08.600 it into data that cannot be converted back into the original password and this is exactly what 11 01:08.600 --> 01:17.710 hashing does; so instead of saving the clear text password the system will calculate its hash and store 12 01:17.800 --> 01:18.730 that hash. 13 01:18.790 --> 01:27.000 This is how Linux, Windows, databases and all other web applications work. Let's see 14 01:27.000 --> 01:31.130 the contents of etc/shadow. 15 01:31.280 --> 01:34.230 Let's take a look at this line. 16 01:34.250 --> 01:43.470 This is the saved hash of the password of user Kali. No one can find out what is its password. By 17 01:43.470 --> 01:44.030 the way 18 01:44.070 --> 01:50.790 there is another dedicated lecture where I have explained in great detail how the Linux systems stores 19 01:50.820 --> 01:57.470 the user's passwords. I'd recommend you to watch that video as well. Okay! 20 01:57.480 --> 02:03.540 Later when the user wants to authenticate he will enter his clear text password. 21 02:03.870 --> 02:12.540 The system will immediately calculate the hash of the entered password and compare it to the hash saved 22 02:12.660 --> 02:17.190 in the password file; if the hashes are equal 23 02:17.190 --> 02:21.390 it means that the user has entered the correct password. 24 02:21.630 --> 02:30.330 Remember that each input, in this case password, has its own unique hash which is deterministic. A wrong 25 02:30.380 --> 02:33.120 password would have given another hash. 26 02:36.160 --> 02:42.400 Sometimes the password is combined to another piece of random data called salt. 27 02:42.460 --> 02:51.070 In this example this is the salt and hashed altogether. Due to rainbow tables hashing alone is not sufficient 28 02:51.130 --> 02:54.760 to protect passwords for mass exploitation. 29 02:54.800 --> 03:01.690 There is another section on rainbow tables and I recommend you to watch those videos as well. 30 03:02.720 --> 03:10.880 Let's move on and talk about File Integrity. Hashing is also used to verify the integrity of a file after 31 03:10.880 --> 03:18.170 it has been transferred from one place to another. When downloading an important document, 32 03:18.170 --> 03:25.760 new software or when updating existing one you want to ensure that your installation is safe, unaltered 33 03:26.060 --> 03:29.820 and from a reputable source. Let's 34 03:29.820 --> 03:35.190 imagine you want to install a new Linux distribution on your computer. 35 03:35.190 --> 03:43.930 Normally you go and download data from a mirror which is an http server closer to you geographically. 36 03:43.990 --> 03:50.710 It would be easy for a malicious entity to modify the installation kit so that it contains exploits 37 03:50.950 --> 03:56.230 or malware and host it unofficially. To prevent this issue 38 03:56.380 --> 04:03.730 you'll generate the hash or checksum of your downloaded ISO file and verify that it matches the 39 04:03.730 --> 04:06.300 hash published on the official website. 40 04:07.960 --> 04:17.170 In this example with Kali Linux - this hash. If the hashes are equal it confirms the ISO file hasn't 41 04:17.170 --> 04:20.020 been tampered with or corrupted. 42 04:20.020 --> 04:24.670 But if they are not equal it means your downloaded images 43 04:24.670 --> 04:27.400 integrity is compromised. 44 04:27.400 --> 04:31.500 Maybe it was corrupted during the download process. 45 04:31.570 --> 04:38.970 Remember that if a single bit is different in your downloaded file the file has a different hash than 46 04:39.040 --> 04:44.390 the one calculated by the development team and published online. 47 04:46.500 --> 04:49.530 Let me show you how this works! 48 04:49.540 --> 04:57.510 I've already downloaded CentOS 8 boot ISO file and I want to check its integrity. 49 04:57.780 --> 04:59.310 It's on my desktop. 50 04:59.370 --> 05:08.600 This is the file! I want to be sure that the file hasn't been tampered with or corrupted. 51 05:08.600 --> 05:10.430 Cenos.org has listed 52 05:10.430 --> 05:16.340 checksum information via https in the release notes for each software release. 53 05:17.550 --> 05:20.350 My release is 8.1911 54 05:20.400 --> 05:30.410 and this is the web page for that release. I will calculated the sha256 hash of the downloaded 55 05:30.410 --> 05:34.220 file and then compare the hash to the one published online. 56 05:34.880 --> 05:41.900 If they are the same I have verified the integrity of the ISO file and they can install it without any 57 05:41.900 --> 05:42.510 risk. 58 05:43.390 --> 05:53.960 Sha256sum and the file! it's calculating the hash!Okay. 59 05:54.010 --> 06:00.440 This is the hash; let's check it against the hash published online. 60 06:07.110 --> 06:07.630 Okay. 61 06:07.650 --> 06:13.320 It starts with 7fea13202bf and so on 62 06:19.040 --> 06:28.490 It's the same hash! Another application of hashes is the process of digital signing! Hashing is a critical 63 06:28.490 --> 06:31.520 component of the digital signing process. 64 06:31.520 --> 06:38.960 In fact the digital signature of a file is the hash of the file encrypted with the private key of the 65 06:38.960 --> 06:41.250 one that signs. 66 06:41.340 --> 06:45.210 Let's take a look at the digital certificate of Google.com. 67 06:49.430 --> 07:01.940 We see that the certificate authority signed it by using the same sha256 and RSA algorithms. The last 68 07:01.940 --> 07:03.620 application of hashes 69 07:03.620 --> 07:12.090 I'm gonna talk about in this lecture is related to data structures of programming languages Python dictionaries 70 07:12.180 --> 07:17.460 and sets or go maps are implemented as hash tables. 71 07:17.580 --> 07:19.810 We won't go deeper into it now. 72 07:19.890 --> 07:25.390 You should only know that they provide constant time look up and insertion time on average.