1 00:00:02,170 --> 00:00:06,640 Hey, everyone, and welcome back to this class natural language processing in Python. 2 00:00:11,550 --> 00:00:16,770 In this lecture, we are going to begin looking at the code to demonstrate our cipher decryption algorithm. 3 00:00:17,520 --> 00:00:22,170 This particular lecture will focus on how to create the original substitution cipher. 4 00:00:22,890 --> 00:00:24,990 Remember that this is a simulation. 5 00:00:25,500 --> 00:00:28,890 Normally, we don't know the true answer, but in this case we do. 6 00:00:29,490 --> 00:00:35,730 The purpose is that we can compare our answer to the true answer to assess the efficacy of our algorithm. 7 00:00:36,330 --> 00:00:41,790 So our goal by the end of this code is to see if we get the correct answer and we will know what the 8 00:00:41,790 --> 00:00:44,940 correct answer is because that's what we started with. 9 00:00:46,800 --> 00:00:50,810 First, we're going to do all of our usual imports, Nampai and map plot lib. 10 00:00:51,280 --> 00:00:56,310 We'll also need the string library, which provides us with a convenient way to load all the valid ASCII 11 00:00:56,310 --> 00:00:57,030 characters. 12 00:00:57,600 --> 00:01:02,520 We'll need the random library to shuffle the ASCII characters and create our substitution cipher. 13 00:01:03,180 --> 00:01:08,820 We'll need the regex library to do some string operations later on in the scripts, and we'll also need 14 00:01:08,820 --> 00:01:12,870 the O's and Tex rep libraries to do some miscellaneous tasks later on. 15 00:01:17,770 --> 00:01:24,160 So this lecture will focus on this first block of code where we create the substitution cipher to start, 16 00:01:24,160 --> 00:01:27,280 we'll just create two lists of letters, letters one and letters two. 17 00:01:27,880 --> 00:01:30,190 When you call string that ASCII lowercase. 18 00:01:30,520 --> 00:01:34,690 This returns all 26 letters in lowercase in alphabetical order. 19 00:01:35,200 --> 00:01:38,770 You can print them out yourself in order to confirm what I've said is correct. 20 00:01:39,370 --> 00:01:41,500 We also cast the Strings two lists. 21 00:01:41,950 --> 00:01:46,990 This is helpful so we can have one element of each list represent a single character at a time. 22 00:01:48,130 --> 00:01:51,460 Next, we initialize an empty dictionary called True Mapping. 23 00:01:52,150 --> 00:01:57,790 The rest of this code will populate this dictionary, so the next step is to randomize the second list 24 00:01:57,790 --> 00:01:59,500 of letters letters to. 25 00:02:00,130 --> 00:02:04,990 There's no need to randomize the first list since those will just be the keys of the dictionary, and 26 00:02:04,990 --> 00:02:06,940 they'll map to the letters of the second list. 27 00:02:07,780 --> 00:02:10,660 Next, we loop through both lists in corresponding order. 28 00:02:11,230 --> 00:02:13,690 We can accomplish that by using the zip function. 29 00:02:14,320 --> 00:02:19,870 So in the first iteration of this loop, K will be the first element of letters one, and we will be 30 00:02:19,870 --> 00:02:21,430 the first element of letters to. 31 00:02:22,400 --> 00:02:27,290 On the second iteration, Kay will be the second element of letters one, and we will be the second 32 00:02:27,290 --> 00:02:30,260 element of letters to inside the loop. 33 00:02:30,270 --> 00:02:34,790 We assign Kay to be the key of the true mapping and a V to be the value. 34 00:02:35,660 --> 00:02:40,860 Once we complete this loop, the true mapping dictionary will represent our substitution cipher. 35 00:02:41,660 --> 00:02:47,360 Conceptually, only the sender and receiver know what the true mapping is and the intruder does not. 36 00:02:48,050 --> 00:02:51,230 The intruder will use the genetic algorithm we described previously. 37 00:02:53,110 --> 00:02:58,270 So the rest of his lectures are under the assumption that you are the intruder and you're trying to 38 00:02:58,270 --> 00:03:01,840 decrypt the message without knowing what this true mapping is.