WEBVTT 00:00.350 --> 00:06.260 All right, it's finally time to tackle our final project, the Caesar cipher. 00:06.320 --> 00:15.590 As I've mentioned before, Caesar cipher is this way of encoding text that was seen as early as during 00:15.590 --> 00:16.880 the times of Julius Caesar. 00:16.880 --> 00:24.560 So when he had these top secret military messages, what he would do is he would shift each letter of 00:24.560 --> 00:28.460 the alphabet by a certain predetermined amount. 00:28.490 --> 00:30.980 I want to show you quickly how this works. 00:30.980 --> 00:35.990 So we've got the alphabet here, and let's say we wanted to encode the letter E. 00:36.350 --> 00:40.910 So now we can line up the alphabet with a new set of alphabet. 00:40.910 --> 00:48.110 And at the moment the shift is zero because the top alphabet and the bottom alphabet are lined up with 00:48.110 --> 00:49.400 a zero difference. 00:49.430 --> 00:52.400 Now let's say that we had a shift of one. 00:52.400 --> 00:59.300 Well, then the alphabet moves to the left and A becomes B, B becomes C, etc. and we can keep going 00:59.350 --> 01:02.770 until we get to the amount of shift that we wanted. 01:02.770 --> 01:07.180 So let's say we're going to encode all of our text with the shift of three. 01:07.210 --> 01:13.750 We'll then E becomes h, f becomes I, g becomes J, etc. and so on and so forth. 01:13.750 --> 01:20.860 And they've actually discovered these artifacts from ages ago where people have created these sort of 01:20.890 --> 01:21.730 dials. 01:21.730 --> 01:29.020 And by simply rotating the dial to a certain amount of shift, then you can line up the letters with 01:29.020 --> 01:29.740 each other. 01:29.740 --> 01:34.570 So A becomes N and Z becomes O, and so on and so forth. 01:34.570 --> 01:38.320 And this is a way that people actually encoded top secret messages. 01:38.500 --> 01:45.280 By the end of this day, you have also built a digital form of the Caesar cipher. 01:45.370 --> 01:50.560 And all you have to do is to type encode to start encoding a message. 01:50.560 --> 01:53.590 Let's say something like hello World. 01:53.680 --> 01:59.820 And once we hit enter, we get to type the shift number, which I'm just going to choose a random one 01:59.820 --> 02:03.150 and it gives us the encoded result. 02:03.180 --> 02:11.160 So now if I take this encoded result and I go ahead and type yes to restart my program, and I'm going 02:11.190 --> 02:15.600 to use the decode function to decode my message. 02:16.080 --> 02:22.710 Now when I use the same shift number, then I should be able to get back the decoded result. 02:22.740 --> 02:27.540 So if there's people monitoring your phone or you're trying to throw a message to your friend in a paper 02:27.540 --> 02:34.710 ball, then this is an easy way to ensure that if your message was intercepted, then it won't be understood 02:34.740 --> 02:36.420 by the other person. 02:36.600 --> 02:41.070 All right, so once you're ready, let's get started building out the Caesar cipher. 02:41.100 --> 02:44.550 Now you'll notice that I've split it into three parts. 02:44.550 --> 02:47.880 So three, four, five are all the Caesar cipher. 02:47.880 --> 02:50.160 And that's because there's quite a few things to do. 02:50.160 --> 02:55.230 And I thought it might be a good idea to break it down into separate lessons so that we can go through 02:55.230 --> 02:59.660 them individually and hopefully it'll make this very large problem. 02:59.690 --> 03:00.860 A little bit easier. 03:00.890 --> 03:07.100 And we practice that key engineering skill, which is breaking down a large problem into smaller problems, 03:07.100 --> 03:11.180 and then taking a small problem, breaking it down into even smaller problems. 03:11.180 --> 03:15.680 That is the only way that we can get through really, really hard problems. 03:15.710 --> 03:16.280 All right. 03:16.280 --> 03:22.160 So you'll notice that in Caesar cipher one I've split the tasks into four to dos. 03:22.160 --> 03:25.460 You'll notice that order is a little bit weird because it goes four then three. 03:25.460 --> 03:30.500 But they're roughly at the places where I expect you to write the relevant code. 03:30.500 --> 03:32.330 So it's not a mistake. 03:32.330 --> 03:34.760 It's actually very much on purpose. 03:34.790 --> 03:40.580 Now, what I want you to do is to go through each of the to dos and read through their descriptions 03:40.580 --> 03:44.030 on the right hand pane and try to solve them. 03:44.030 --> 03:48.140 If you get stuck, there are some hints in there that will hopefully help you. 03:48.140 --> 03:52.670 But essentially the really important thing is you actually read through the to dos, because some of 03:52.670 --> 03:55.210 the things we might not have seen before. 03:55.390 --> 04:02.950 For example, in order to complete the task, you will need to use a built in function called index. 04:02.980 --> 04:11.080 And index is really handy because it allows us to find out the position of any item in a list. 04:11.080 --> 04:15.970 So if we had a list of fruits and we know that they start counting from zero. 04:15.970 --> 04:20.170 So apple is at position zero, pear is at one, orange is at two. 04:20.200 --> 04:24.160 Then we can say fruits, the name of the list dot index. 04:24.160 --> 04:29.740 And then in between the parentheses of this index function we pass in whatever it is we want to check. 04:29.740 --> 04:34.180 So in this case pear would give us a position of one. 04:34.180 --> 04:37.450 And we can verify this by printing it out. 04:37.450 --> 04:44.080 So I can show this to you in the Python console, which is basically something that's completely separate 04:44.080 --> 04:45.610 from the rest of our code. 04:45.610 --> 04:54.680 And if I go ahead and now print this value, you can see it gives me one, which is what we would expect. 04:54.680 --> 05:02.210 So this is going to be key for you to be able to solve this challenge, especially to do number two. 05:02.240 --> 05:04.280 So look through the description. 05:04.280 --> 05:05.840 It's there to help you. 05:05.840 --> 05:09.650 And hopefully you'll be able to tackle each of these to do's. 05:09.650 --> 05:13.310 And in order to figure out what it is that you're trying to aim for. 05:13.340 --> 05:19.070 As always, you can right click on solution and run it without viewing the actual solution. 05:19.070 --> 05:25.220 So we already have some pre-built in code which I've added in for you. 05:25.250 --> 05:32.270 I wanted to save you the tedious task of typing out all the 26 letters of the alphabet, as well as 05:32.390 --> 05:34.460 all the various inputs that we need. 05:34.460 --> 05:41.150 So we need the user to tell us whether, if they want to encode or decode a message, and then we want 05:41.150 --> 05:46.370 to actually ask them for the message, and then we want to ask them for the shift number. 05:46.370 --> 05:51.310 So what you want to do is by the time you have entered these three pieces of information. 05:51.340 --> 05:55.300 The next time I hit enter, it should give me a printed output. 05:55.330 --> 06:00.940 Here is the encoded result and it gives me the encoded output. 06:00.940 --> 06:02.620 So we're encoding. 06:02.620 --> 06:03.130 Hello. 06:03.130 --> 06:06.790 So h plus three letters I j k. 06:06.820 --> 06:12.640 So h becomes k e plus three letters is f g h. 06:12.640 --> 06:15.700 So this becomes h and so on and so forth. 06:15.700 --> 06:21.940 Until we complete the entire text that we are supposed to encode by the shift number. 06:21.970 --> 06:23.800 So that's all there is to it. 06:23.800 --> 06:29.800 Look through the to dos, look through the description and try to complete part one of Caesar cipher. 06:29.830 --> 06:35.350 I'll go quiet now and you can pause the video and tackle this challenge. 06:36.100 --> 06:37.480 Cths. 06:40.660 --> 06:41.260 All right. 06:41.290 --> 06:45.880 Hopefully you have completed this challenge in its entirety. 06:45.910 --> 06:51.240 If you haven't, I'm going to go through the solution together with you and hopefully by viewing the 06:51.240 --> 06:57.480 solution, some things will become more obvious and you will be able to go back and tackle the code 06:57.480 --> 06:58.590 by yourself. 06:58.590 --> 07:02.430 So the first step is to create a function called encrypt. 07:02.430 --> 07:08.580 I'm going to hide this right hand side pane just so that I can show you the to do in the entire line. 07:08.580 --> 07:14.580 So we create a function with def, and then the function is going to be called encrypt. 07:14.580 --> 07:17.250 And then we add a set of parentheses. 07:17.250 --> 07:21.030 And in this case we want to have two inputs. 07:21.030 --> 07:26.190 One is called original text and the other one is called shift amount. 07:26.190 --> 07:30.210 And we add those if you remember, into the parentheses. 07:30.210 --> 07:35.490 Finally we finish this function definition with a colon. 07:35.490 --> 07:39.570 And then we can write something inside the function. 07:39.570 --> 07:43.800 So to do number two is going to go inside the function. 07:43.800 --> 07:47.490 So I'm just going to bring it up here to make it easier to see. 07:48.110 --> 07:55.340 So inside the encrypt function we're going to shift each letter of the original text forwards in the 07:55.340 --> 07:59.990 alphabet by the shift amount and print the encrypted text. 07:59.990 --> 08:02.240 So there's a couple of things that we need to do. 08:02.240 --> 08:05.060 So we're going to do them in several steps. 08:05.630 --> 08:12.470 And the first step is to loop through each of the letters in this text. 08:12.470 --> 08:14.900 So we can do that using a for loop. 08:14.900 --> 08:27.170 So for letter in the original text we're going to go ahead and figure out what is the shifted position 08:27.170 --> 08:28.460 for that letter. 08:28.460 --> 08:35.360 So if we take the current alphabet and we use dot index. 08:35.360 --> 08:40.130 And inside here we pass in the letter that we're currently looping through. 08:40.130 --> 08:43.400 So let's start with a concrete example. 08:43.400 --> 08:46.390 That way we can go through it line by line. 08:46.390 --> 08:51.640 So if the original text was hello, and if the shift amount. 08:51.670 --> 08:57.430 Was two, then the letter that we first loop through is h. 08:57.430 --> 09:04.720 So h is going to be checked against this list called alphabet for its index. 09:04.720 --> 09:11.860 So h is at position 01234567. 09:11.860 --> 09:15.340 So h is at position seven right. 09:15.340 --> 09:18.280 And this becomes seven. 09:18.550 --> 09:25.000 And once we've gotten this number what we want to do is we want to shift it upwards. 09:25.000 --> 09:25.540 Right. 09:25.570 --> 09:30.640 And we want to shift it by the shift amount, which in our case we said was going to be two. 09:30.670 --> 09:33.640 So h plus two is I j. 09:33.640 --> 09:35.170 So it becomes j. 09:35.170 --> 09:38.140 So how do we make it go to that position. 09:38.140 --> 09:44.290 Well we take this index seven and we add the shift amount to it. 09:44.310 --> 09:52.020 And then this means seven, then becomes nine, seven plus two becomes nine, and position nine is the 09:52.020 --> 09:54.270 letter J, which is what we want. 09:54.270 --> 10:00.150 So how can we turn this into the actual letter? 10:00.180 --> 10:07.770 Well, firstly we need to probably save this shifted position into a variable so that we can use it 10:07.770 --> 10:09.210 on the next line. 10:09.210 --> 10:19.110 And what we're going to do is we're going to create a letter by using the alphabet list, using the 10:19.110 --> 10:24.120 square brackets, and tapping to the item at this shifted position. 10:24.270 --> 10:26.640 Now what do we do with this letter. 10:26.640 --> 10:28.890 We actually need to accumulate it. 10:28.890 --> 10:36.270 So in order to accumulate it we need to have some sort of empty string, which I will call ciphertext. 10:36.300 --> 10:39.930 We started out as just an empty string with nothing in it. 10:39.930 --> 10:45.710 And as always, it's very important that it's outside the for loop because we don't want it to reset 10:45.740 --> 10:48.530 each time if it is inside the for loop. 10:48.530 --> 10:56.720 So now going inside the for loop so indented inside here I'm going to set the ciphertext to plus equal 10:56.720 --> 11:02.330 this new letter that we've found by this shifting process. 11:02.330 --> 11:09.350 So at each of these steps I recommend printing out the values that you've got just to understand what 11:09.350 --> 11:11.480 is going on behind the scenes. 11:11.480 --> 11:16.310 So at this point we've now got the letter J from this. 11:16.310 --> 11:23.030 And this J is being added to the ciphertext as the first value in there. 11:23.030 --> 11:26.570 So it should look something like this. 11:27.290 --> 11:36.050 Now the next step is to go ahead and for this loop to continue to the next cycle. 11:36.050 --> 11:38.150 So we've now done H. 11:38.180 --> 11:44.110 It's going to go to the next letter in the original text, which is E, and it's going to repeat this 11:44.110 --> 11:50.020 process until we end up with all of the letters of the ciphertext. 11:50.110 --> 11:54.340 And once it's done that, then we want to print it out. 11:54.340 --> 11:57.640 So making sure that we are outside of the for loop. 11:57.640 --> 12:04.330 So once the for loop has completed then we will go ahead and make a print statement. 12:04.360 --> 12:08.440 And what we want to print is something like this. 12:08.440 --> 12:11.680 Here is the encoded result. 12:11.710 --> 12:20.620 Now if I go ahead and turn this into an F string then I can replace this with my ciphertext. 12:20.620 --> 12:29.620 And now all I need to do is to actually call this function so that it gets triggered and it passes in 12:29.620 --> 12:33.010 all of the user inputs from up here. 12:33.040 --> 12:38.890 Now remember with functions, just defining it and running your code is not going to do anything. 12:38.890 --> 12:42.420 It's just going to sit there until it is called upon. 12:42.420 --> 12:47.160 So let's complete to do number three and complete this process. 12:47.160 --> 12:52.650 So let's call this function encrypt and pass in the user inputs. 12:52.650 --> 12:58.050 So I'm going to use a keyword arguments just to make it a little bit more clear. 12:58.050 --> 13:04.860 So I'm going to say original text is equal to the text that was entered by the user. 13:05.070 --> 13:12.780 And then the next value that I need to add in is the shift amount. 13:12.780 --> 13:16.320 So here let's go ahead and add the shift amount. 13:16.320 --> 13:21.630 And that's going to be set equal to whatever the user typed in here as an input. 13:22.020 --> 13:23.850 So it's called shift. 13:23.880 --> 13:31.530 And now I can run this file and go ahead and go through the process of encoding the word hello. 13:31.530 --> 13:34.020 And I'm going to shift it by two again. 13:34.020 --> 13:37.530 So now you can see the result is printed out. 13:37.550 --> 13:41.780 And the first letter indeed does start with J. 13:42.470 --> 13:47.000 And then it continues working out all of the other letters. 13:47.030 --> 13:53.990 And you can see that there's two ends, because there are two L's in the original text. 13:53.990 --> 13:57.290 So that all seems to be working just fine. 13:57.320 --> 14:02.540 Now there is one slight problem which is highlighted by this to do number four. 14:02.570 --> 14:08.570 What happens if you try to shift the letter Z forwards by nine? 14:08.600 --> 14:10.250 Well let's try it. 14:10.250 --> 14:14.210 Let's encode and we're going to encode just the letter z. 14:14.240 --> 14:16.040 We're going to shift it by nine. 14:16.070 --> 14:22.970 Well we get a classic error index error list index out of range. 14:22.970 --> 14:24.890 So what's going on here. 14:24.890 --> 14:32.510 Well if we take a look at our alphabet we know that Z has position 25. 14:32.540 --> 14:35.450 And I know that because there's 26 letters in the alphabet. 14:35.480 --> 14:39.430 If we start from zero, then Z is at 25. 14:39.460 --> 14:43.210 So now we've got that position. 14:43.210 --> 14:44.800 This is 25. 14:44.800 --> 14:46.930 And then we add a shift amount. 14:46.960 --> 14:48.790 We try to add nine to it. 14:48.790 --> 14:53.080 So nine plus 25 is 34. 14:53.110 --> 15:00.610 And then we're going to try and take 34 as the shifted position to go into the alphabet. 15:00.640 --> 15:07.480 Now of course that is going to give us a index out of range error because there is nothing at position 15:07.510 --> 15:08.110 34. 15:08.140 --> 15:10.450 It ends at 25. 15:10.450 --> 15:14.050 So how can we solve this problem? 15:14.050 --> 15:21.430 Well, if you think about this problem, there's actually many, many ways that you can solve it. 15:21.430 --> 15:25.540 Um, some are easier and some are more difficult. 15:25.540 --> 15:34.880 But if you think back to when we learned about the modulo, you might remember that we can use the modulo 15:34.880 --> 15:36.890 to get the remainder. 15:36.950 --> 15:40.850 So let's go into the Python console again and let me show you. 15:40.880 --> 15:52.010 So if we had the number four and I tried to modulo it or divide it by 25, it's going to give me four 15:52.010 --> 15:56.180 because four does not divide by 25. 15:56.180 --> 16:01.280 So the remainder is what we had at the beginning, which is four. 16:01.310 --> 16:04.850 Now let's see what happens if we go over 25. 16:04.850 --> 16:11.990 Let's say that we had 34 and we tried to modulo it by 25. 16:12.020 --> 16:16.220 Well what's going to happen here is 34 divided by 25. 16:16.250 --> 16:19.670 There is one set of 25 within 34. 16:19.670 --> 16:22.190 And then it's going to give us the remainder. 16:22.190 --> 16:25.580 So that means it's going to give us nine. 16:25.610 --> 16:32.920 And this is really really useful because in our case this is exactly the behavior we want. 16:32.920 --> 16:41.080 If we go outside the range just by modulating by the length of the alphabet, we can make sure that 16:41.080 --> 16:51.490 no matter how far outside of the range of the list, we're always going to adjust it to within the length 16:51.490 --> 16:54.190 from 0 to 25. 16:54.220 --> 17:05.380 So to do this, all we need to do is take this shifted position and we're going to set it to equal the 17:05.380 --> 17:10.450 previous version of shifted position modulo by the length. 17:10.450 --> 17:14.650 So the number of items in our list called alphabet. 17:14.680 --> 17:23.200 Now just as what you can do with plus equals where you take the ciphertext and add this alphabet to 17:23.230 --> 17:27.700 it and then set it as the new value, you can do the same with modulo. 17:27.700 --> 17:34.410 So we can actually change this to modulo equals to make our code even more short. 17:34.740 --> 17:44.880 So shifted position modulo equals the length of the alphabet is going to make sure that we are always 17:44.880 --> 17:48.480 within the range of 0 to 25. 17:48.660 --> 17:53.070 And this is going to prevent that problem that we had earlier. 17:53.070 --> 18:02.100 So if I go ahead and run the code and type Z and I shift it up by nine, you'll see that it's performing 18:02.100 --> 18:09.210 the correct implementation, which is taking it all the way around and starting over here. 18:09.210 --> 18:15.990 So Z plus nine is 123456789. 18:15.990 --> 18:19.500 And it should as expected give us I. 18:19.530 --> 18:21.840 Now this can be quite confusing. 18:21.840 --> 18:29.840 This idea of the modulo um because it does involve a little bit of maths, but it's nothing that you 18:29.870 --> 18:30.800 can't understand. 18:30.800 --> 18:37.190 If you just try out some principles, try out some different letters, try out some different numbers, 18:37.190 --> 18:42.740 experiment with it, and I'm sure that you'll eventually understand what's going on. 18:42.800 --> 18:47.240 Don't move on until you actually understand what the code does. 18:47.270 --> 18:55.010 Alternatively, you could have tried some of the other methods that I gave in the hint, and maybe one 18:55.010 --> 19:00.590 of the other ones would be a little bit easier to understand for you, but I think this one should be 19:00.590 --> 19:03.740 doable by most of the students. 19:03.740 --> 19:08.330 As long as you invest a bit of time understanding how it works. 19:08.420 --> 19:12.020 So that concludes part one of the Caesar cipher. 19:12.020 --> 19:16.970 And we've got a functioning cipher that can now encode messages. 19:16.970 --> 19:24.020 In the next part we're going to tackle how to decode messages and how to combine the encryption and 19:24.020 --> 19:26.270 decryption functionalities together. 19:26.270 --> 19:28.670 So for all of that and more I'll see you there.