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This is a transcript for a video linked here: Introduction to Cryptography part 1.
welcome to this video on an introduction to cryptography 00:00:04.080 --> 00:00:08.700 it's an important field of cyber security providing a way of protecting data 00:00:08.700 --> 00:00:13.800 this video is going to explain what cryptography is and towards the end I'll give an example of 00:00:13.800 --> 00:00:19.620 how you can Implement a simple Cipher in Python in future videos I'll show how cryptography is used 00:00:19.620 --> 00:00:25.860 today and how we can use to protect information so good Communications and prove your identity 00:00:29.160 --> 00:00:34.740 the word cryptography comes from ancient Greek cryptos meaning hidden secret and graphy 00:00:34.740 --> 00:00:40.740 meaning to write a way to turn some readable information plain text into unintelligible 00:00:40.740 --> 00:00:46.560 nonsense Cipher text although text is used in this definition it doesn't need to be 00:00:46.560 --> 00:00:51.300 text could be any information that needs to be headed such as images or other binary data 00:00:54.120 --> 00:00:59.160 cryptography is far from a new idea another attempts to break the codes for thousands of 00:00:59.160 --> 00:01:04.800 years kings queens and other rulers have used cryptography to protect messages and for just 00:01:04.800 --> 00:01:09.240 as long others have tried to break those codes perhaps one of the most famous from history is 00:01:09.240 --> 00:01:13.860 that of Mary Queen of Scots who used the form of cryptography to protect her secret messages 00:01:13.860 --> 00:01:18.720 to her allies this included her instructions for Queen Elizabeth the first to be killed 00:01:19.320 --> 00:01:23.700 her Cipher was broken and messages used as evidence in her trial leading to her 00:01:23.700 --> 00:01:29.700 execution in 1586. showing how cryptography can literally be a case of life or death 00:01:31.560 --> 00:01:37.260 the use of cryptography has changed the course of Wars significantly during World War II Germany 00:01:37.260 --> 00:01:42.240 made use of the Enigma machine for encrypted messages used for orders and status reports to 00:01:42.240 --> 00:01:47.400 and from the Army and Navy as explained in the film Enigma the Enigma machine could be used 00:01:47.400 --> 00:01:54.420 to protect the messages the Enigma machine Germans have thousands of them what to do 00:01:55.500 --> 00:02:00.540 it turns plain text messages onto gobbledygook then the gobbledygook is transmitted in Morse 00:02:01.260 --> 00:02:05.040 at the receiving end there's another Enigma machine to turn it back into the original message 00:02:06.000 --> 00:02:08.880 press the same key any number of times it will always come out different 00:02:10.080 --> 00:02:13.620 some of the top mathematicians in the UK came together at Bletchley Park with 00:02:13.620 --> 00:02:17.520 the aim of breaking Enigma and other types of encryption used by the Germans 00:02:17.520 --> 00:02:24.060 the bombe shown here was designed by Alan Turin based on the Polish bomba it was further refined 00:02:24.060 --> 00:02:29.220 by Golden Welshman with an appropriate crib supplied by the cryptographers the bombes 00:02:29.220 --> 00:02:32.580 were used to work through different machine settings to try and find that day's settings 00:02:34.440 --> 00:02:39.480 another snippet of video this time from the imitation game sums up the tasks they faced 00:02:40.980 --> 00:02:46.380 and what is it that we're really doing we're going to break an unbreakable Nazi code and win the war 00:02:47.820 --> 00:02:54.480 oh one of the outcomes from Bexley Park was the creation of the world's first electronic computer 00:02:54.480 --> 00:03:01.380 Colossus the aim to decipher the Lorenz encrypted tunny messages between Hitler and his generals 00:03:01.980 --> 00:03:05.100 Colossus was designed and built by Tommy flowers and colleagues 00:03:05.100 --> 00:03:09.780 and was closely guarded secret until 1975 when the files were finally Declassified 00:03:13.260 --> 00:03:17.940 I'll just cover a few key aspects of terminology here before we move on to the examples 00:03:22.260 --> 00:03:28.020 so this shows a typical example of cryptography in use this is in fact a real example which we'll see 00:03:28.020 --> 00:03:33.720 later it's based around rot 13 but that's not important at this stage we start on the left 00:03:33.720 --> 00:03:38.940 and follow the flow to the right so we start with the plain text which may also be known as 00:03:38.940 --> 00:03:45.180 the unencrypted or raw data this is turned into ciphertext through a process known as encryption 00:03:46.080 --> 00:03:53.580 this is known as encrypting the data which results in the encrypted data shown here as the ciphertext 00:03:55.020 --> 00:03:59.040 the software that performs the encryption is based upon an algorithm and usually some form 00:03:59.040 --> 00:04:06.120 of key although the key may just be part of the algorithm to be able to read the ciphertext or 00:04:06.120 --> 00:04:11.640 encrypted data then it needs to be decrypted this is through a process known as decryption 00:04:11.640 --> 00:04:17.280 which again uses an algorithm and often some form of key depending upon the type of the 00:04:17.280 --> 00:04:22.080 encryption this may be the same key as used for the encryption or is often a different key 00:04:23.340 --> 00:04:29.580 this then returns the plain text or an encrypted data which can be read again it is the complexity 00:04:29.580 --> 00:04:35.460 of the algorithm and if appropriate the key which determines how secure the data is there are also 00:04:35.460 --> 00:04:40.020 different types of algorithms which depends upon the situation that the data will be used and can 00:04:40.020 --> 00:04:44.220 result in different levels of security this is something that will be covered in future videos 00:04:46.920 --> 00:04:53.040 I'd also like to introduce some characters this is Alice she's the one wanting to send a protected 00:04:53.040 --> 00:04:57.900 message and here is Bob he's the intended recipient who wants to read the messages 00:04:59.460 --> 00:05:04.560 and here is Eve who is trying to eavesdrop on the message to find out what Alice is telling Bob 00:05:05.460 --> 00:05:10.500 these are commonly used as fictional characters when discussing cryptography you'll often see them 00:05:10.500 --> 00:05:15.480 in other explanations and academic papers so it makes sense to continue to use those names here 00:05:18.660 --> 00:05:24.540 in my practical example I'm going to use rot 13. this is not a secure way of protecting information 00:05:24.540 --> 00:05:32.400 we're still used today for obscuring information rather than securing it rot 13 is based on the 00:05:32.400 --> 00:05:38.340 Caesar shift Cipher used by Julius Caesar in the first century BC so I'm going to first show how 00:05:38.340 --> 00:05:43.980 the Caesar shift Cipher works and then move on to the specific implementation with rot13. 00:05:46.560 --> 00:05:51.780 the Caesar shift Cipher is easy to understand basically shift the letter of the alphabet along 00:05:51.780 --> 00:05:57.240 the number of the characters in the key the table shown above shows the alphabet with a key of zero 00:05:58.380 --> 00:06:02.700 the ciphertext letter on the top lines up directly with the same letter below it 00:06:03.840 --> 00:06:09.300 clearly this does not hide the message Cipher is not case sensitive I've used 00:06:09.300 --> 00:06:13.500 lowercase for the plaintext and uppercase for ciphertext just to make it easy to follow 00:06:16.440 --> 00:06:20.460 with a shift of one character then the ciphertext moves along one position 00:06:21.060 --> 00:06:27.180 this is the key value of one shift in a total of three characters gives this layout 00:06:29.280 --> 00:06:34.620 if I take our message from earlier we apply one character at a time so the H translates to E 00:06:35.940 --> 00:06:40.380 then the E to B the Ls translate to eyes 00:06:41.880 --> 00:06:47.940 and the O becomes an l as we're only using the alphabet the punctuation is left as it is 00:06:49.440 --> 00:06:55.260 we can continue with the rest of the characters this gives the cipher text string to on the 00:06:55.260 --> 00:07:01.680 bottom line which Alice can pass to Bob when Bob receives the message then he needs to know the key 00:07:03.720 --> 00:07:09.480 with that he can then create his own table which is the opposite of the one used by Alice shifted 00:07:09.480 --> 00:07:11.640 -3 instead of plus three characters 00:07:14.280 --> 00:07:20.880 then Bob can use the same technique to convert from the ciphertext to the plain text so e becomes 00:07:20.880 --> 00:07:27.300 h then Bob can continue to use the same technique to convert from the ciphertext to the plain text 00:07:29.400 --> 00:07:35.760 given the original text if he was able to see the message being transmitted then she would 00:07:35.760 --> 00:07:40.980 have only seen the encrypted text without knowing the algorithm and the key she would not have been 00:07:40.980 --> 00:07:45.960 able to understand the message of course this is not a very secure form of encryption and 00:07:45.960 --> 00:07:50.580 anyone that is familiar with code word puzzles would be able to break this code very quickly 00:07:54.540 --> 00:08:00.780 which brings us along to rot13. this is not a secure way of protecting information but 00:08:00.780 --> 00:08:03.840 it's still used today for obscuring information rather than securing it 00:08:04.560 --> 00:08:09.180 sometimes used on internet forums to hide the punchline of a joke or to provide an answer 00:08:09.180 --> 00:08:15.060 to a question in a way that is not immediately obvious we can also avoid Simple Text filters such 00:08:15.060 --> 00:08:21.420 as those used to prevent inappropriate language on forums Windows XP even used it to obfuscate 00:08:21.420 --> 00:08:26.760 some of the values in the registry the reason I'm using it here is that it gives an example 00:08:26.760 --> 00:08:31.680 that can be easily seen and understood as modern cryptography uses far more complicated ciphers 00:08:35.520 --> 00:08:39.420 the rot 13 stands for rotate by 13 places 00:08:40.860 --> 00:08:47.760 it's a special case of the Caesar Cipher shifting 13 places so effectively the key is integrated 00:08:47.760 --> 00:08:54.000 into the algorithm there are 26 characters in the alphabet so shift in 13 places means that 00:08:54.000 --> 00:08:59.580 to go back to the plain text you just need to apply the algorithm again so instead of having 00:08:59.580 --> 00:09:06.720 to shift in the opposite direction minus 13 is the same as shifting plus 13 places so here is 00:09:06.720 --> 00:09:13.980 the same phrase encoded using rot 13. if you put the ciphertext through the algorithm again as 00:09:13.980 --> 00:09:19.740 though it was plaintext then you'll get the plain text output this is known as a reciprocal Cipher 00:09:22.440 --> 00:09:27.240 you may also have noticed that I've done all this in lowercase Rot 13 is normally 00:09:27.240 --> 00:09:32.520 implemented with a 13 shift for the lowercase and a 13 shift for the uppercase so it keeps the case 00:09:33.360 --> 00:09:37.140 using rot 13 digits and punctuation is often left intact 00:09:37.980 --> 00:09:44.160 rot 47 is an alternative that uses 94 characters including digits and some punctuation 00:09:45.420 --> 00:09:51.360 but none of these features make any of these any more secure in fact because these are common Keys 00:09:51.360 --> 00:09:57.540 13 and 47 means they're more likely to be tried making them less secure as I said previously 00:09:57.540 --> 00:10:03.360 these are only used to prevent you accidentally seeing the answers similar to puzzle answers being 00:10:03.360 --> 00:10:07.140 written upside down it should not be used for anything that needs to be kept secret 00:10:09.660 --> 00:10:13.320 I'm going to show you how this can be implemented in Python I'm going to do 00:10:13.320 --> 00:10:16.200 this manually as a demonstration of how it could be implemented 00:10:17.040 --> 00:10:22.800 but the algorithm is also included in the python codecs module so you don't need to 00:10:22.800 --> 00:10:27.600 implement it yourself just import that module and you can use it from standard libraries 00:10:30.780 --> 00:10:35.820 here's the code it's only 40 lines long and some of these are comments empty lines or 00:10:35.820 --> 00:10:40.740 print statements the code is available on my website see the description for details 00:10:41.460 --> 00:10:44.340 also note this isn't necessarily the most efficient way of doing 00:10:44.340 --> 00:10:48.060 this it's been broken down step by step so you can see what happens 00:10:49.680 --> 00:10:56.100 this is all kept enclosed in a while loop so it keeps running constantly until a blank string is 00:10:56.100 --> 00:11:04.440 entered it uses the input statement to get the information from the user as the platin text 00:11:06.780 --> 00:11:11.580 if that's blank then it breaks out of this while open finishes 00:11:12.180 --> 00:11:19.680 but assuming that we've entered some string it's going to go through the plain text string storing 00:11:19.680 --> 00:11:25.740 each character at a time in a variable called pt_char which stands for plain text character 00:11:28.260 --> 00:11:35.220 it's going to convert that into an integer pt_int and because then we can perform mathematical 00:11:35.220 --> 00:11:45.060 operations on it it's good to check that it's whether it's a capital letter i bigger or equal 00:11:45.060 --> 00:11:53.580 to a and less than Z assuming it is it's going to increment by 13. if it's gone beyond the Z 00:11:53.580 --> 00:12:03.840 then it's going to subtract 26 characters so this effectively becomes a ring it moves around and 00:12:03.840 --> 00:12:10.140 then it's going to convert this number back into a character and add that to the ciphertext string 00:12:13.260 --> 00:12:17.880 the lowercase is exactly the same but we're checking for the lowercase letters here instead 00:12:19.440 --> 00:12:25.500 and finally if it's not a text character then it will just add it 00:12:25.500 --> 00:12:29.880 as it is so this will include numbers or punctuation will not be changed 00:12:33.720 --> 00:12:36.240 and then finally we print the result 00:12:38.640 --> 00:12:43.380 so here we've got the code in the Thonny editor I'm going to set it running 00:12:44.820 --> 00:12:49.500 and you can see that it's asked for was to enter the string which is from 00:12:49.500 --> 00:12:56.220 the start of that program so we've just put the same phrase we used before hello friend 00:12:57.240 --> 00:13:03.000 and you'll see the output is here which is the same as we used in the previous example 00:13:04.560 --> 00:13:10.020 and we can show that we can use the same algorithm by copying and pasting that 00:13:14.100 --> 00:13:19.980 onto here and then that has been converted back to Hello friends so 00:13:19.980 --> 00:13:24.540 you can see how it moves backwards and forwards between the strings 00:13:26.760 --> 00:13:30.120 and you'll also see that the U is capital 00:13:30.900 --> 00:13:36.900 to match the H which is capital and the punctuation which is the space is unchanged 00:13:39.960 --> 00:13:45.300 this has just been a very basic introduction to cryptography I briefly covered some of the 00:13:45.300 --> 00:13:50.520 history of cryptography and I've demonstrated you this in example of the rot 13 Cipher 00:13:51.180 --> 00:13:55.500 in future videos I'll be explaining some other features of cryptography and how 00:13:55.500 --> 00:14:00.240 they can be used to keep your data secure if you'd like to know more please subscribe and 00:14:00.240 --> 00:14:05.520 enable notifications to get notified about my future videos if this has been useful 00:14:05.520 --> 00:14:09.780 please give it a like thanks for watching and I look forward to seeing you on a future video
