1 00:00:02,180 --> 00:00:04,600 We'd like to apply encryption to the plain text. 2 00:00:04,600 --> 00:00:14,540 Now we know that to encode some data we need a key parameter as well as the plain text the encryption 3 00:00:14,540 --> 00:00:24,240 result is the ciphertext to reverse the encryption process to convert the ciphertext to plain text really 4 00:00:24,250 --> 00:00:27,310 to use the same shared key. 5 00:00:27,320 --> 00:00:32,840 This explains the name of the working principles of symmetric ciphers. 6 00:00:33,040 --> 00:00:39,150 The same key is used for encryption and decryption. 7 00:00:39,240 --> 00:00:41,740 What are the strengths of this simple solution. 8 00:00:43,390 --> 00:00:51,040 Above all symmetric algorithms are extremely fast that can be 1000 to 10000 times faster than public 9 00:00:51,040 --> 00:00:56,960 key asymmetric ciphers 10000 times makes a large difference. 10 00:00:57,790 --> 00:01:04,330 It could mean for example that to encrypt some text using a symmetric algorithm you need a second while 11 00:01:04,330 --> 00:01:08,530 for an asymmetric algorithm the same operation would take 5000 seconds. 12 00:01:09,630 --> 00:01:13,770 The difference is obvious. 13 00:01:13,940 --> 00:01:19,320 Other than that symmetric ciphers are based on a simple operation. 14 00:01:19,430 --> 00:01:21,050 We've mentioned that earlier. 15 00:01:21,290 --> 00:01:28,090 It's the exclusive disjunction with Samit or the exclusive Ekso are the following mechanism is the Ekso 16 00:01:28,090 --> 00:01:32,090 our operation has the advantage of being very easy to implement in hardware. 17 00:01:32,140 --> 00:01:34,290 X can be of any arbitrary link. 18 00:01:34,300 --> 00:01:38,880 This operation seems custom designed for microprocessors a very complex city. 19 00:01:40,970 --> 00:01:46,300 You can implement devices that use symmetric encryption and decryption. 20 00:01:46,330 --> 00:01:49,650 There's a large number of available options. 21 00:01:49,880 --> 00:01:59,130 For example Network Interface Cards you can purchase nics that natively support the basic protocol did 22 00:01:59,210 --> 00:02:03,860 encryption and decoding is then delegated from an operating system to this hardware. 23 00:02:07,490 --> 00:02:10,400 This solution has its share of disadvantages as well. 24 00:02:12,390 --> 00:02:19,470 The first and foremost is setting up a key you've witnessed this problem when we discussed one time 25 00:02:19,470 --> 00:02:22,000 pad. 26 00:02:22,130 --> 00:02:26,310 How can you provide a key to a message recipient to make them decode the message. 27 00:02:29,460 --> 00:02:37,590 If the message is intended for the recipient only the recipient must have their own key symmetric algorithms 28 00:02:37,590 --> 00:02:41,690 require the use of a huge amount of keys. 29 00:02:41,880 --> 00:02:48,070 The key number formula is directly derived from the number of system users. 30 00:02:48,240 --> 00:02:54,140 If there are 50 users you'll need over 1200 and 20 keys. 31 00:02:54,150 --> 00:03:00,440 The point is to allow everyone to be able to exchange data with another user in a secure manner. 32 00:03:00,600 --> 00:03:07,200 You can exchange a single piece of information with everyone managing that many keys is no trifling 33 00:03:07,200 --> 00:03:08,160 matter. 34 00:03:09,300 --> 00:03:12,170 This is why symmetric ciphers are not widely used. 35 00:03:13,920 --> 00:03:20,100 Moreover if a potential attacker seizes a key they will not only be able to decode confidential data 36 00:03:20,310 --> 00:03:26,670 but also freely modify it and encode it back using the same key. 37 00:03:26,700 --> 00:03:32,700 This will be unnoticeable and will jeopardize not only the confidentiality but also the authenticity 38 00:03:32,700 --> 00:03:34,500 of transmitted information. 39 00:03:37,100 --> 00:03:46,080 Our symmetric ciphers implemented the concept behind them is very straightforward even if an operation 40 00:03:46,080 --> 00:03:52,770 does not provide absolute security if it's repeated enough times it can at last prove practically secure 41 00:03:55,390 --> 00:04:04,770 this idea is the basis for round's symmetric encryption is performed in rounds an operation is repeated 42 00:04:04,770 --> 00:04:13,140 a specified number of times the number of rounds depends on a particular algorithm and its configuration 43 00:04:14,560 --> 00:04:18,580 attacks on symmetric ciphers usually succeed in breaking an algorithm. 44 00:04:18,670 --> 00:04:28,340 If 16 rounds of repetition are used the default round number is however 25 giving you a 9 round fallback 45 00:04:30,830 --> 00:04:33,730 the algorithm can be considered secure. 46 00:04:33,930 --> 00:04:36,810 It is passed required tests and is widely used. 47 00:04:39,180 --> 00:04:44,430 If the principle behind the symmetric key cipher is to work you can't repeat operations in each round 48 00:04:44,430 --> 00:04:52,770 exactly using the x or operation all over won't change the security. 49 00:04:52,980 --> 00:04:56,840 You have to introduce an element of randomness in individual rounds. 50 00:04:56,910 --> 00:04:58,720 You need different round keys. 51 00:05:03,020 --> 00:05:08,930 If you still remember the differentiation between confusion and diffusion in light of these terms symmetric 52 00:05:08,930 --> 00:05:15,620 ciphers to be considered more or less secure need to be the mix of ciphertext increasing the diffusion 53 00:05:15,620 --> 00:05:18,610 as much as possible. 54 00:05:18,680 --> 00:05:30,030 They should distribute the plaintext over the ciphertext. 55 00:05:30,060 --> 00:05:34,600 The basic division of symmetric key algorithms can be made according to the following principle. 56 00:05:36,520 --> 00:05:44,420 Imagine that an algorithm can encrypt a plaintext an arbitrary string as a whole. 57 00:05:44,430 --> 00:05:46,160 This is how a stream ciphers operate.