**Lecture 10 Chapter 02 Cryptographic Tools section 2.1 Confidentiality with Symmetric Encryption — Transcript & Summary | SozAI** Source: https://sozai.app/transcript/lecture-10-cryptographic-tools-confidentiality-symmetric-encryption/ Lecture on symmetric (single-key) encryption: components, how it works, DES/3DES/AES, block vs stream ciphers, and key security requirements in cryptographic tools. ## Key Takeaways - Symmetric encryption uses one shared secret key for both encryption and decryption. - Security depends on keeping the secret key secret, since algorithms are usually public. - DES is outdated due to its short 56-bit key; AES with 128+ bit keys is the modern standard. - Block cipher modes like ECB leak patterns, so the mode of operation matters as much as the cipher. ## What the video covers - Chapter 2 introduces cryptographic tools, explaining encryption as obscuration to maintain confidentiality, the first pillar of the CIA triad. - Outlines the chapter roadmap: encryption basics, symmetric vs asymmetric keys, message authentication, hash functions, digital signatures, key distribution, random numbers, and stored-data encryption. - Defines symmetric encryption as single-key (conventional) encryption where the same secret key is used for both encryption and decryption. - Describes five core components: plaintext, encryption algorithm, secret key, ciphertext, and decryption algorithm. - Explains open design: algorithms are typically public, so security depends entirely on keeping the secret key confidential. - Presents Caesar cipher as a simple substitution example (shift of three letters, 'hello' becomes 'khoor'). - Covers block ciphers, which split data into fixed-size blocks, versus stream ciphers that encrypt a continuous keystream. - Discusses DES (56-bit key, 64-bit blocks), its weakness, Triple DES as a stopgap, and AES with 128/192/256-bit keys. - Explains modes of operation such as ECB and its weakness: identical plaintext blocks produce identical ciphertext blocks, revealing patterns. - Emphasizes that the security of symmetric encryption rests on secure key storage and the key-distribution problem between sender and receiver. ## Chapters 1. 00:00 Introduction to Cryptographic Tools and Chapter Overview 2. 01:44 Encryption Methods: DES, AES, Block and Stream Ciphers 3. 03:33 Symmetric Encryption Concept and Single-Key Mechanism 4. 04:58 Five Components of Symmetric Encryption and Open Design 5. 08:10 Security Requirements and the Role of the Secret Key 6. 13:33 Caesar Cipher Example and Substitution Techniques 7. 16:55 Block Ciphers, DES Key Size, and Triple DES 8. 28:27 Block Cipher Modes, ECB Weaknesses, and Stream Ciphers Answers ## Questions about this video Why is symmetric encryption also called single-key encryption? Because the same key is used for both encryption and decryption. Unlike asymmetric systems that use two keys, the one secret key handles both processes. Why does the security of symmetric encryption depend on the secret key rather than the algorithm? Encryption algorithms are typically public under the open design principle, so anyone can inspect them. Security holds only as long as the secret key stays confidential and is shared only between the parties involved. What is the main weakness of DES that led to Triple DES and AES? DES uses a 56-bit key, which is too short for modern computing power and can be brute-forced. Triple DES extends the key size, and AES offers 128, 192, or 256-bit keys for stronger long-term security. ## Full Transcript — Download SRT & Markdown 00:01 Speaker A In the name of God, the Most Gracious, the Most Merciful. Welcome to the second chapter. In our computer and network course. In this chapter, we will talk, God willing, about cryptographic tools. Which are encryption tools. Why do we use 00:15 Speaker A encryption tools? Encryption means obscuration. Obscuration means that I have text that I want. To create text. 00:21 Speaker A Or I have traffic. I want to create traffic, so that any person in the path . Who does not possess the decryption key will not understand anything from the transmitted data. Okay, so I will use encryption to maintain the 00:36 Speaker A confidentiality of data. Thus, encryption is considered one of the tools used for, who is it? Achieving confidentiality, which is the first pillar of the CIA triad. Which is computer security. Okay, back to encryption. What will we talk about in 00:54 Speaker A this chapter? What will we go through? We will talk about encryption basics. We will distinguish between symmetric and asymmetric keys. We will also distinguish, or describe, the applications related to message authentication protocols and digital signatures. We will talk about hash 01:10 Speaker A functions, message authentication codes , and digital certificates. Okay, we will also talk about the key distribution mechanism. For my encryption keys, in order to exchange encryption keys between the sender and receiver, I need a mechanism for, who is it, key distribution. Okay, the last 01:30 Speaker A thing, I will talk about some common cryptographic applications. The famous ones that exist and their role in securing computer systems. The first section in this chapter is 2.1. 01:44 Speaker A Encryption and its mechanisms. And its methods, like DES, which is the Data Encryption Standard. And AES, which is the Advanced Encryption Standard. We will talk about block and stream ciphers. We will talk about ECB and some other modes. In section 2.2, we 02:06 Speaker A will talk about message authentication and hash functions. How do I have a message? I want to ensure this message reached me correctly. It was not tampered with. How will we use the hash function to achieve this goal and 02:17 Speaker A purpose? We will talk about all of this in section 2.2. In section 2.3, we said that we have two types of encryption. 02:23 Speaker A We have symmetric, and the second is asymmetric, which is non-symmetrical. Which we will talk about in public-key encryption. In section 2.4, we will talk about digital signatures and key management. In section 2.5, we will talk about random and pseudo-random 02:40 Speaker A numbers. In section 2.6, we will talk about the encryption of stored data. Okay, let's start with section 2.1: Confidentiality with symmetric key encryption. By its name, symmetric, what does symmetric mean? It means one key, a symmetrical key. It is the same 02:59 Speaker A key used for encryption, and it is the same used for decryption. Okay, it says symmetric encryption is also called conventional encryption. Why? Because it is the first encryption that appeared. Okay, and it is the traditional encryption mechanism that 03:16 Speaker A existed until we had public-key encryption. Okay, two things. What does it say to me? It is also called single-key encryption. Why? Because here I have only one key for encryption . The key used for encryption is the same key used for decryption. That's 03:33 Speaker A why we call it a single key. While in other types, we have two keys. Exactly, while in symmetric encryption, it is only one key that I use for the encryption process and for the decryption process. Okay, the core idea 03:48 Speaker A we have behind the encryption key, or the core idea in encryption keys, is that I use the same key, I use what? 03:58 Speaker A The same secret key, I will use the same key whether for the encryption process or the decryption process. So, where do we use symmetric key encryption? He told me that, to this day, it is used in very wide 04:13 Speaker A applications. Despite some weaknesses it has. We use it in the encryption process. We use it in the process of. 04:19 Speaker A Message authentication protocols, we use it for encrypting messages between each other. Which are the software we have. Meaning it is still used very extensively. So, how does the symmetric key mechanism work? He told me, quite simply. I have one key for encryption. 04:39 Speaker A And I have a text I want to encrypt. After I encrypt it. On the other side, I want to decrypt it, right? So, I have a text that needs to be encrypted. 04:47 Speaker A Consequently, I need a key to encrypt with and an algorithm to encrypt with. And I have a text that, after it is encrypted, I want to transmit. After I transmitted it, the other party wants to decrypt it. Therefore, I have a 04:58 Speaker A decryption algorithm. Since we are talking about symmetric encryption, that means the key I encrypted with is the same key I will decrypt with. So I have a text, this is the first component. I have an encryption key, this is the second component. We have 05:14 Speaker A an encryption algorithm, or what we call an encryption algorithm, this is the third component. We have a text resulting from encrypting the original text with the key, we call it what? 05:25 Speaker A Ciphertext, which is the encrypted text . Okay, now I have the ciphertext. I transmitted it from the sender to the receiver. The receiver wants to know what the original data is. Because the ciphertext is an unintelligible text. 05:39 Speaker A So he is forced to use an algorithm, called the decryption algorithm. Okay, for the decryption algorithm to decrypt the text and return it to its original plaintext state, what will it use? Does it need a new key for decryption or is 05:52 Speaker A it the same key I encrypted with? It is the same key that I encrypted with. 05:58 Speaker A Right. After I decrypt it, what will I get in the end? The original plaintext that we had. Where? So again. The method or the basic process, the basic process, it is very simple in symmetric encryption, it contains 5 components. 06:16 Speaker A We have the text I want to encrypt. The encryption algorithm. The encryption key. The text after encryption, which is the ciphertext. And what? And the decryption algorithm. These are the five ingredients, the five main components in the recipe of encryption 06:33 Speaker A and decryption. Right. What is it in symmetric encryption? It is the plaintext, the original message or the data that I want to encrypt. Component two is the encryption algorithm, which will perform the process of substitution and transformation of the 06:48 Speaker A original text to convert it into what? for the ciphertext, I have the encryption key, okay? We call it the secret key, and we have the ciphertext, which is the text resulting after the encryption process, and we have the 07:04 Speaker A decryption algorithm that returns the text to its original state, which is? the plaintext. Okay, it tells me that one of the principles of open design we talked about in the first chapter is that, for me, I keep encryption 07:19 Speaker A algorithms open-source. So that anyone or any person wanting to contribute can . Wants to inspect the algorithm. 07:27 Speaker A Identify its problems and report them to me. Okay. Since the encryption algorithm or the decryption algorithm is public. In most cases, it will be public for me. That means the public is able to see it. And see its structure. 07:41 Speaker A So, how in this case do I maintain data security? He said the key point for us is the secret key, the encryption key. 07:49 Speaker A As long as my encryption key is stored securely. And only possessed by two people, or two parties, or two processes. The process that encrypts, and the process that decrypts. Your data is secure. Once security is lost at this point, the security of the text 08:06 Speaker A is lost. The encrypted text transmitted via the key, once it is revealed, is clear. So, I have five components: the original text. Encryption algorithm. 08:18 Speaker A Encryption key. Ciphertext. And the decryption algorithm. He mentions a point that. The encryption algorithm is usually public. What does public mean? 08:28 Speaker A Available to the public. Consequently, everyone knows how the encryption process is carried out. So what is the solution for me? He said the solution for me is that the secret key, the encryption key. I will use in this 08:40 Speaker A algorithm. Must remain secret, so that no person can discover it easily. And I lose the confidentiality of my information. Okay. This is the same diagram we talked about before, but shown in a better way. Here I have the 08:57 Speaker A original text, I will input it as an entry to the system. So I enter the text named X. Okay. For X, I will use an encryption key called K, using an encryption algorithm like. Data Encryption Standard, okay. It will 09:12 Speaker A produce the ciphertext, which is Y. I will transmit it, okay. After transmitting it, it will be received by the decryption algorithm. It will perform the reverse process of the encryption operation. Using whom? Using the same key. So I will get X after D. 09:30 Speaker A Which is decrypting Y. Using whom? Using the decryption key we have. Which is the same encryption key that was here with us. I will get in the end the plaintext. Which is the original text I had. As we said, what does security in 09:50 Speaker A symmetric encryption depend on? It depends on my secret key remaining secret and trustworthy. So what do I need to build a symmetric encryption system that is secure? He said I have two important things. The first thing. 10:13 Speaker A The encryption algorithm must be strong . The second thing is that the key management process must be secure. 10:22 Speaker A Let's talk about the first part: the attacker...My algorithm must be strong, meaning I don't give the attacker a chance to recover the original text. 10:34 Speaker A Once patterns are discovered, or they are able to deduce my secret key. Okay, so my secret key must be large. And my algorithm, when it generates text. It should not produce it in clear patterns . Meaning, if I told him the word " 10:48 Speaker A hello," and I want to encrypt it the following way. The encryption output tells me: for every letter, give me the next one. The word hello: H, E, L, L, O. Okay. H-I, E-F. Okay. L-M. Okay. O-P . If I encrypted it, isn't this now 11:08 Speaker A ciphertext? Yes, ciphertext. Okay. But what is the problem with it? The problem is that it is easy to predict. 11:15 Speaker A For the word "hello," which is the known plaintext, the encryption became "IFMMP." I simply took the next letter, and therefore I was able to link a pattern that will confirm this information if another text follows, such as the word "no." Now, after it, 11:33 Speaker A what? N-O, O-P. If he finds "no" followed by "op." That means this encryption process confirmed the original text with the ciphertext. 11:46 Speaker A Because my encryption algorithm actually does what? It gives me only the next letter. And thus, the pattern is discovered. My encryption algorithm must, when it generates ciphertext. 11:59 Speaker A Ensure that there is no clear pattern to the encryption process. Otherwise, it becomes easy to discover the ciphertext. If I possess the, which is the secret key, it is clear. So this is what is meant by the first principle. 12:14 Speaker A That my text should not be something I have the ability to. Deduce, and another thing, and another thing within the key algorithm. That the encryption key, also, I should not have the ability to quickly deduce it. Like if 12:28 Speaker A the key has, well, a small number of characters: 1, 3, 10, okay, then the search options are limited. The second matter is secure key management. What does secure key management mean? It says the sender and receiver must exchange keys between them securely. 12:55 Speaker A What does securely mean? In a 100% guaranteed way. That no one in the middle of the path can take the key and identify it. Okay, the key must remain secret. Why must the key remain secret? 13:10 Speaker A Because we said one of the principles of design is open. Open design, meaning my algorithm is already available to the public. Since it is available to the public, I must maintain the secret key. Okay, because quite simply, if any 13:23 Speaker A attacker knew what my secret key in the encryption was. My entire encrypted communication would become exposed. Is that clear? So I have requirements for secure...We have. We have two basic requirements. The first requirement is the encryption algorithm. My text, or 13:48 Speaker A rather my algorithm, must not produce clear patterns from the encrypted texts . So that if I have both the plaintext and the ciphertext, I can figure out what the algorithm does. Consequently, a weak algorithm is created. I would be 14:07 Speaker A able to deduce the plaintext from the ciphertext. The second thing is the key . My algorithm must have a long key so that it is very difficult, or requires many attempts, to derive the key. The second requirement is that the key 14:26 Speaker A exchange between the sender and receiver must be done securely. Because if the key is actually exposed, then every encrypted connection using that key is what? It is exposed, exactly. 14:39 Speaker A Okay, it tells me about attacks on symmetric encryption; we have two main types of attacks on it. Symmetric encryption, the first type is called cryptanalysis. Analysis, okay, cryptanalysis. Analysis, and the second type is called brute force. Let's come 14:56 Speaker A to the first one, cryptanalysis. This talks about the first part we mentioned . That I am able to know the properties of my algorithm. By knowing some of the texts I have. And linking the plaintext with its corresponding ciphertext. 15:12 Speaker A Thereby deducing the patterns and the method of encryption. For example, if my plaintext was a word. If my plaintext was the word "hello". Right. 15:27 Speaker A And its ciphertext was "khoor". Right. The 'o' and the 'l', l, m, n, o. So the 'o' is three places ahead in the alphabet. The 'r' after 'o' is also three places ahead. The 'k' after 'h' is also three places ahead. If I manage 15:45 Speaker A to link this pattern together, I know that each letter is...The ciphertext letter is the third letter in alphabetical order after the letter I am encrypting. Right. The same applies to, for example, "fdw". The 'c' which is in "cat", c, d, e, f. The 'f' after 16:05 Speaker A 'c' is three places ahead. The 'w' after 't' is three places ahead. Therefore, in this case, what has happened to me? I have gained knowledge that there is a specific pattern in the encryption process. That for every letter, I take the letter three places 16:23 Speaker A after it. Consequently, for me, the algorithm has become useless. Right. This is the first type of attack. That I try to obtain pairs of ciphertext and plaintext to understand the nature of the encryption algorithm. We have a second type called a brute force attack 16:45 Speaker A . This is simply trying every possible encryption key until I find the original plaintext. So I try all possible encryption keys until I get the plaintext. Now, if our encryption key consisted of one character, how many attempts would it need? Attempts 17:06 Speaker A equal to the number of characters. Right. Well, what if the key consisted of two characters? What if it consisted of 50 characters? See how much I am complicating the process for him. 17:16 Speaker A Therefore, to overcome a brute-force attack, my encryption key must be very large. And thus, one would not be able to deduce my encryption key. Okay, block ciphers tell us that in symmetric keys, what do most operations do when 17:35 Speaker A encrypting? They don't take character by character or byte by byte, but rather divide the data I want to encrypt into whom? Blocks of a uniform size across all blocks. Here is my data . The message is split into blocks, 17:51 Speaker A which are P1, P2, P3, and so on. I apply the encryption process to them block by block and obtain the ciphertext; after obtaining the ciphertext, its size will be who? About the same size as the text that went 18:08 Speaker A into the encryption process. So, what happens to us? We get the ciphertext. I collect it together or send it packet by packet over the network. Okay, over whom? The network. So, what do block ciphers do? They take the original text 18:23 Speaker A we have, which is the message. They take the original text we have, which is the message? And what do they do with it? They divide it into a group of equal-sized blocks. I first take a group of blocks of a certain size. I 18:39 Speaker A take each block from this set separately. I perform encryption on it. And it produces the resulting ciphertext. We will discover that this method has a problem. We will talk about it a little further on. Okay. Now , let's note a point: the size of the 19:03 Speaker A encrypted packet is equal to the size of the original packet. Okay. So, what encryption algorithms do we have? Which are symmetric and well-known. Here we have DES, 3DES, and the AES. Let's talk about them in detail. Data Encryption 19:24 Speaker A Standard, which is who, DES? And its standard was set in 1977 by whom? NIST and the NBS, okay, and it is a... 19:39 Speaker A Encryption method, how large is its block? 64 bits. Since the block I want to encrypt is 64 bits, the ciphertext will also be how much? 64 bits. My encryption key must be 56 bits in size. 19:55 Speaker A The key in this case is 56 bits. Its main weakness is that its encryption key is how much? 56 bits. Therefore, it is considered very short. So any modern brute-force system can break this key. 20:12 Speaker A And I will know the original text. Okay . There is an important piece of information. This algorithm has been studied very extensively. And so far, they haven't found a practical, real error. That reveals a weakness in the crypto. Which is the ciphertext after 20:43 Speaker A the encryption process. But, the problem is...Actually, it turns out the issue is that the key size is inadequate. The key size is inadequate. 20:54 Speaker A Okay. Why did DES, or the Data Encryption Standard, become deprecated? It tells me that with the development we've had in computer systems. In computational capabilities or arithmetic operations. The computer calculates it if my key is...bits. So, how many possible values are there? It 21:22 Speaker A is 2 to the power of 56. And it equals approximately 7.2 x 10 ^ 16. Which is a probability. And that is with systems. 21:36 Speaker A With advances in modern computing systems, quantum computing, it has become possible to discover the encryption key basically, so a 56-bit encryption key has become insecure. So, what is the solution? The solution, sir , is that you need to make me 21:55 Speaker A algorithms that use higher encryption keys. Okay, let's use something like what? Like a key of length 128 bits or higher. Okay, it said to solve this problem, we have another term called Triple DES. Okay, what is the Triple? 22:16 Speaker A It said the basic idea is that I will take my text, okay, apply the DES algorithm to it. Which is the Data Encryption Standard. Three times. I will take the block. Encrypt it for the first time. The second encryption 22:31 Speaker A process. And the second decoding process for whom. For the encryption process. The third, and thus I get the encrypted text. It tells me the keys here in this process, in Triple DES, in this algorithm, can be two keys or 22:44 Speaker A three keys. In the case of two keys, I use the first key for encryption here, the second key here, and the third key here. In the case of two keys, the total encryption key length would be, what is it? Which is 56. Which is 112 23:01 Speaker A bits. Okay, 112 bits. Okay, what if they were three encryption keys? Each encryption operation in 3DES is done with an independent key, which is 56, 56, and 56, giving us 168. Okay. What are its advantages? Much stronger than 23:18 Speaker A DES compared to what? To brute force attacks. However, and it is based primarily on what? On this algorithm, which is DES. Well-studied and researched. And attempts were made to see if it had vulnerabilities or not. 23:33 Speaker A And if it has vulnerabilities. Actually , its main problem is that it is very slow because the encryption process happens three times and the decryption process happens three times, so it is considered slow. It also tells me it 23:49 Speaker A uses a 64-bit block. Just like whom? Like DES, okay. And what does it tell me? Unsuitable for what? Long-term modern use, okay, why? Because I might say: why not tell me the third type of symmetric, which is what? Advanced 24:10 Speaker A Encryption Standard, which is the abbreviation for what, "AES". Okay, it was also discovered. Or developed and replaced what in the DES 3 or Triple DES? Okay, what are the most important features of the Advanced Encryption protocol, it says. First, it is a 24:27 Speaker A symmetric block cipher. So, just like whom? Like a normal symmetric key. The second thing it tells me is 128-bit block size. My block size in 3DES was how much? 56, in the Advanced Encryption Standard or AES. How much? 24:46 Speaker A 128. Bits? Okay. It also says it has keys like this. 128 bits for the key. 24:56 Speaker A 192 bits also for another key. And 256 bits for a key of another type. Okay. 25:03 Speaker A So, the design. The goal is to achieve very high security, high performance, and optimal hardware utilization. 25:13 Speaker A Together, we have a somewhat flexible key distribution process. Clear? If we compare the three encryption methods we’ve encountered—symmetric encryption—we had DES, 3DES, and AES, okay? The block size in DES and 3DES is 64 bits. While in AES, it is 128 bits. 25:47 Speaker A The key size in DES is 56 bits, and in 3DES, it is up to 168 bits. In AES, we have 128, 192, and 256 bits. 25:59 Speaker A Performance is natural. It is the oldest and easiest. Consequently, it is faster than 3DES. And 3DES is slow. 26:10 Speaker A While AES is fast. Okay, it asks about the current usage. DES is used fundamentally, or let's say 3DES is considered legacy, and therefore, what about AES? It is used very extensively. 26:34 Speaker A Security in DES, which is the basic one here, is inadequate. 3DES is legacy, meaning it is old and needs improvements. And that is. Which is AES -128, what about it here? Faster. Used much more widely. And it is a new 26:52 Speaker A standard. Okay. Now, here, who did we say encrypts? It is the symmetric key. Great. Let’s move to the second point . Or another point. Which is practical security. That is Electronic Codebook ( ECB). It tells me that I usually do 27:23 Speaker A this. I divide the text into a set of sub-texts. Each sub-text will be encrypted using a key. I will assemble the encrypted text. Where we have the original key. Accepted codebook. Okay. 27:45 Speaker A Does the encryption of P1 depend on the encryption of P2? No. This has an encryption key. It differs from this. 27:54 Speaker A It differs from this. Things are clear. The block itself will be encrypted separately. So what. Are the problems we face? It says identical plaintext blocks mean. They result in identical ciphertext blocks. Another thing, patterns must remain, which is bad. And 28:19 Speaker A an attacker could exploit this point. And try to reconstruct the message realistically. And the issues or the. 28:30 Speaker A The problems as we said. We have identical plaintext blocks. Therefore, patterns might remain. Thus, I might be able to reveal the original text from them. They might attack the original text or the ciphertext to extract the relationship between them. The problems 28:47 Speaker A are under, or unsuitable for legacy structures. It is inadequate for the algorithm. Separated from other algorithms, okay. So, what are the modes? Modes of operation; it says every message, if our message were larger than the block size, okay? It 29:17 Speaker A says every block I have, when I encrypt it, has a mode of operation, which is the Electronic Codebook. We mentioned it a little while ago. Each block is taken individually, encrypted, and sent alone. And what is the benefit of this? 29:31 Speaker A It provides me a faster option and I am able to...Encrypt more than one block. 29:35 Speaker A All right. Well, our goal with this model is to overcome the problems of the Electronic Codebook. So we have CBC , CFB, OFB, CTR, and GCM left. We will talk about them, or more specifically, we will talk about the CBC. All right. 29:57 Speaker A Well, let's talk about something called a stream cipher. I will have blocks in the form of a stream that I want to encrypt. In return, I have blocks in the form of a stream of keys to encrypt with. The process is...It uses or 30:10 Speaker A receives data from me synchronously. All right. Not while I'm standing in one place, of course. All this data will be handled as fixed-size blocks. 30:21 Speaker A All right, the block taken for the encryption process must be loaded. What is the key? Used in the encryption process; its encryption process will use an encryption key. It will encrypt the first block. I will take the data 30:35 Speaker A from the first and second blocks, merge them, and encrypt. And so on until I reach a complete block at the end. A full block encrypted using whose method ? The stream. We have a stream of bytes , or in return, we have...Generating a 30:50 Speaker A key stream of keys. All right. How will I encrypt the message? XORing it with the key stream will give me the ciphertext. The encryption is the same encrypted message, but XORed with... 31:03 Speaker A With the key stream. I will get the ciphertext. What is the difference between block ciphers and stream ciphers? It tells me that, quite simply , in the processing, a block cipher has a fixed size. Consequently, I can...And therefore, it's easy to finish the 31:19 Speaker A cipher. It says a continuous stream. I have text that is always being renewed. What does streaming do? The output will be a block. The output for us could be an element. It could be a byte. It could be a bit, in the case of a stream 31:33 Speaker A cipher. The strength of the block cipher is strong. But in a stream cipher, if designed correctly, it will be strong. Its performance, generally its performance...The block cipher's is considered high. But in a stream cipher , it can be very high. All right. 31:55 Speaker A Typical use of a block cipher: I will use it for files and databases. They use the block. As for the stream cipher , I will use it for continuous data streams, like network communication. 32:08 Speaker A Both provide strong encryption. All right. But the storage method and my usage method determine which one I use. 32:19 Speaker A That is clear. With this, we have finished the first part, related to confidentiality with symmetric keys. We talked about symmetric keys. We talked about blocks. We talked about the encryption method using blocks. We talked about the different types, like 32:37 Speaker A DES, Triple DES, and AES. All right, thank you. You're welcome, and we will continue in the next lecture, God willing. 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