20210226 Computer Science Crash Course 40 Episodes (31-33)

31. Cybersecurity: The scope of computer security is as fast as the development of computing power. We can regard computer security as the protection of systems and data: confidentiality, integrity, and availability. (1) Secrecy/Confidentiality means that only authorized persons can read computer systems and data. (2) Integrity Intergrity means that only people with limited authority can use and modify the system and data. (3) Availability Avalibaility means that people with authority should be able to access the system and data at any time. Denial of Service Attacks (DDOS) means that hackers send a large number of fake requests to the server to make the website slow or hang up. This is an attack on availability. Establish a threat analysis model. There are three types of identity authentication, each with pros and cons. What you know? The account password is easy to guess, so it needs to be more complicated to set, including symbols. What you have? It is based on the user having certain objects, such as keys and locks. This can avoid being "guessed", and usually requires the presence of people, so long-range attacks are more difficult. What you are? This verification is based on you, you show the characteristics to the computer for verification. Biometric verification, such as fingerprints, iris. And the data from the sensor will be different every time. What you know&What you have are deterministic, but biometrics are probabilistic, and another problem with biometrics is that they cannot be reset. After "identity authentication", it comes to "Access Control". Once the system knows who you are, it needs to know what you can access. There should be a specification stating what you can access, modify, and use. This can be It is achieved through "Permissions" or "Access Control List (ACL)", which describes the user's access rights to each file, folder, and program. The model that cannot be read up or written down is called the BELI-LaPaDual model, which was formulated for the "Multilayer Security Policy" of the US Department of Defense. There are many other access control models, such as the Chinese Wall Model and Biba model. "Authentication" and "Access Control" help the computer know who you are and what you can access, but the software that does this must be trustworthy. isolation. sandbox.
32. Hackers & Cyber ​​Attacks. (1) The most common attack is Phishing. (2) Another common method is to pretend to be a person in the company’s IT department to trick you into setting up your computer, and the changes are easily attacked. (3) The torjan horses in the emails are also common methods, and the email photos contain the malware Malware. (4) Recently, a new attack method called NAND mirroring has appeared. If you can physically touch the computer, you can connect a few wires to the memory, copy the entire memory, and then violently try the password until the device asks you to wait. You only need to overwrite the memory with the copied content. In essence, the memory is reset. There is no need to wait, and you can continue to try the password. (5) If there is no physical access and only a remote attack, the attacker generally needs to use system vulnerabilities to gain certain capabilities or access rights. This is called "vulnerability" exploit. A common exploit is "buffer overflow". Buffer is a general term that refers to a reserved memory space. (6) Another classic method is called code injection. It is often used to attack websites that use databases. We will use Structured Query Language, SQL, a popular data API.
33. Cryptography: In order to encrypt information, the encryption algorithm Cipher is used to convert plain text into ciphertext. This process is called encryption, and the translation of ciphertext into plaintext is called decryption. "Caesar encryption" moves the letters in the letter 3 positions backward, for example, A becomes D. There is a large class of encryption algorithms called "replacement encryption" substitution ciphers, and "Caesar encryption" is one of them. A huge disadvantage is that the frequency of letters is the same. For example, the letter E appears most frequently. If you replace it with X, then the experienced codebreaker Cryptanalyst can find the regularity from the statistics. Another type of encryption algorithm is called "shift encryption" Permutation ciphers. The most famous German Ingmar in World War II used complex substitution encryption. Among the early encryption algorithms, the most widely used is the "Data Encryption Standard" developed by IBM and NSA in 1977. DES originally used a 56-bit binary key, but by 1999, it could be cracked by computer brute force. Therefore, in 2001, Advanced Encryption Standard, Advanced Encryption Standard, AES , adopted a longer key, 128/192/256bit, which made brute force cracking more difficult. AES cuts the data into blocks of 16 bytes each, and then uses a key to perform a series of replacement encryption and shift encryption, plus some other operations to further encrypt the information. Each block of data will repeat this process more than 10 times. In the Internet age, we need some way to pass the key to the other party on the open Internet. This does not seem to be safe. Once intercepted by a hacker, the hacker can decrypt the password. The solution is " KEY EXCHANGE""It is an algorithm that does not send the key but still allows the two computers to reach a consensus on the key. We can use the "one-way-function" one-way-function to do it. The one-way function is a mathematical operation and it is easy to calculate the result. , But it is very difficult to infer the input from the result, just like mixing colors is easy, but it is difficult to know what colors are mixed. We can use "Diffie-Hellman key exchange" in Diffie-Hellman Among them, the one-way function is a modular exponentiation operation, which is to do the exponentiation first, take a number as the base, take a number as the exponent, and then divide by the third number, and finally get the remainder we want. Diffie-Hellman key Exchange is a method of establishing a shared key. Both parties use the same key to encrypt and decrypt messages, which is called "symmetric encrytion". Caesars encryption, Ingmar, and AES are all symmetric encryption. There is also "asymmetric encryption". Encryption", there are two different keys, one is public and the other is private. People use the public key to encrypt messages, and only those who have the private key can decrypt it. In other words, know that the public key can only encrypt but not Decryption is "asymmetric". The reverse is also possible, that is, encrypt with a private key and decrypt with a public key. This method is used for signing, and the server can also be encrypted with the private key, and anyone can decrypt with the public key of the server. The most popular asymmetric encryption technology is RSA, whose name comes from the inventors Rivest, Shamir, and Adleman.