网络安全技术英文习题集网络安全技术文档格式.docx

1、 release of message contents and traffic analysis. Active attacks: masquerade, replay, modification of messages, and denial of service.1.4 Lists and briefly define categories of security service?Authentication: The assurance that the communicating entity is the one that it claims to be. Access contr

2、ol: The prevention of unauthorized use of a resource （i.e., this service controls who can have access to a resource, under what conditions access can occur, and what those accessing the resource are allowed to do）.Data confidentiality: The protection of data from unauthorized disclosure.Data integri

3、ty: The assurance that data received are exactly as sent by an authorized entity （i.e., contain no modification, insertion, deletion, or replay）.Nonrepudiation: Provides protection against denial by one of the entities involved in a communication of having participated in all or part of the communic

4、ation.Availability service: The property of a system or a system resource being accessible and usable upon demand by an authorized system entity, according to performance specifications for the system （i.e., a system is available if it provides services according to the system design whenever users

5、request them）.Chapter2 Symmetric Encryptionand Message Confidentiality2.1 What are the essential ingredients of a symmetric cipher?Plaintext, encryption algorithm, secret key, ciphertext, decryption algorithm.2.2 What are the two basic functions used in encryption algorithms?Permutation and substitu

6、tion.2.3 How many keys are required for two people to communicate via a symmetric cipher?One secret key.2.4 What is the difference between a block cipher and a stream cipher?A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. A block cipher is one in which a blo

7、ck of plaintext is treated as a whole and used to produce a ciphertext block of equal length.2.5 What are the two general approaches to attacking a cipher?Cryptanalysis and brute force.2.6 Why do some block cipher modes of operation only use encryption while others use both encryption and decryption

8、?In some modes, the plaintext does not pass through the encryption function, but is XORed with the output of the encryption function. The math works out that for decryption in these cases, the encryption function must also be used.2.7 What is triple encryption?With triple encryption, a plaintext blo

9、ck is encrypted by passing it through an encryption algorithm; the result is then passed through the same encryption algorithm again; the result of the second encryption is passed through the same encryption algorithm a third time. Typically, the second stage uses the decryption algorithm rather tha

10、n the encryption algorithm.2.8 Why is the middle portion of 3DES a decryption rather than an encryption?There is no cryptographic significance to the use of decryption for the secondstage. Its only advantage is that it allows users of 3DES to decrypt data encrypted by users of the older single DES b

11、y repeating the key.2.9 What is the difference between link and end-to-end encryption?With link encryption, each vulnerable communications link is equipped on both ends with an encryption device. With end-to-end encryption, the encryption process is carried out at the two end systems. The source hos

12、t or terminal encrypts the data; the data in encrypted form are then transmitted unaltered across the network to the destination terminal or host.2.10 List ways in which secret keys can be distributed to two communicating parties.For two parties A and B, key distribution can be achieved in a number

13、of ways, as follows:（1）A can select a key and physically deliver it to B.（2）A third party can select the key and physically deliver it to A and B.（3）If A and B have previously and recently used a key, one party can transmit the new key to the other, encrypted using the old key.（4）If A and B each has

14、 an encrypted connection to a third party C, C can deliver a key on the encrypted links to A and B.2.11 What is the difference between a session key and a master key?A session key is a temporary encryption key used between two principals. A master key is a long-lasting key that is used between a key

15、 distribution center and a principal for the purpose of encoding the transmission of session keys. Typically, the master keys are distributed by noncryptographic means.2.12 What is a key distribution center?A key distribution center is a system that is authorized to transmit temporary session keys t

16、o principals. Each session key is transmitted in encrypted form, using a master key that the key distribution center shares with the target principal.ANSWERS NSWERS TO PROBLEMS2.1 What RC4 key value will leave S unchanged during initialization? That is, after the initial permutation of S, the entrie

17、s of S will be equal to the values from 0 through 255 in ascending order.Use a key of length 255 bytes. The first two bytes are zero; that is K0 = K1 = 0. Thereafter, we have: K2 = 255; K3 = 254; K255= 2.2.2 If a bit error occurs in the transmission of a ciphertext character in 8-bit CFB mode, how f

18、ar does the error propagate? Nine plaintext characters are affected. The plaintext character corresponding to the ciphertext character is obviously altered. In addition, the altered ciphertext character enters the shift register and is not removed until the next eight characters are processed.2.3 Ke

19、y distribution schemes using an access control center and/or a key distribution center have central points vulnerable to attack. Discuss the security implications of such centralization.The central points should be highly fault-tolerant, should be physically secured, and should use trusted hardware/

20、software.Chapter 3 Public-Key Cryptography and Message Authentication3.1 List three approaches to message authentication.Message encryption, message authentication code, hash function.3.2 What is message authentication code?An authenticator that is a cryptographic function of both the data to be aut

21、henticated and a secret key.3.3 Briefly describe the three schemes illustrated in Figture3.2.（a） A hash code is computed from the source message, encrypted using symmetric encryption and a secret key, and appended to the message. At the receiver, the same hash code is computed. The incoming code is

22、decrypted using the same key and compared with the computed hash code. （b） This is the same procedure as in （a） except that public-key encryption is used; the sender encrypts the hash code with the senders private key, and the receiver decrypts the hash code with the senders public key. （c） A secret

23、 value is appended to a message and then a hash code is calculated using the message plus secret value as input. Then the message （without the secret value） and the hash code are transmitted. The receiver appends the same secret value to the message and computes the hash value over the message plus

24、secret value. This is then compared to the received hash code.3.4 What properties must a hash function have to be useful for message authentication?（1）H can be applied to a block of data of any size.（2）H produces a fixed-length output.（3）H（x） is relatively easy to compute for any given x, making bot

25、h hardware and software implementations practical.（4）For any given value h, it is computationally infeasible to find x such that H（x） = h. This is sometimes referred to in the literature as the one-way property.（5）For any given block x, it is computationally infeasible to find y x with H（y） =H（x）.（6

26、）It is computationally infeasible to find any pair （x, y） such that H（x） = H（y）.3.5 In the context of a hash function, what is a compression function?The compression function is the fundamental module, or basic building block, of a hash function. The hash function consists of iterated application of

27、 the compression function.3.6 What are the principal ingredients of a public-key cryptosystem?Plaintext: This is the readable message or data that is fed into the algorithm as input. Encryption algorithm: The encryption algorithm performs various transformations on the plaintext. Public and private

28、keys: This is a pair of keys that have been selected so that if one is used for encryption, the other is used for decryption. The exact transformations performed by the encryption algorithm depend on the public or private key that is provided as input. Ciphertext: This is the scrambled message produ

29、ced as output. It depends on the plaintext and the key. For a given message, two different keys will produce two different ciphertexts. Decryption algorithm: This algorithm accepts the ciphertext and the matching key and produces the original plaintext.3.7 List and briefly define three uses of a public-key cryptosystem.Encryption/decryption: The sender encrypts a message with the recipients public key. Digital signature: The sender signs a message with its private key.