网络安全技术英文习题集_网络安全技术

《网络安全技术》英文习题集Chapter 1 In troductio nANSWERS NSWERS TO QUESTIONS1.1What is the OSI security architecture?The OSI Security Architecture is a framework that provides a systematic way of defi ning the requireme nts for security and characteriz ing the approaches to satisfy ing those requireme nts. The docume nt defi nes security attacks, mecha ni sms, and services, and the relati on ships among these categories.1.2What is the differe nee betwee n passive and active security threats? Passive attacks have to do with eavesdropping on, or monitoring,tran smissi ons. Electro nic mail, file tran sfers, and clie nt/server excha nges are examples of tran smissi ons that can be moni tored. Active attacks in clude the modificati on of tran smitted data and attempts to gai n un authorized access to computer systems.1.3Lists and briefly define categories of passive and active security attacks?Passive attacks: release of message contents and traffic an alysis. Active attacks: masquerade, replay, modificati on of messages, and denial of service.1.4Lists and briefly define categories of security service?Authe nticati on : The assura nee that the com muni cati ng en tity is the one that it claims to be.Access contr ol: The preve nti on of un authorized use of a resource (i.e., this service con trols who can have access to a resource, un der what con diti ons access can occur, and what those access ing the resource are allowed to do). Data con fide ntiality : Theprotecti on of data from un authorized disclosure.Data integrity : The assuranee that data received are exactly as sent by an authorized entity (i.e., contain no modification, insertion, deletion, or replay). Non repudiati on : Provides protecti on aga inst denial by one of the en tities invo Ived in a com muni cati on of hav ing participated in all or part of the com muni cati on.Availability service : The property of a system or a system resource being accessible and usable upon dema nd by an authorized system en tity, accord ing to performa nee specificati ons for the system (i.e., a system is available if it provides services accordi ng to the system desig n whe never users request them).Chapter2 Symmetric En crypti onand MessageCon fide ntialityANSWERS NSWERS TO QUESTIONS2.1What are the essential ingredients of a symmetric cipher?Plain text, en crypti on algorithm, secret key, ciphertext, decrypti on algorithm.2.2What are the two basic fun cti ons used in en crypti on algorithms? Permutati on and substituti on.2.3How many keys are required for two people to com muni cate via a symmetric cipher?One secret key.2.4What 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 block of pla in text is treated as a whole and used to produce a ciphertext block of equal le ngth.2.5What are the two gen eral approaches to attack ing a cipher? Crypta nalysis and brute force.2.6Why do some block cipher modes of operati on only use en crypti on while others use both en crypti on and decrypti on?In some modes, the pla in text does not pass through the en crypti on function, but is XORed with the output of the en crypti on fun cti on. The math works out that for decrypti on in these cases, the en crypti on fun cti on must also be used.With triple en cryptio n, a pla in text block is en crypted by pass ing 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 decryptio n algorithm rather tha n the en crypti on algorithm.2.8Why is the middle porti on of 3DES a decrypti on rather tha n an en cryptio n?There is no cryptographic sig nifica nee to the use of decrypti on for the sec ond stage. Its only advantage is that it allows users of 3DES to decrypt data en crypted by users of the older sin gle DES by repeati ng the key.2.9What is the differe nee betwee n link and en d-to-e nd en crypti on?With link en cryptio n, each vuln erable com muni cati ons link is equipped on both ends with an en crypti on device. With en d-to-e nd en crypti on, the en crypti on process is carried out at the two end systems. The source host or terminal en crypts the data; the data in en crypted form are the n tran smitted un altered across the network to the destination terminal or host.2.10List ways in which secret keys can be distributed to two com muni cat ing parties.For two parties A and B, key distributi on can be achieved in a nu mber 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 tothe other, en crypted using the old key.(4)If A and B each has an en crypted connection to a third party C, C can deliver a key on the en crypted links to A and B.2.11What is the difference between a session key and a master key?A sessi on key is a temporary en crypti on key used betwee n two prin cipals. A master key is a Iong-lasting key that is used between a key distribution center and a principal for the purposeof encoding the transmission of session keys. Typically, the master keys are distributed by non cryptographic means.A key distributen center is a system that is authorized to transmit temporary session keys to principals. Each session key is transmitted in encrypted form, using a master key that the key distributio n cen ter shares with the target prin cipal.ANSWERS NSWERS TO PROBLEMS2.1What RC4 key value will leave S unchanged during initialization? Thatis, after the initial permutation of S, the entries of S will be equal to the values from 0 through 255 in asce nding order.Use a key of len gth 255 bytes. The first two bytes are zero; that is K[0] = K[1]= 0. Thereafter, we have: K[2] = 255; K[3] = 254; …K[255]= 2.2.2If a bit error occurs in the tran smissi on of a ciphertext character in8-bit CFB mode, how far does the error propagate?Ni ne pla in text characters are affected. The pla in text character corresp ondingto the ciphertext character is obviously altered. In addition, the altered ciphertext character en ters the shift register and is not removed un til the n ext eight characters are processed.2.3Key distributi on schemes using an access con trol cen ter an d/or a key distributio n cen ter have cen tral points vuln erable to attack. Discuss the security implications of such centralization.The cen tral points should be highly fault-tolera nt, should be physically secured, and should use trusted hardware/software.Chapter 3 Public-Key Cryptography and MessageAuthe nticati onANSWERS NSWERS TO QUESTIONS3.1List three approaches to message authe nticati on.Message en crypti on, message authe nticati on code, hash function.An authenticator that is a cryptographic function of both the data to be authe nticated and a secret key.3.3Briefly describe the three schemes illustrated in Figture3.2.(а)A hash code is computed from the source message, en crypted using symmetric en crypti on and a secret key, and appe nded to the message. At the receiver, the same hash code is computed. The incoming code is 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 en crypti on is used; the sen der en crypts the hash code with the sen der's private key, and the receiver decrypts the hash code with the sender's public key. (c) A secret value is appe nded to a message and the n a hash code is calculated using the message plus secret value as in put. Then the message (without the secret value) and the hash code are tran smitted. The receiver appe nds the same secret value to the message and computes the hash value over the message plus secret value. This is the n compared to the received hash code.3.4What properties must a hash function have to be useful for message authe nticatio n?(1)H can be applied to a block of data of any size.(2)H produces a fixed-le ngth output.(3)H(x) is relatively easy to compute for any given x, making both hardware and software impleme ntatio ns practical.(4)For any give n value h, it is computati on ally in feasible to find x such that H(x) =h. This is sometimes referred to in the literature as the on e-way property.(5)For any give n block x, it is computati on ally in feasible to find y 丰 x with H(y) =H(x). (б)It is computati on ally in feasible to find any pair (x, y) such that H(x) = H(y).3.5In the con text of a hash fun ctio n, what is a compressi on function?The compressi on fun cti on is the fun dame ntal module, or basic buildi ng block, of a hash function. The hash function consists of iterated application of the compressi on fun cti on.3.6What are the prin cipal in gredie nts of a public-key cryptosystem?Plai ntext: This is the readable message or data that is fed in to the algorithm as in put. En crypti on algorithm: The en crypti on algorithm performs various tran sformati ons on the pla in text. Public and private keys: This is a pair of keys that have been selected so that if one is used for encryption, the other is used for decrypti on. The exact tran sformatio ns performed by the en crypti on algorithm depe nd on the public or private key that is provided as in put. Ciphertext: This is the scrambled message produced as output. It depe nds on the pla in text and the key. For a give n message, two differe nt keys will produce two differe nt ciphertexts. Decryptio n algorithm: This algorithm accepts the ciphertext and the match ing key and produces theorig inal pla in text.3.7List and briefly define three uses of a public-key cryptosystem.En crypti on/decrypti on: The sen der en crypts a message with the recipie nt's public key. Digital sig nature: The sen der "sig ns" a message with its private key. Sig ning is achieved by a cryptographic algorithm applied to the message or to a small block of data that is a fun cti on of the message. Key excha nge: Two sides cooperate to excha nge a sessi on key. Several differe nt approaches are possible, involving the private key(s) of one or both parties.3.8What is the differe nee betwee n a private key and a secret key?The key used in conven tio nal en crypti on is typically referred to as a secret key. The two keys used for public-key en crypti on are referred to as the public key and the private key.3.9What is digital sig nature?A digital sig nature is an authe nticatio n mecha nism that en ables the creator of a message to attach a code that acts as a sig nature. The sig nature is formed by tak ing the hash of the message and en crypti ng the message with the creator's private key. The sig nature guara ntees the source and in tegrity of the message.3.10What is a public-key certificate?A pubic-key certificate con sists of a public key plus a User ID of the key owner, with the whole block signed by a trusted third party. Typically, the third party is a certificate authority (CA) that is trusted by the user community, such as a government agency or a financial institution.3.11How can public-key en crypti on be used to distribute a secret key? Several differe nt approaches are possible, involving the private key(s) of one or both parties. One approach isDiffie-Hellma n key excha nge. Ano ther approach is for the sen der to en crypt a secret key with the recipie nt's public key.ANSWERS NSWERS TO PROBLEMS3.1Con sider a 32-bit hash function defi ned as the con cate nati on of two16-bit fun ctio ns: XOR and RXOR, defi ned in Section 3.2 as “ two simplehash function. ”a.Will this checksum detect all errors caused by an odd nu mber of error bits? Expla in.b.Will this checksum detect all errors caused by an eve n nu mber of error bits? If not, characterize the error patter ns that will cause the checksum to fail.me nts on the effective ness of this function for use a hashfun cti ons for authe nticati on.a.Yes. The XOR function is simply a vertical parity check. If there is an odd nu mber of errors, the n there must be at least one colu mn that contains an odd number of errors, and the parity bit for that column will detect the error. Note that the RXOR function also catches all errors caused by an odd nu mber of error bits. Each RXOR bit is a function of a unique "spiral" of bits in the block of data. If there is an odd nu mber of errors, the n there must be at least one spiral that contains an odd nu mber of errors, and the parity bit for that spiral will detect the error.b.No. The checksum will fail to detect an eve n nu mber of errors whe n both the XOR and RXOR functions fail. In order for both to fail, the pattern of error bits must be at in tersecti on points betwee n parity spirals and parity colu mns such that there is an eve n nu mber of error bits in each parity colu mn and an eve n nu mber of error bits in each spiral.c.It is too simple to be used as a secure hash function; finding multiple messages with the same hash function would be too easy.3.2Suppose H ( m) is a collisi on resista nt hash function that maps amessage of arbitrary bit length into an n-bit hash value. Is it true that, forall messages x, x' withx孜',we have HX)工H<')?Explain your answer.The stateme nt is false. Such a fun ctio n cannot be on e-to-one because the nu mber of in puts to the fun cti on is of arbitrary, but the nu mber of unique outputs is 2n. Thus, there are multiple in puts that map into the same output.3.3 Perform encryption and decryption using the RSA algorithm, as in Figture3.9, for thefollowing:a. p=3;q=11;e=7;M=5b. p=5;q=11;e=3;M=9c. p=7;q=11;e=17;M=8d. p=11;q=13;e=11;M=7e. p=17;q=31;e=7;M=2. Hint: D ecrypti on is not as hard as you thi nk; use some fin esse. a. n ==33; (n)=20; d = 3 ;C = 26. b. n = =55; (n)=40; d = 27; C = 14. c. n =:77; (n)= 60; d = 53; C = 57. d. n = =143; (n) =120; d = :11; C = 106.e. n =:527; (n): =480; d = 343; C = 128. For decryption, we have 128343 mod 527 =128256 12864 12816 1284 1282 1281 mod 527=35 256 35 101 47 128 = 2 mod 527 =2 mod 2573.4 In a public-key system using RSA, you intercept the cipher text sent to a userwhose public key is e=5, n =35.What is the pla in text M = 5 3.5 In an RSA system, the public key of a give n user is e=31, n=3599.What is the private key of this user?d = 30313.6 Suppose we have a set of blocks en coded with the RSA algorithmand we don ' t have the private key, Assume n=pq, e is the public key Suppose also some one tells us they know one of the pla in text blocks has a com mon factor with n. Does this help us in any way?Yes. If a pla in text block has a com mon factor with n modulo n the n the en coded block will also have a com mon factor with n modulo n. Because we en code blocks that are smaller tha n pq, the factor must be p or q and the pla in text block must be a multiple of p or q. We can test each C=10 M?block for primality. If prime, it is p or q. In this case we divide into n to find the other factor. If not prime, we factor it and try the factors as divisors of n.3.7Con sider a Diffie-Hellma n scheme with a com mon prime q=11 and aprimitive root a=2.a.If user A has public key YA=9, what is A ' s private key XA?b.If user B has public key YB=3, what is the shared secret key K?a.XA = 6b.K = 3Chapter 4 Authe nticati on Applicati onsANSWERS NSWERS TO QUESTIONS4.1What problem was Kerberos desig ned to address?The problem that Kerberos addresses is this: Assume an ope n distributed en vir onment in which users at workstatio ns wish to access services on servers distributed throughout the n etwork. We would like for servers to be able to restrict access to authorized users and to be able to authe nticate requests for service. In this environment, a workstation cannot be trusted to identify its users correctly to n etwork services.4.2What are three threats associated with user authe nticati on over an etwork or Intern et?A user may gai n access to a particular workstati on and prete nd to be ano ther user operat ing from that workstati on. 2. A user may alter the n etwork address of a workstati on so that the requests sent from the altered workstati on appear to come from the impers on ated workstati on. 3.A user may eavesdrop on excha nges and use a replay attack to gain entrance to a server or to disrupt operati ons.4.3List three approaches to secure user authentication in a distributed en vir onment.Rely on each in dividual clie nt workstati on to assure the ide ntity of its user or users and rely on each server to en force a security policy based on user iden tificati on (ID). 2. Require that clie nt systems authe nticate themselves to servers, but trust the client system concerning the identity of its user. 3. Require the user to prove identity for each service invoked. Also require that servers prove their ide ntity to clie nts.4.4What four requireme nts are defi ned for Kerberos?Secure: A n etwork eavesdropper should not be able to obta in the n ecessary in formatio n to impers on ate a user. More gen erally, Kerberos should be stro ng eno ugh that a pote ntial opp onent does not find it to be the weak link. Reliable: For all services that rely on Kerberos for access con trol, lack of availability of the Kerberos service means lack of availability of the supported services. Hen ce, Kerberos should be highly reliable and should employ a distributed server architecture, with one system able to back up ano ther. Tran spare nt: Ideally, the user should not be aware that authe nticati on is tak ing place, bey ond the requireme nt to en ter a password. Scalable: The system should be capable of support ing large nu mbers of clie nts and servers. This suggests a modular, distributed architecture.4.5What en tities con stitute a full-service Kerberos en vir onment?A full-service Kerberos en vir onment con sists of a Kerberos server, a nu mber of clie nts, and a nu mber of applicatio n servers.4.6In the con text of Kerberos, what is a realm?A realm is an en vir onment in which: 1. The Kerberos server must have the user ID (UID) and hashed password of all participating users in its database. All users are registered with the Kerberos server. 2. The Kerberos server must share a secret key with each server. All servers are registered with the Kerberos server.4.7What are the prin cipal differe nee betwee n vers ion 4 and vers ion 5 of Kerberos?Versi on 5 overcomes some en vir onmen tal shortco mings and some tech ni cal deficie ncies in Vers ion 4.X.509 defi nes a framework for the provisi on of authe nticati on services by the X.500 directory to its users. The directory may serve as a repository of public-key certificates. Each certificate contains the public key of a user and is sig ned with the private key of a trusted certificatio n authority .In additi on, X.509 defi nes alter native authe nticati on protocols based on the use of public-key certificates.4.9What is a chain of certificates?A cha in of certificates con sists of a seque nee of certificates created by differe ntcertificatio n authorities (CAs) in which each successive certificate is a certificate by one CA that certifies the public key of the next CA in the chain.4.10How is an X.509 certificate revoked?The owner of a public-key can issue a certificate revocati on list that revokes one or more certificates.ANSWERS NSWERS TO PROBLEMS4.1Show that a ran dom error in block of cipher text is propagated to all subseque nt blocks of pla in text in PCBC mode (Figure 4.9).An error in Ci affects Pi because the encryption of C is XORed with IV to produceP i. Both Ci and P affect P2, which is the XOR of the encryption of C2 with the XOR of Ci and Pi. Bey ond that, FN - is one of the XORed in puts to forming FN.4.2The i988 version of X.509 lists properties that PSA keys must satisfy to be secure, give n curre nt kno wledge about the difficulty of factori ng large nu mbers. The discussi on con cludes with a con stra int on the public exp onent and the modulus n: It must be en sured that e>log2 (n) to preve nt attack by tak ing the eth root mod n to disclose the pla in text. Although the constraint is correct, the reason given for requiring it isin correct. What is wrong with the reas on give n and what is the correct reas on?Taking the eth root mod n of a ciphertext block will always reveal the pla in text, no matter what the values of e and n are. In general this is a very difficult problem, and in deed is the reas on why RSA is secure. The point is that, if e is too small, then taking the normal integer eth root will be the same as taking the eth root mod n, and tak ing in teger eth roots is relatively easy.Chapter 5 Electr onic Mail SecurityANSWERS NSWERS TO QUESTIONS5.1What are the five prin cipal services provided by PGP?Authe nticati on, con fide ntiality, compressi on, e-mail compatibility, and segme ntati on5.2What is the utility of a detached sig nature?A detached sig nature is useful in several con texts. A user may wish to mai nta in a separate sig nature log of all messages sent or received. A detached sig nature of an executable program can detect subseque nt virus infection. Fi nally, detached sig natures can be used whe n more tha n one party must sig n a docume nt, such as a legal con tract. Each pers on's sig nature is in depe ndent and therefore is applied only to the docume nt. Otherwise, sig natures would have to be n ested, with the sec ond sig ner sig ning both the docume nt and the first sig nature, and so on.5.3Why does PGP gen erate a sig nature before appl ying compressi on?a.It is preferable to sig n an un compressed message so that one can store only the un compressed message together with the sig nature for future verificatio n. If one signed a compressed document, then it would be necessary either to store a compressed versi on of the message for laterverificatio n or to recompress the message when verification is required. b. Even if one were willi ng to gen erate dyn amically a recompressed message for verificati on, PGP's compression algorithm presents a difficulty. The algorithm is not determi nistic; various impleme ntatio ns of the algorithm achieve differe nt tradeoffs in running speed versus compressi on ratio and, as a result, produce differe nt compressed forms. However, these differe nt compressi on algorithms are in teroperable because any versi on of the algorithm can correctly decompress the output of any other versi on. Appl ying the hash fun cti on and sig nature after compressi on would con strain all PGP impleme ntatio ns to the same vers ion of the compressi on algorithm.5.4What is R64co nversio n?R64 con verts a raw 8-bit binary stream to a stream of prin table ASCII characters. Each group of three octets of bi nary data is mapped into four ASCII characters.5.5Why is R64 conversion useful for an e-mail application?Whe n PGP is used, at least part of the block to be tran smitted is en crypted. If only the sig nature service is used, the n the message digest is en crypted (with the sen der's private key). If the con fide ntiality service is used, the message plus sig nature (if prese nt) are en crypted (with a on e-time symmetric key). Thus, part or all of the resulting block consists of a stream of arbitrary 8-bit octets. However, many electro nic mail systems only permit the use of blocks consisti ng of ASCII text.5.6Why is the segme ntati on and reassembly fun ctio n in PGP n eeded? E-mail facilities ofte n are restricted to a maximum message len gth.5.7How does PGP use the con cept of trust?PGP in cludes a facility for assig ning a level of trust to in dividual sig ners and to keys.5.8What is RFC822?RFC 822 defi nes a format for text messages that are sent using electr onic mail.5.9What is MIME?MIME is an exte nsion to the RFC 822 framework that is inten ded to address some of the problems and limitations of the use of SMTP (Simple Mail Transfer Protocol) or some other mail tran sfer protocol and RFC 822 for electr onic mail.5.10What is S/MIME?S/MIME (Secure/Multipurpose In ternet Mail Exte nsio n) is a security enhan ceme nt to the MIME Internet e-mail format sta ndard, based on tech no logy from RSA Data Security.ANSWERS NSWERS TO PROBLEMS5.1In the PGP scheme, what is the expected number of session keys gen erated before a previously created key is produced?This is just ano ther form of the birthday paradox discussed in Appe ndix 11A. Let us state the problem as one of determ ining what nu mber of sessi on keys must be gen erated so that the probability of a duplicate is greater tha n 0.5. From Equati on (11.6) in Appe ndix 11A, we have the approximati on:k 1.18 nFor a 128-bit key, there are 228 possible keys. Thereforek 1.18 2128 1.18 2645.2The first 16 bits of the message digest in a PGP sig nature are tran slated in the clear.a.To what exte nt does this compromise the security of the hash algorithm?b.To what exte nt does it in fact perform its in ten ded fun cti on, n amely, to help determine if the correct RSA key was used to decrypt the digest?a.Not at all. The message digest is en crypted with the sen der's private key. Therefore, anyonein possessi on of the public key can decrypt it and recover the en tire message digest.b.The probability that a message digest decrypted with the wrong key wouldhave an exact match in the first 16 bits with the original message digest is 2 -65.3In Figure 5.4, each en try in the public-key ring contains an owner trust field that indicates the degree of trust associated with this public-key owner. Why is that not eno ugh? Thatis, if this owner istrusted and this is supposed to be the owne r' s public key, why is not that trust eno ugh to permit PGP to use this public key?We trust this owner, but that does not n ecessarily mean that we can trust that we are in possession of that own er's public key.5.4Consider radix-64 conversion as a form of encryption. In this case, there is no key. Butsuppose that an opp onent knew only that some form of substitution algorithm was being used toencrypt English text and did not guess it was R64. How effective would this algorithm be againstcrypta nalysis?It certa inly provides more security tha n a mono alphabetic substituti on. Because we are treating the pla in text as a stri ng of bits and en crypt ing 6 bits at a time, we are not en crypt ingin dividual characters. Therefore, the freque ncy information is lost, or at least significantlyobscured.5.5Phil Zimmermann chose IDEA, three-key triple DES, and CAST-128 as symmetric en crypti onalgorithms for PGP.Give reas ons why each of the follow ing symmetric en crypti on algorithms fordescribed in this book is suitable or un suitable for PGP: DES, two-key triple DES, and AES.DES is un suitable because of its short key size. Two-key triple DES, which has a key len gth of112 bits, is suitable. AES is also suitable.Chapter 6 IP SecurityANSWERS NSWERS TO QUESTIONS6.1Give examples of applicati ons of IPSec.Secure branch office conn ectivity over the Intern et: A compa ny can build a secure virtual。