计算机网络第四版课后英文题目

Chapter 1. Introduction Section 1.1. Uses of Computer Networks Section 1.2. Network Hardware Section 1.3. Network Software Section 1.4. Reference Models Section 1.5. Example Networks Section 1.6. Network Standardization Section 1.7. Metric Units Section 1.8. Outline of the Rest of the Book Section 1.9. Summary Chapter 2. The Physical Layer

Section 2.1. The Theoretical Basis for Data Communication Section 2.2. Guided Transmission Media Section 2.3. Wireless Transmission Section 2.4. Communication Satellites Section 2.5. The Public Switched Telephone Network Section 2.6. The Mobile Telephone System Section 2.7. Cable Television Section 2.8. Summary Chapter 3. The Data Link Layer Section 3.1. Data Link Layer Design Issues Section 3.2. Error Detection and Correction Section 3.3. Elementary Data Link Protocols Section 3.4. Sliding Window Protocols Section 3.5. Protocol Verification Section 3.6. Example Data Link Protocols Section 3.7. Summary

Chapter 4. The Medium Access Control Sublayer Section 4.1. The Channel Allocation Problem Section 4.2. Multiple Access Protocols Section 4.3. Ethernet Section 4.4. Wireless LANs Section 4.5. Broadband Wireless Section 4.6. Bluetooth Section 4.7. Data Link Layer Switching Section 4.8. Summary

Chapter 5. The Network Layer Section 5.1. Network Layer Design Issues Section 5.2. Routing Algorithms Section 5.3. Congestion Control Algorithms

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Section 5.4. Quality of Service Section 5.5. Internetworking

Section 5.6. The Network Layer in the Internet Section 5.7. Summary

Chapter 6. The Transport Layer Section 6.1. The Transport Service

Section 6.2. Elements of Transport Protocols Section 6.3. A Simple Transport Protocol

Section 6.4. The Internet Transport Protocols: UDP Section 6.5. The Internet Transport Protocols: TCP Section 6.6. Performance Issues Section 6.7. Summary Chapter 7. The Application Layer Section 7.1. DNS—The Domain Name System Section 7.2. Electronic Mail Section 7.3. The World Wide Web Section 7.4. Multimedia Section 7.5. Summary Chapter 8. Network Security Section 8.1. Cryptography Section 8.2. Symmetric-Key Algorithms Section 8.3. Public-Key Algorithms Section 8.4. Digital Signatures Section 8.5. Management of Public Keys Section 8.6. Communication Security Section 8.7. Authentication Protocols Section 8.8. E-Mail Security Section 8.9. Web Security Section 8.10. Social Issues Section 8.11. Summary Chapter 9. Reading List and Bibliography Section 9.1. Suggestions for Further Reading Section 9.1.1. Introduction and General Works Section 9.2. Alphabetical Bibliography

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计算机网络课后题目

第一章

1. Imagine that you have trained your St. Bernard, Bernie, to carry a box of three 8mm tapes instead of a flask of brandy. (When your disk fills up, you consider that an emergency.) These tapes each contain 7 gigabytes. The dog can travel to your side, wherever you may be, at 18 km/hour. For what range of distances does Bernie have a higher data rate than a transmission line whose data rate (excluding overhead) is 150 Mbps?

2. An alternative to a LAN is simply a big timesharing system with terminals for all users. Give two advantages of a client-server system using a LAN.

3. The performance of a client-server system is influenced by two network factors: the bandwidth of the network (how many bits/sec it can transport) and the latency (how many seconds it takes for the first bit to get from the client to the server). Give an example of a network that exhibits high bandwidth and high latency. Then give an example of one with low bandwidth and low latency.

4. Besides bandwidth and latency, what other parameter is needed to give a good characterization of the quality of service offered by a network used for digitized voice traffic?

5. A factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If switching time is 10 μsec, is this likely to be a major factor in the response of a client-server system where the client is in New York and the server is in California? Assume the

propagation speed in copper and fiber to be 2/3 the speed of light in vacuum. 6. A client-server system uses a satellite network, with the satellite at a height of 40,000 km. What is the best-case delay in response to a request?

7. In the future, when everyone has a home terminal connected to a computer network, instant public referendums on important pending legislation will become possible. Ultimately, existing

legislatures could be eliminated, to let the will of the people be expressed directly. The positive aspects of such a direct democracy are fairly obvious; discuss some of the negative aspects.

8. A collection of five routers is to be connected in a point-to-point subnet. Between each pair of routers, the designers may put a high-speed line, a medium-speed line, a low-speed line, or no line.

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If it takes 100 ms of computer time to generate and inspect each topology, how long will it take to inspect all of them?

9. A group of 2n - 1 routers are interconnected in a centralized binary tree, with a router at each tree node. Router i communicates with router j by sending a message to the root of the tree. The root then sends the message back down to j. Derive an approximate expression for the mean number of hops per message for large n, assuming that all router pairs are equally likely.

10.A disadvantage of a broadcast subnet is the capacity wasted when multiple hosts attempt to access the channel at the same time. As a simplistic example, suppose that time is divided into discrete slots, with each of the n hosts attempting to use the channel with probability p during each slot. What fraction of the slots are wasted due to collisions?

11.What are two reasons for using layered protocols?

12.The president of the Specialty Paint Corp. gets the idea to work with a local beer brewer to produce an invisible beer can (as an anti-litter measure). The president tells her legal department to look into it, and they in turn ask engineering for help. As a result, the chief engineer calls his counterpart at the other company to discuss the technical aspects of the project. The engineers then report back to their respective legal departments, which then confer by telephone to arrange the legal aspects. Finally, the two corporate presidents discuss the financial side of the deal. Is this an example of a multilayer protocol in the sense of the OSI model? 13.What is the principal difference between connectionless communication and connection-oriented communication?

14.Two networks each provide reliable connection-oriented service. One of them offers a reliable byte stream and the other offers a reliable message stream. Are these identical? If so, why is the distinction made? If not, give an example of how they differ. 15.What does ''negotiation'' mean when discussing network protocols? Give an example.

16.In Fig. 1-19, a service is shown. Are any other services implicit in this figure? If so, where? If not, why not?

17.In some networks, the data link layer handles transmission errors by requesting damaged frames to be retransmitted. If the probability of a frame's being damaged is p, what is the mean number of transmissions required to send a frame? Assume that acknowledgements are never lost.

18.Which of the OSI layers handles each of the following:

a. (a) Dividing the transmitted bit stream into frames. b. (b) Determining which route through the subnet to use.

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19.If the unit exchanged at the data link level is called a frame and the unit exchanged at the network level is called a packet, do frames encapsulate packets or do packets encapsulate frames? Explain your answer.

20.A system has an n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h-byte header is added. What fraction of the network bandwidth is filled with headers?

21.List two ways in which the OSI reference model and the TCP/IP reference model are the same. Now list two ways in which they differ. 22.What is the main difference between TCP and UDP?

23.The subnet of Fig. 1-25(b) was designed to withstand a nuclear war. How many bombs would it take to partition the nodes into two disconnected sets? Assume that any bomb wipes out a node and all of the links connected to it.

24.The Internet is roughly doubling in size every 18 months. Although no one really knows for sure, one estimate put the number of hosts on it at 100 million in 2001. Use these data to compute the expected number of Internet hosts in the year 2010. Do you believe this? Explain why or why not.

25.When a file is transferred between two computers, two acknowledgement strategies are possible. In the first one, the file is chopped up into packets, which are individually acknowledged by the receiver, but the file transfer as a whole is not acknowledged. In the second one, the packets are not acknowledged individually, but the entire file is acknowledged when it arrives. Discuss these two approaches.

26.Why does ATM use small, fixed-length cells?

27.How long was a bit on the original 802.3 standard in meters? Use a transmission speed of 10 Mbps and assume the propagation speed in coax is 2/3 the speed of light in vacuum.

28.An image is 1024 x 768 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a 10-Mbps Ethernet? Over 100-Mbps Ethernet?

29.Ethernet and wireless networks have some similarities and some differences. One property of Ethernet is that only one frame at a time can be transmitted on an Ethernet. Does 802.11 share this property with Ethernet? Discuss your answer. 30.Wireless networks are easy to install, which makes them inexpensive since installation costs usually far overshadow equipment costs. Nevertheless, they also have some disadvantages. Name two of them. 31.List two advantages and two disadvantages of having international standards for network protocols.

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all the stations whose addresses are prime numbers suddenly become ready at once, how many bit slots are needed to resolve the contention?

11.A collection of 2n stations uses the adaptive tree walk protocol to arbitrate access to a shared cable. At a certain instant, two of them become ready. What are the minimum, maximum, and mean number of slots to walk the tree if 2n 1?

12.The wireless LANs that we studied used protocols such as MACA instead of using CSMA/CD. Under what conditions, if any, would it be possible to use CSMA/CD instead?

13.What properties do the WDMA and GSM channel access protocols have in common? See Chap. 2 for GSM.

14.Six stations, A through F, communicate using the MACA protocol. Is it possible that two transmissions take place simultaneously? Explain your answer.

15.A seven-story office building has 15 adjacent offices per floor. Each office contains a wall socket for a terminal in the front wall, so the sockets form a rectangular grid in the vertical plane, with a separation of 4 m between sockets, both horizontally and

vertically. Assuming that it is feasible to run a straight cable between any pair of sockets, horizontally, vertically, or

diagonally, how many meters of cable are needed to connect all sockets using

a. (a) a star configuration with a single router in the middle? b. (b) an 802.3 LAN?

16.What is the baud rate of the standard 10-Mbps Ethernet?

17.Sketch the Manchester encoding for the bit stream: 0001110101. 18.Sketch the differential Manchester encoding for the bit stream of the previous problem. Assume the line is initially in the low state. 19.A 1-km-long, 10-Mbps CSMA/CD LAN (not 802.3) has a propagation speed of 200 m/μsec. Repeaters are not allowed in this system. Data frames are 256 bits long, including 32 bits of header, checksum, and other overhead. The first bit slot after a successful transmission is reserved for the receiver to capture the channel in order to send a 32-bit acknowledgement frame. What is the effective data rate, excluding overhead, assuming that there are no collisions?

20.Two CSMA/CD stations are each trying to transmit long (multiframe) files. After each frame is sent, they contend for the channel, using the binary exponential backoff algorithm. What is the probability that the contention ends on round k, and what is the mean number of rounds per contention period?

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21.Consider building a CSMA/CD network running at 1 Gbps over a 1-km cable with no repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size?

22.An IP packet to be transmitted by Ethernet is 60 bytes long, including all its headers. If LLC is not in use, is padding needed in the Ethernet frame, and if so, how many bytes?

23.Ethernet frames must be at least 64 bytes long to ensure that the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64-byte minimum frame size but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size? 24.Some books quote the maximum size of an Ethernet frame as 1518 bytes instead of 1500 bytes. Are they wrong? Explain your answer.

25.The 1000Base-SX specification states that the clock shall run at 1250 MHz, even though gigabit Ethernet is only supposed to deliver 1 Gbps. Is this higher speed to provide for an extra margin of safety? If not, what is going on here?

26.How many frames per second can gigabit Ethernet handle? Think carefully and take into account all the relevant cases. Hint: the fact that it is gigabit Ethernet matters.

27.Name two networks that allow frames to be packed back-to-back. Why is this feature worth having?

28.In Fig. 4-27, four stations, A, B, C, and D, are shown. Which of the last two stations do you think is closest to A and why?

29.Suppose that an 11-Mbps 802.11b LAN is transmitting 64-byte frames back-to-back over a radio channel with a bit error rate of 10-7. How many frames per second will be damaged on average?

30.An 802.16 network has a channel width of 20 MHz. How many bits/sec can be sent to a subscriber station?

31.IEEE 802.16 supports four service classes. Which service class is the best choice for sending uncompressed video?

32.Give two reasons why networks might use an error-correcting code instead of error detection and retransmission. 33.From Fig. 4-35, we see that a Bluetooth device can be in two piconets at the same time. Is there any reason why one device cannot be the master in both of them at the same time?

34.Figure 4-25 shows several physical layer protocols. Which of these is closest to the Bluetooth physical layer protocol? What is the biggest difference between the two?

35.Bluetooth supports two types of links between a master and a slave. What are they and what is each one used for?

36.Beacon frames in the frequency hopping spread spectrum variant of 802.11 contain the dwell time. Do you think the analogous beacon

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frames in Bluetooth also contain the dwell time? Discuss your answer.

37.Consider the interconnected LANs showns in Fig. 4-44. Assume that hosts a and b are on LAN 1, c is on LAN 2, and d is on LAN 8. Initially, hash tables in all bridges are empty and the spanning tree shown in Fig 4-44(b) is used. Show how the hash tables of different bridges change after each of the following events happen in sequence, first (a) then (b) and so on.

a. (a) a sends to d. b. (b) c sends to a. c. (c) d sends to c.

d. (d) d moves to LAN 6. e. (e) d sends to a.

38.One consequence of using a spanning tree to forward frames in an extended LAN is that some bridges may not participate at all in forwarding frames. Identify three such bridges in Fig. 4-44. Is there any reason for keeping these bridges, even though they are not used for forwarding?

39.Imagine that a switch has line cards for four input lines. It frequently happens that a frame arriving on one of the lines has to exit on another line on the same card. What choices is the switch designer faced with as a result of this situation?

40.A switch designed for use with fast Ethernet has a backplane that can move 10 Gbps. How many frames/sec can it handle in the worst case?

41.Consider the network of Fig. 4-49(a). If machine J were to suddenly become white, would any changes be needed to the labeling? If so, what?

42.Briefly describe the difference between store-and-forward and cut-through switches.

43.Store-and-forward switches have an advantage over cut-through switches with respect to damaged frames. Explain what it is. 44.To make VLANs work, configuration tables are needed in the switches and bridges. What if the VLANs of Fig. 4-49(a) use hubs rather than multidrop cables? Do the hubs need configuration tables, too? Why or why not?

45.In Fig. 4-50 the switch in the legacy end domain on the right is a VLAN-aware switch. Would it be possible to use a legacy switch there? If so, how would that work? If not, why not?

第五章

Problems

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1. Give two example computer applications for which

connection-oriented service is appropriate. Now give two examples for which connectionless service is best.

2. Are there any circumstances when connection-oriented service will (or at least should) deliver packets out of order? Explain.

3. Datagram subnets route each packet as a separate unit, independent of all others. Virtual-circuit subnets do not have to do this, since each data packet follows a predetermined route. Does this observation mean that virtual-circuit subnets do not need the capability to route isolated packets from an arbitrary source to an arbitrary destination? Explain your answer. 4. Give three examples of protocol parameters that might be negotiated when a connection is set up. 5. Consider the following design problem concerning implementation of virtual-circuit service. If virtual circuits are used internal to the subnet, each data packet must have a 3-byte header and each router must tie up 8 bytes of storage for circuit identification. If datagrams are used internally, 15-byte headers are needed but no router table space is required. Transmission capacity costs 1 cent per 106 bytes, per hop. Very fast router memory can be purchased for 1 cent per byte and is depreciated over two years, assuming a 40-hour business week. The statistically average session runs for 1000 sec, in which time 200 packets are transmitted. The mean packet requires four hops. Which implementation is cheaper, and by how much?

6. Assuming that all routers and hosts are working properly and that all software in both is free of all errors, is there any chance, however small, that a packet will be delivered to the wrong destination? 7. Consider the network of Fig. 5-7, but ignore the weights on the lines. Suppose that it uses flooding as the routing algorithm. If a packet sent by A to D has a maximum hop count of 3, list all the routes it will take. Also tell how many hops worth of bandwidth it consumes. 8. Give a simple heuristic for finding two paths through a network from a given source to a given destination that can survive the loss of any communication line (assuming two such paths exist). The routers are considered reliable enough, so it is not necessary to worry about the possibility of router crashes. 9. Consider the subnet of Fig. 5-13(a). Distance vector routing is used, and the following vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6, 0, 9, 10); and from E: (7, 6, 3, 9, 0, 4). The measured delays to B, D, and E, are 6, 3, and 5, respectively. What is C's new routing table? Give both the outgoing line to use and the expected delay.

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10.If delays are recorded as 8-bit numbers in a 50-router network, and delay vectors are exchanged twice a second, how much bandwidth per (full-duplex) line is chewed up by the distributed routing algorithm? Assume that each router has three lines to other routers. 11.In Fig. 5-14 the Boolean OR of the two sets of ACF bits are 111 in every row. Is this just an accident here, or does it hold for all subnets under all circumstances? 12.For hierarchical routing with 4800 routers, what region and cluster sizes should be chosen to minimize the size of the routing table for a three-layer hierarchy? A good starting place is the hypothesis that a solution with k clusters of k regions of k routers is close to optimal, which means that k is about the cube root of 4800 (around 16). Use trial and error to check out combinations where all three parameters are in the general vicinity of 16.

13.In the text it was stated that when a mobile host is not at home, packets sent to its home LAN are intercepted by its home agent on that LAN. For an IP network on an 802.3 LAN, how does the home agent accomplish this interception?

14.Looking at the subnet of Fig. 5-6, how many packets are generated by a broadcast from B, using

a. (a) reverse path forwarding? b. (b) the sink tree?

15.Consider the network of Fig. 5-16(a). Imagine that one new line is added, between F and G, but the sink tree of Fig. 5-16(b) remains unchanged. What changes occur to Fig. 5-16(c)?

16.Compute a multicast spanning tree for router C in the following subnet for a group with members at routers A, B, C, D, E, F, I, and K.

17.In Fig. 5-20, do nodes H or I ever broadcast on the lookup shown starting at A? 18.Suppose that node B in Fig. 5-20 has just rebooted and has no routing information in its tables. It suddenly needs a route to H. It sends

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30.In Fig. 7-25, www.aportal.com keeps track of user preferences in a cookie. A disadvantage of this scheme is that cookies are limited to 4 KB, so if the preferences are extensive, for example, many stocks, sports teams, types of news stories, weather for multiple cities, specials in numerous product categories, and more, the 4-KB limit may be reached. Design an alternative way to keep track of preferences that does not have this problem. 31.Sloth Bank wants to make on-line banking easy for its lazy customers, so after a customer signs up and is authenticated by a password, the bank returns a cookie containing a customer ID number. In this way, the customer does not have to identify himself or type a password on future visits to the on-line bank. What do you think of this idea? Will it work? Is it a good idea?

32.In Fig. 7-26, the ALT parameter is set in the tag. Under what conditions does the browser use it, and how?

33.How do you make an image clickable in HTML? Give an example. 34.Show the tag that is needed to make the string ''ACM'' be a hyperlink to http://www.acm.org. 35.Design a form for a new company, Interburger, that allows hamburgers to be ordered via the Internet. The form should include the customer's name, address, and city, as well as a choice of size (either gigantic or immense) and a cheese option. The burgers are to be paid for in cash upon delivery, so no credit card information is needed.

36.Design a form that requests the user to type in two numbers. When the user clicks on the submit button, the server returns their sum. Write the server side as a PHP script.

37.For each of the following applications, tell whether it would be (1) possible and (2) better to use a PHP script or JavaScript and why.

a. (a) Displaying a calendar for any requested month since

September 1752.

b. (b) Displaying the schedule of flights from Amsterdam to New

York.

c. (c) Graphing a polynomial from user-supplied coefficients

38.Write a program in JavaScript that accepts an integer greater than 2 and tells whether it is a prime number. Note that JavaScript has if and while statements with the same syntax as C and Java. The modulo operator is %. If you need the square root of x, use Math.sqrt (x).

39.An HTML page is as follows:

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If the user clicks on the hyperlink, a TCP connection is opened and a series of lines is sent to the server. List all the lines sent. 40.The If-Modified-Since header can be used to check whether a cached page is still valid. Requests can be made for pages containing images, sound, video, and so on, as well as HTML. Do you think the effectiveness of this technique is better or worse for JPEG images as compared to HTML? Think carefully about what ''effectiveness'' means and explain your answer.

41.On the day of a major sporting event, such as the championship game in some popular sport, many people go to the official Web site. Is this a flash crowd in the same sense as the Florida election in 2000? Why or why not?

42.Does it make sense for a single ISP to function as a CDN? If so, how would that work? If not, what is wrong with the idea? 43.Under what conditions is using a CDN a bad idea?

44.Wireless Web terminals have low bandwidth, which makes efficient coding important. Devise a scheme for transmitting English text efficiently over a wireless link to a WAP device. You may assume that the terminal has several megabytes of ROM and a moderately powerful CPU. Hint: think about how you transmit Japanese, in which each symbol is a word.

45.A compact disc holds 650 MB of data. Is compression used for audio CDs? Explain your reasoning.

46.In Fig. 7-57(c) quantization noise occurs due to the use of 4-bit samples to represent nine signal values. The first sample, at 0, is exact, but the next few are not. What is the percent error for the samples at 1/32, 2/32, and 3/32 of the period?

47.Could a psychoacoustic model be used to reduce the bandwidth needed for Internet telephony? If so, what conditions, if any, would have to be met to make it work? If not, why not?

48.An audio streaming server has a one-way distance of 50 msec with a media player. It outputs at 1 Mbps. If the media player has a 1-MB buffer, what can you say about the position of the low-water mark and the high-water mark?

49.The interleaving algorithm of Fig. 7-60 has the advantage of being able to survive an occasional lost packet without introducing a gap in the playback. However, when used for Internet telephony, it also has a small disadvantage. What is it?

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50.Does voice over IP have the same problems with firewalls that streaming audio does? Discuss your answer.

51.What is the bit rate for transmitting uncompressed 800 x 600 pixel color frames with 8 bits/pixel at 40 frames/sec?

52.Can a 1-bit error in an MPEG frame affect more than the frame in which the error occurs? Explain your answer.

53.Consider a 100,000-customer video server, where each customer watches two movies per month. Half the movies are served at 8 P.M. How many movies does the server have to transmit at once during this time period? If each movie requires 4 Mbps, how many OC-12 connections does the server need to the network?

54.Suppose that Zipf's law holds for accesses to a 10,000-movie video server. If the server holds the most popular 1000 movies on magnetic disk and the remaining 9000 on optical disk, give an expression for the fraction of all references that will be to magnetic disk. Write a little program to evaluate this expression numerically.

第八章

Problems

1. Break the following monoalphabetic cipher. The plaintext,

consisting of letters only, is a well-known excerpt from a poem by Lewis Carroll.

kfd ktbd fzm eubd kfd pzyiom mztx ku kzyg ur bzha kfthcm ur mftnm zhx mfudm zhx mdzythc pzq ur ezsszcdm zhx gthcm zhx pfa kfd mdz tm sutythc fuk zhx pfdkfdi ntcm fzld pthcm sok pztk z stk kfd uamkdim eitdx sdruid pd fzld uoi efzk rui mubd ur om zid uok ur sidzkf zhx zyy ur om zid rzk hu foiia mztx kfd ezindhkdi kfda kfzhgdx ftb boef rui kfzk 2. Break the following columnar transposition cipher. The plaintext is taken from a popular computer textbook, so ''computer'' is a probable word. The plaintext consists entirely of letters (no spaces). The ciphertext is broken up into blocks of five characters for readability.

aauan cvlre rurnn dltme aeepb ytust iceat npmey iicgo gorch srsoc

nntii imiha oofpa gsivt tpsit lbolr otoex

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3. Find a 77-bit one-time pad that generates the text ''Donald Duck'' from the ciphertext of Fig. 8-4.

4. Quantum cryptography requires having a photon gun that can, on demand, fire a single photon carrying 1 bit. In this problem, calculate how many photons a bit carries on a 100-Gbps fiber link. Assume that the length of a photon is equal to its wavelength, which for purposes of this problem, is 1 micron. The speed of light in fiber is 20 cm/nsec. 5. If Trudy captures and regenerates photons when quantum cryptography is in use, she will get some of them wrong and cause errors to appear in Bob's one-time pad. What fraction of Bob's one-time pad bits will be in error, on average? 6. A fundamental cryptographic principle states that all messages must have redundancy. But we also know that redundancy helps an intruder tell if a guessed key is correct. Consider two forms of redundancy. First, the initial n bits of the plaintext contain a known pattern. Second, the final n bits of the message contain a hash over the message. From a security point of view, are these two equivalent? Discuss your answer.

7. In Fig. 8-6, the P-boxes and S-boxes alternate. Although this arrangement is esthetically pleasing, is it any more secure than first having all the P-boxes and then all the S-boxes?

8. Design an attack on DES based on the knowledge that the plaintext consists exclusively of upper case ASCII letters, plus space, comma, period, semicolon, carriage return, and line feed. Nothing is known about the plaintext parity bits.

9. In the text we computed that a cipher-breaking machine with a billion processors that could analyze a key in 1 picosecond would take only 1010 years to break the 128-bit version of AES. However, current machines might have 1024 processors and take 1 msec to analyze a key, so we need a factor of 1015 improvement in performance just to obtain the AES-breaking machine. If Moore's law (computing power doubles every 18 months) continues to hold, how many years will it take to even build the machine?

10.AES supports a 256-bit key. How many keys does AES-256 have? See if you can find some number in physics, chemistry, or astronomy of about the same size. Use the Internet to help search for big numbers. Draw a conclusion from your research.

11.Suppose that a message has been encrypted using DES in ciphertext block chaining mode. One bit of ciphertext in block Ci is

accidentally transformed from a 0 to a 1 during transmission. How much plaintext will be garbled as a result?

12.Now consider ciphertext block chaining again. Instead of a single 0 bit being transformed into a 1 bit, an extra 0 bit is inserted

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into the ciphertext stream after block Ci. How much plaintext will be garbled as a result?

13.Compare cipher block chaining with cipher feedback mode in terms of the number of encryption operations needed to transmit a large file. Which one is more efficient and by how much?

14.Using the RSA public key cryptosystem, with a = 1, b = 2, etc.,

a. If p = 7 and q = 11, list five legal values for d. b. If p = 13, q = 31, and d = 7, find e.

c. Using p = 5, q = 11, and d = 27, find e and encrypt

''abcdefghij''.

15.Suppose a user, Maria, discovers that her private RSA key (d 1, n 1) is same as the public RSA key (e 2, n 2) of another user, Frances. In other words, d 1 = e 2 and n 1 = n 2. Should Maria consider changing her public and private keys? Explain your answer.

16.Consider the use of counter mode, as shown in Fig. 8-15, but with IV = 0. Does the use of 0 threaten the security of the cipher in general?

17.The signature protocol of Fig. 8-18 has the following weakness. If Bob crashes, he may lose the contents of his RAM. What problems does this cause and what can he do to prevent them?

18.In Fig. 8-20, we see how Alice can send Bob a signed message. If Trudy replaces P, Bob can detect it. But what happens if Trudy replaces both P and the signature?

19.Digital signatures have a potential weakness due to lazy users. In e-commerce transactions, a contract might be drawn up and the user asked to sign its SHA-1 hash. If the user does not actually verify that the contract and hash correspond, the user may inadvertently sign a different contract. Suppose that the Mafia try to exploit this weakness to make some money. They set up a pay Web site (e.g., pornography, gambling, etc.) and ask new customers for a credit card number. Then they send over a contract saying that the customer wishes to use their service and pay by credit card and ask the customer to sign it, knowing that most of them will just sign without verifying that the contract and hash agree. Show how the Mafia can buy diamonds from a legitimate Internet jeweler and charge them to unsuspecting customers.

20.A math class has 20 students. What is the probability that at least two students have the same birthday? Assume that nobody was born on leap day, so there are 365 possible birthdays.

21.After Ellen confessed to Marilyn about tricking her in the matter of Tom's tenure, Marilyn resolved to avoid this problem by dictating the contents of future messages into a dictating machine and having her new secretary just type them in. Marilyn then planned to examine the messages on her terminal after they had been typed in to make

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