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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