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computer science
computer networking

Questions and Answers of Computer Networking

Assume a router receives packets of size 400 bits every 100 ms, which means with the data rate of 4 kbps. Show how we can change the output data rate to less than 1 kbps by using a leaky bucket
Which of the following applications are classified as VBR-RT in ATM?a. HTTPb. SNMPc. SMTPd. VoIP
In a switch using the token bucket algorithm, tokens are added to the bucket at a rate of r = 5 tokens/second. The capacity of the token bucket is c = 10. The switch has a buffer that can hold only
Which of the following applications are classified as UBR in ATM?a. HTTPb. SNMPc. SMTPd. VoIP
To understand how the token bucket algorithm can give credit to the sender that does not use its rate allocation for a while but wants to use it later, let€™s repeat the previous problem with r =
Which of the following technique(s) is (are) used for scheduling?a. FIFO queuingb. Priority queuingc. Leaky Bucket
Which of the following technique(s) is (are) used for traffic shaping?a. Token bucketb. Priority queuingc. Leaky Bucket
A program to simulate a leaky bucket.
Assume an ISP uses three leaky buckets to regulate data received from three customers for transmitting to the Internet. The customers send fixed-size packets (cells). The ISP sends 10 cells per
Define the parts of flow specification in IntServ.
Assume that the ISP in the previous problem decided to use token buckets (of capacity c = 20 and rate r = 10) instead of leaky buckets to give credit to the customer that does not send cells for a
Distinguish between guaranteed services and controlled-load services in IntServ.
IntServ is normally called a destination-based service. Explain the reason.
DiffServ is normally called a source-based service. Explain the reason.
If a communication is unicast, how can we use RSVP, which is designed for multicast in IntServ?
How can multicasting be achieved using DiffServ?
Why do we need Path and Resv messages in RSVP?
Figure 30.12 shows a router using FIFO queuing at the input port.The arrival and required service times for seven packets are shown below; timeans that the packet has arrived or departed i ms after a
How many per-hop behaviors have been defined for DiffServ? Name them.
List the components of a traffic conditioner in DiffServ.
Is the flow label in IPv6 more appropriate for IntServ or DiffServ?
Which of the following attacks is a threat to confidentiality?a. Snoopingb. Masqueradingc. Repudiation
A general program to implement substitution (additive) cipher (encryption and decryption). The input to the program is a flag that demands encryption or decryption, the symmetric key, and the
Define the type of attack in each of the following cases:a. A student breaks into a professor’s office to obtain a copy of the next test.b. A student gives a check for $10 to buy a used book. Later
Which of the following attacks is a threat to integrity?a. Modificationb. Replayingc. Denial of service
A general program to implement transposition cipher (encryption and decryption). The input to the program is a flag that demands encryption or decryption, the symmetric key, and the plaintext or
Which of the following attacks is a threat to availability?a. Repudiationb. Denial of servicec. Modification
A general program to implement RSA cryptosystem. The input to the program is a flag that demands encryption or decryption, the value of p and q, the value of e, and the plaintext or ciphertext. The
Which of the following words means “secret writing”? Which one means “covered writing”?a. Cryptographyb. Steganography
Atbash was a popular cipher among Biblical writers. In Atbash, “A” is encrypted as “Z”, “B” is encrypted as “Y”, and so on. Similarly, “Z” is encrypted as “A”, “Y” is
A substitution cipher does not have to be a character-to-character transformation. In a Polybius cipher, each letter in the plaintext is encrypted as two integers. The key is a 5 Ã— 5
Alice can use only the additive cipher on her computer to send a message to a friend. She thinks that the message is more secure if she encrypts the message two times, each time with a different key.
One of the attacks an intruder can apply to a simple cipher like an additive cipher is called the ciphertext attack. In this type of attack, the intruder intercepts the cipher and tries to find the
Alice and Bob exchange confidential messages. They share a very large number as the encryption and decryption key in both directions. Is this an example of symmetric-key or asymmetric-key
Another method used in a ciphertext attack (see previous problem) is called the statistical approach, in which the intruder intercepts a long ciphertext and tries to analyze the statistics of the
What are the two protocols defined by IPSec?
Assume we have a very simple message digest. Our unrealistic message digest is just one number between 0 and 25. The digest is initially set to 0. The cryptographic hash function adds the current
A permutation block (P-box) in a modern block cipher is an example of a keyless transposition cipher. What does this statement mean? (See Figure 31.8 in the text.)Figure 31.8 2 3. 2 Straight
Assume you want to write a program to simulate the permutation boxes in Figure 31.8 in the text.Figure 31.8a. Show how you represent each box as a table.b. Show the inversion of each box as a table.
The circular shift operation is one of the components of the modern block ciphers.a. Show the result of a 3-bit circular left shift on the word (10011011)2.b. Show the result of a 3-bit circular
Distinguish between a substitution cipher and a transposition cipher.
In a transposition cipher the encryption and decryption keys are often represented as two one-dimension tables (arrays) and the cipher is represented as a piece of software (a program).a. Show the
In a cipher, all As in the plaintext have been changed to Ds in the ciphertext and all Ds in the plaintext have been changed to Hs in the ciphertext. Is this a monoalphabetic or polyalphabetic
The swap operation is one of the components of the modern block ciphers.a. Swap the word (10011011)2.b. Swap the word resulting from part a.c. Compare the results of part a and part b to show that
Which cipher can be broken more easily, monoalphabetic or polyalphabetic?
A very common operation in block ciphers is the XOR operation. Find the results of the following operations. Interpret the results.a. (01001101) ⊕ (01001101)b. (01001101) ⊕ (00000000)
Assume Alice and Bob use an additive cipher in modulo 26 arithmetic. If Eve, the intruder, wants to break the code by trying all possible keys (bruteforce attack), how many keys should she try on
If we have a single integer key in Example 31.1 and 31.2 in the text, how many integer keys do we have in Example 31.3 in the text?
Assume we have a keyless substitution box (S-box) with three inputs (x1, x2, and x3) and two outputs (y1and y2). The relation between the inputs and outputs is defined as follows (⊕ means XOR):y1 =
Assume we have a plaintext of 1000 characters. How many keys do we need to encrypt or decrypt the message in each of the following ciphers?a. Additiveb. Monoalphabeticc. Autokey
Each round in a block cipher should be invertible to make the whole block invertible. Modern block ciphers use two approaches to achieve this. In the first approach, each component is invertible; in
According to the definitions of stream and block ciphers, find which of the following ciphers is a stream cipher.a. Additiveb. Monoalphabeticc. Autokey
In Figure 31.9 we have a swapper in each round. What is the use of this swapper?Figure 31.9 64-bit plaintext DES 32 bits 32 bits 41 Initial permutation K1 Round I 48-bit f( R1. K ) K; Round i 56-bit
A permutation block (P-box) in a modern block cipher has five inputs and five outputs. This is a _____ permutation?a. Straightb. Compressionc. Expansio
In Figure 31.9, we have two straight permutation operations: initial permutation and final permutation. Experts believe these operations are useless and do not help to make the cipher stronger. Can
The key in DES is 56 bits. Assume Eve, the intruder, tries to find the key using a brute-force attack (tries all of the keys one by one). If she can try one million keys (almost 220) in each second
In a modern block cipher, we often need to use a component in the decryption cipher that is the inverse of the component used in the encryption cipher. What is the inverse of each of the following
Assume Bob, using the RSA cryptosystem, selects p = 11, q = 13, and d = 7, which of the following can be the value of public key e?a. 11b. 103c. 19
In RSA, given p = 107, q = 113, e = 13, and d = 3653, encrypt the message “THIS IS TOUGH” using 00 to 26 (A: 00 and space: 26) as the encoding scheme. Decrypt the ciphertext to find the original
In each round of DES, we have all components defined in Figure 31.8 in the text. Which components use a key and which components do not?Figure 31.8 4 2. 3. Straight permutation Compression
A cryptographic hash function needs to be second preimage resistant, which means that given the message M and the message digest d, we should not be able to find any other message, M′, whose digest
In Figure 31.10 in the text, why do we need an expansion P-box? Why can€™t we use a straight or a compression P-box?Figure 31.10 In 32 bits Expansion P-box 48 bits XOR 4 - K; (48 bits) [48 bits
Explain why private-public keys cannot be used in creating a MAC.
Figure 31.9 in the text shows that DES creates 16 different 48-bit keys, one for each round. Why do we need 16 different keys? Why can€™t we use the same key in each round?Figure 31.9 64-bit
The nonce in Figure 31.22 is to prevent a replay of the third message. Eve cannot replay the third message and pretend that it is a new request from Alice, because when Bob receives the response, the
If the one-time pad cipher (Figure 31.12 in the text) is the simplest and most secure cipher, why is it not used all of the time?Figure 31.12 Random sequence bit generator Secure key-exchange channel
Explain why encryption is used in the second message (from Bob to Alice) in Figure 31.23, but signing is done in the third message (from Alice to Bob) in Figure 31.24.Figure 31.23Figure 31.24 Alice
If Alice and Bob need to communicate using asymmetric-key cryptography, how many keys do they need? Who needs to create these keys?
Figure 31.22 shows a unidirectional authentication that authenticates Alice for Bob. Change this figure to provide bidirectional authentication: to authenticate Alice for Bob and then Bob for Alice.
Why do you think asymmetric-key cryptography is used only with small messages.
Change Figure 31.23 to allow bidirectional authentication. Alice needs to be authenticated for Bob and Bob for Alice.
In an asymmetric public key cipher, which key is used for encryption? Which key is used for decryption?a. Public keyb. Private key
Change Figure 31.24 to allow bidirectional authentication. Alice needs to be authenticated for Bob and Bob for Alice.
In RSA, why can’t Bob choose 1 as the public key e?
You may have noticed that there is a flaw in Figure 31.26. Eve, the intruder, can replay the third message and, if she can somehow get access to the session key, can pretend to be Alice and exchange
What is the role of the secret key added to the hash function in Figure 31.17 in the text (MAC)? Explain.Figure 31.17 Figure 31.17 Message authentication code Alice Bob M: Message K: A shared secret
Distinguish message authentication and entity authentication.
To understand the concept of secret-key distribution, assume a small private club has only 100 members (excluding the president). Answer the following questions:a. How many secret keys are needed if
Alice signs the message she sends to Bob to prove that she is the sender of the message. Which of the following keys does Alice need to use?a. Alice’s public keyb. Alice’s private key
Alice needs to send a message to a group of fifty people. If Alice needs to use message authentication, which of the following schemes do you recommend?a. MACb. Digital signature
Which of the following services are not provided by digital signature?a. Message authenticationb. Confidentialityc. Nonrepudiation
Assume Alice needs to send a confidential signed document to 100 people. How many keys does Alice need to use to prepare 100 copies if she uses asymmetric-key confidentiality? Explain.
In a club with 50 members, how many secret keys are needed to allow secret messages to be exchanged between any pair of members?
A key distribution center (KDC) is designed to solve the problem of distributing __________ keys.a. Secretb. Publicc. Private
A certification authority (CA) is designed to solve the problem of distributing __________ keys.a. Secretb. Publicc. Private
Why does IPSec need a security association?
Host A and host B use IPSec in the transport mode. Can we say that the two hosts need to create a virtual connection-oriented service between them? Explain.
How does IPSec create a set of security parameters?
When we talk about authentication in IPSec, do we mean message authentication or entity authentication? Explain.
If Alice and Bob are continuously sending messages to each other, can they create a security association once and use it for every packet exchanged? Explain.
What does AH add to the IP packet?
Can we use SSL with UDP? Explain.
What does ESP add to the IP packet?
Why is there no need for a Security Association with SSL?
Are both AH and ESP needed for IP security? Why or why not?
Compare and contrast PGP and S/MIME. What are the advantages and disadvantages of each?
What are the two protocols discussed in this chapter that provide security at the transport layer?
Should the handshaking in SSL occur before or after the three-way handshaking in TCP? Can they be combined? Explain.
What is IKE? What is its role in IPSec?
We defined two security services for e-mail (PGP and S/MIME). Explain why e-mail applications cannot use the services of SSL/TLS and need to use either PGP or S/MIME.

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