Questions and Answers of Computer Network

Consider the adaptive playout strategy described in Section 7.3. a. How can two successive packets received at the destination have timestamps that differ by more than 20 msecs when the two packets
What are the differences between message confidentiality and message integrity? Can you have confidentiality without integrity? Can you have integrity without confidentiality? Justify your answer.
Can you "decrypt" a hash of a message to get the original message? Explain your answer.
Consider a variation of the MAC algorithm (Figure 8.9) where the sender sends (m, H(m) + s), where H(m) + s is the concatenation of H(m) and s. Is this variation flawed? Why or why not?
What does it mean for a signed document to be verifiable and non-forgeable?
In what way does the public-key encrypted message hash provide a better digital signature than the public-key encrypted message?
Suppose Alice has a message that she is ready to seod to anyone who asks. Thousands of people want to obtain Alice's message, but each wants to be sure of the integrity of the message. In this
What is the man-in-the-middle attack? Can this attack occur when symmetric keys are used?
Suppose Bob initiates a TCP connection to Trudy who is pretending to be Alice. During the handshake, Trudy sends Bob Alice's certificate. In what step of the SSL handshake algorithm will Bob discover
From a service perspective, what is an important difference between a symmetric-key system and a public-key system?
Why must an application gateway work in conjunction with a router filter to be effective?
Suppose N people want to communicate with each of N - I other people using symmetric key encryption. All communication between any two people, i and j, is visible to all other people in this group of
Suppose n = 10,000, a = 10,023, and b = 10,004. Use an identity of modular arithmetic to calculate in your head (a • b) mod n.
In what way does a hash provide a better message integrity check than a checksum (such as the Internet checksum)?
Suppose Alice wants to communicate with Bob using symmetric key cryptography using a session key KS. In Section 8.2, we learned how public-key cryptography can be used to distribute the session key
Compute a third message, different from the two messages in Figure 8.8, that has the same checksum as the messages in Figure 8.8.
Suppose Alice and Bob share two secret keys: an authentication key S1 and a symmetric encryption key S2 Augment Figure 8.9 so that both integrity and confidentiality are provided.
In the BitTorrent P2P file distribution protocol (see Chapter 2), the seed breaks the file into blocks, and the peers redistribute the blocks to each other. Without any protection, an attacker can
The OSPF routing protocol uses a MAC rather than digital signatures to provide message integrity. Why do you think a MAC was chosen over digital signatures?
Consider our authentication protocol in Figure 8.16 in which Alice authenticates herself to Bob, which we saw works well (i.e., we found no flaws in it). Now suppose that while Alice is
In the man-in-the-middle attack in Figure 8.19, Alice has not authenticated Bob. If Alice were to require Bob to authenticate himself using the publickey authentication protocol, would the
Figure 8.20 shows the operations that Alice must perform with PGP to provide confidentiality, authentication, and integrity. Diagram the corresponding perations that Bob must perform on the package
Suppose Alice wants to send an e-mail to Bob. Bob has a public-private key pair (K+B,K-B)' and Alice has Bob's certificate. But Alice does not have a public, private key pair. Alice and Bob (and the
Consider the Wircshark output below for a portion of an SSL session.a. Is Wireshark packet 112 sent by the client or server?b. What is the server's IP address and port number?c. Assuming no loss and
Show that Trudy's known-plaintext attack, in which she knows the (ciphertext, plaintext) translation pairs for seven letters, reduces the number of possible substitutions to be checked in the example
In Section 8.5.1. it is shown that without sequence numbers. Trudy (a woman-in-the middle) can wreak havoc in an SSL session by interchanging TCP segments. Can Tmdy do something similar by deleting a
The following True/False questions pertain to Figure 8.29. a. When a host in 172.16.1124 sends a datagram to an Amazon.com server, the router R I will encrypt the datagram using IPsec. b. When a host
Consider the example in Figure 8.29. Suppose Trudy is a woman-in-themiddle, who can insert datagrams into the stream of datagrams going from R1 and R2. As part of a replay attack. Trudy sends a
Consider the following pseudo-WEP protocol. The key is 4 bits and the IV is 2 bits . The IV is appended to the end of the key when generating the keystream. Suppose that the shared secret key is 10
Provide a filter table and a connection table for a stateful firewall that is restrictive as possible but accomplishes the following:a. Allows all internal users to establish Telnet sessions with
Suppose Alice wants to visit the Web site activist.com using TOR-like service. This service uses two non-colluding proxy servers Proxy I and Proxy2. Alice first obtains the certificates (each
Consider the polyalphabetic system shown in Figure 8.4. Will a chosenplaintext attack that is able to get the plaintext encoding of the message "The quick brown fox jumps over the lazy dog." be
Consider the block cipher in Figure 8.5. Suppose that each block cipher T; simply reverses the order of the eight input bits (so that, for example, 11110000 becomes 00001111). Further suppose that
Consider the block cipher in Figure 8.5. For a given "key" Alice and Bob would need to keep eight tables, each 8 bits by 8 bits. For Alice (or Bob) to store all eight tables, how many bits of storage
Consider the 3-bit block cipher in Table 8.1. Suppose the plaintext is 100100100. (a) Initially assume that CBC is not used . What is the resulting ciphertext? (b) Suppose Trudy sniffs the
(a) Using RSA, choose p = 3 and q = 11, and encode the word "dog" by encrypting each letter separately. Apply the decryption algorithm to the encrypted version to recover the original plaintext
Consider RSA with p =5 and q = II. a. What are nand z. b. Let e be 3. Why is this an acceptable choice for e? c. Find d such that de = I (mod z) and d < 160. d. Encrypt the message m = 8 using the
In this problem, we explore the Diffie-Hellman (DH) public-key encryption algorithm, which allows two entities to agree on a shared key. The DH algorithm makes use of a large prime number p and
Why would a network manager benefit from having network management tools? Describe five scenarios.
What is the role of ASN.1 in the ISO/OSI reference model 's presentation layer?
What is the difference between network management and service management?
Define the following terms: managing entity, managed device, management agent, MIB, network management protocol.
What is an important difference between a request-response message and a trap message in SNMP?
Consider the two ways in which communication occurs between a managing entity and a managed device: request-response mode and trapping. What are the pros and cons of these two approaches, in terms of
In Section 9.3 we saw that it was preferable to transport SNMP messages in unreliable UDP datagrams. Why do you think the designers of SNMP chose UDP rather than TCP as the transport protocol of

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