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IST Network Security Class Homework Answer
Name: _______________________
1. For each question (5 points), there are multiple choices, but only one is correct
1.1 When a user receives an email sent by the Melissa virus
(a) Even if the user does not open the attachment, the user’s computer will still be
affected by the virus when the email is opened.
(b) The attachment contains only MS WORD macro code, so the attachment is not a
readable when you open it.
(c) If the user does not open the attachment, the user’s computer will not be affected by
the virus.
(d) The attachment is still readable, but when the attachment is opened, the malicious
macro code included in the attachment will not be executed. The macro code will be
executed only when the attachment is saved onto the local disk.
Answer: (c)
1.2 The Internet worm uses one of the following methods to establish a shell on a remote
computer except
(a) rsh
(b) fingerd
(c) sendmail
(d) pingd
Answer: (d)
1.3 When the attacker uses a faked source IP address to send an attacking packet to a
target, the corresponding attack is a_____
(a) scan attack
(b) sniffer attack
(c) spoofing attack
(d) Trojan horse
(e) buffer overflow attack
Answer: (c)
1.4 To establish a listening post, which of the following tools is useful?
(a) remote exploit tools
(b) local exploit tools
(c) stealth tools
(d) backdoor tools
Answer: (d)
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1.5 The key length of RSA is____
(a) 56 bits
(b) 64 bits
(c) 512 bits
(d) 1024 bits
(e) determined by the user
Answer: (e)
1.6 The Diffie-Hellman key exchange protocols needs ___ messages to finish key
exchange.
(a) 1
(b) 2
(c) 3
(d) 4
(e) 5
Answer: (b)
2. Please say TRUE or FALSE to each of the following statements (5 points each)
2.1 ______When an Internet DoS attack is enforced, the real attacker will send a message
to each daemon to instruct the daemon to send packets to the victim.
Answer: False, since the real attacker will ask the master to send messages to daemons.
2.2 ______A Melissa virus can make a mail server unable to send out emails because the
virus corrupts the code of the mail server.
Answer: False, since the correct reason should be denial-of-service due to too many
emails in or out the mail server.
2.3 ______In known plaintext cryptanalysis, the attacker knows the plaintext of any
ciphertext he or she is interested.
Answer: False, since in known plaintext cryptanalysis, the attacker knows the ciphertext
of any plaintext he or she is interested.
2.4 ______Both RSA and DES are a block cipher.
Answer: True
2.5 ______When Vemam one-time pad is used to achieve perfect secrecy, the key must
be as long as the plaintext.
Answer: True
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2.6 ______ Double DES is two times as secure as single DES.
Answer:
(1) If the security is measured by the number of trails needed to break DES, the
answer is True.
(2) If the security is measured by the effective key length, the answer is False, since
the strength of Double DES is equivalent to a 57 bit key instead of 112 bits.
Grading Note: Since the question is not clear, 5 points will be given no matter the
answer is True of False.
2.7 ______Since the attacker can easily figure out Bob’s private key using Bob’s public
key, so when Alice wants to use public key cryptograph to communicate with Bob
securely, Alice needs to keep Bob’s public key confidential.
Answer: False
2.8 ______ The only way to authenticate the sender of a message is to use digital
signatures.
Answer: False, since secret key cryptography can also be used to authenticate the
sender of a message. Please refer to crypto-2.pdf, slide 35.
Grading Note: Since we did not discuss this point in detail, 5 points will be given no
matter the answer is True of False.
3. When letter substitution is used to do encryption: (10points)
3.1 If the plaintext is “west”, and the key is {wx, eu, sc, ti}, what is the
ciphertext?
Answer: xuci
3.2 If the plaintext is “west”, and the cybertext is “gord”, what is the key?
Answer: the key is {wg, eo, sr, td}
4. When KDC and symmetric cryptograph are used to do key change, the key
change can be done by three messages. What are the three messages? (10 points)
Answer: Please refer to net-security-1.pdf, slides 4 and 5.
5. Please show how DES can be used to generate message digests. (10 points)
Answer: Please refer to crypto-2.pdf, slides 39 and 40.