CNT 4704: Computer Communication Networks
Fall 2010
Homework 5
(Assigned 11/23; due: 12/04 , no late submission, submitted via webcourse)
Chapter 8:
1. Suppose N people want to communicate with each of N-1 other
people using symmetric key incryption. All communication between any
two people, i and j, is visible to all other people in this group
of N, and no other people in this group should be able to decode their
comunication. How many keys are required in this system as a whole? Now
suppose that public key encryption is used. How mnay keys are required
in this case?
2. Using the Cesear cipher introduced in lecture, decode the
message "JU'T UJNF OPX.". Also show what is the shifting key used
here? Using the same shifting key, encode message "This is an easy
problem."
3. Using RSA algorithm, choose p=3 and q=11, show what are the
public key and private key? Then encode "hello". Apply decryption
algorithm to the encrypted text to recover the original message. You
must show all steps in encoding and decoding process, just like the
lecture note "Chapter8-part1.ppt", page 27.
4. True or false questions. If a statement is false, you must
explain the reason why it is false:
a). Suppose that TCP is being run over IPsec with
the AH protocol. If TCP retransmits the same packet, then the two
packets will have the same sequence number in the AH header.
b). Suppose certifier.com creates a certificate for
foo.com. Typically the entire certificate would be encrypted with
certifier.com's public key.
c). Consider encrypting a large file with Cipher
Block Chaining. With this mechanism, the source sends an Initialization
Vector (IV) and the secret key in cleartext to the receiver.
d). In the Internet web public key infrastructure, a
client browser needs to contact the CA in order to get the certificate
for a target web server.
e). Firewall can block outside attacks to internal
computers, but it cannot stop internal infected computer to attack
outside computers.