CPE 401/601 Computer Communication Networks

Spring 2010

Homework 2 : Web and Security

Due on Friday, May 7 at 4:00 pm

Part 1: Complete the prelab on SNMP.

Part 2: Answer the following questions.

1 : What is an important difference between a request-response message and a trap message in SNMP?

2 : Why does HTTP run over TCP instead of UDP?

3 : Consider an HTTP client that wants to retrieve a Web document at a given URL. The IP address of the HTTP server is initially unknown. What transport and aplication layer protocols are needed in this scenario?

4 : Using the monoalphabetic cipher in slide 6 of Lecture 23, encode the message "This is an easy problem". Decode the message "rmi jku uamu xyj".

5 : What are the differences between message confidentiality and message integrity? Can you have one without the other? Justify your answer.

6 : Show that Trudy's known-plaintext attack, in which she knows the (ciphertext, plaintext) translation for seven letters, reduces the number of possible substitutions to be checked by approximately 10^9.

7 : Consider an 8-bit block cipher. How many possible input blocks does this cipher have? How many mappings are there? If each mapping is considered as a key, how many possible keys does this cipher have?

8 : Consider the block cipher in slide 16 of Lecture 23. For a given "key" Alice and Bob would need to keep eight tables, each 8 bits by 8 bits. For Alice and Bob to storae all eight tables, how many bits of storage are necessary? How does this number compare with the number of bits required for a full table 64-bit block cipher?

9 : From a service perspective, what are the main differences between symmetric key and public key crypto systems?

10 : Consider RSA with p=5 and q=11.

a - What are n and z values?
b - Is e=3 an acceptable value, why?
c - Find d given e=3.
d - Encrypt message 8 using (n,e) values.

Submitting your files

Submission of your homework is via WebCT. You must submit all the required files in a single tar or zip file containing all the files for your submission.