CS 472
Network and Systems Security
Fall 2007
Midterm Exam
Time 2 & 1/2 hours
Open Book & Notes
Name:
Unix Login:
Each Question is 10 points
2
Question 1:
Dr. Wahab uses the crypto program to encrypt all his files. Not only that but he also use the same
key for encrypting all his files!
Assume that you got hold of one of his plain files and the corresponding cipher file.
Describe how you can find his key and read all his encrypted files?
2
3
Question 2:
Assume that you have the following <plaintext, ciphertext> pairs:
<wahab,xbibc > and <zeil, afjm>.
Can you guess the cipher method used in this encryption?
3
4
Question 3:
The traditional combination lock, T, requires three turns between 1 and 40.
Assume that you have the option of buying new combination locks, N that requires six turns
between numbers 0 and 9.
Which lock (T or N) is the more secure? Explain?
4
5
Question 4:
How many bits are required to store the mapping of 6-bit blocks to other arbitrary 6-bit blocks?
5
6
Question 5:
Assume that Bob and Alice share a secret K. Describe how Alice can authenticate Bob using:
 Secreted Key Cryptography.
 Message Digests Algorithms.
6
7
Question 6:
Describe how UNIX authenticates its users in order to login.
7
8
Question 7:
Consider In the following openssl commands:
% openssl genrsa
-out rsaprivatekey.pem
-des3 1024
What is the option –des3 is used for ?
% openssl rsautl -encrypt -pubin -inkey rsapublickey.pem -in file1 -out file2
What is maximum length of file1?
8
9
Question 8:
A. Show the result, in HEX, of the first 4 octets of the 1st round of DES to encrypt one data
block of all 1s using a key of all 1s.
B. Consider Figure 3-8 (Chunk Transformation).
Assume the 2nd chunk of R is 111000 and the 2nd chunk of K is 100011.
What is the output of the S-Box corresponding for these chunks?
9
10
Question 9:
In the following table, fill in the specified attributes of : DES, IDEA and AEAS-128.
DES
IDEA
AES-128
Block Size
Key Size
Number of Rounds
Number of Round Keys
Round Keys size
10
11
Question 10:
In AES-128: What the result of applying MixColumn to a column:
ff
00
ff
00
11