CS 772/872
Network and Systems Security
Fall 2006
Midterm Exam
Time 2 & 1/2 hours
Open Book & Notes
Name:
Login:
Each Question is 10 points
Question 1:
Dr. Wahab uses the crypto program to encrypt all his files. Not only that but he 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?
Question 2:
Briefly describe how UNIX implements:
1. Authentication.
2. Authorization.
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) and (C) such that:
(N) Requires seven turns between numbers 0 and 9.
2
(C) Requires five turns between charters A and Z.
Which lock (T, N or C) is more secure? Explain?
Question 4:
Assume we have the following encryption scheme:
(C) Caesar-based cipher: Divide the string into 8 bits long blocks and replace each block with
another block K positions away with wrap around.
(M) Monoalphabetic-based cipher: Arbitrary map one 8 bits block with another block.
Which encryption scheme (C or M) is more secure? Explain?
Question 5:
In this question, we will use the following notations:








P and V are the Public and its corresponding priVate keys.
K is a symmetric Key.
M is a Message and C is the corresponding cipher
K {M} : message M is encrypted with K.
K [M] : message M is decrypted with K.
{M}P : message M is encrypted/verified with P.
[M]V : message M is decrypted/signed with V.
H(M): the hash of message M.
Assume that Bob and Alice agree on a shared secret K and eA / dA are the public/private key
pair of Alice.
Bob may authenticate himself to Alice using any of the following methods:
1. Bob sends Alice: C = K{K}
2. Bob sends Alice: C= {K }eA
3. Bob sends Alice: C = H(K)
In each method, describe what Alice should do when she receives C in order to authenticate Bob.
1.
2.
3.
Question 6:
2
3
Assume that Bob and Alice agree on a shared secret K, explain how to use a message digest
algorithm to encrypt a message M sent from Bob to Alice.
Question 7:
A. In the following:
% openssl genrsa -out rsaprivatekey.pem
-des3
1024
1. What is 1024 represents?
2. What –des3 is used for ?
B. In the following:
% openssl rsautl -encrypt -pubin -inkey rsapublickey.pem -in file1 -out file2
1. What is maximum length of file1?
2. How to convert file2 to base64?
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. Show the result, in HEX, of the first 4 octets of 1st round of IDEA to:
Encrypt one data block
of all 1s with a key of all 1s.
Question 9:
Show the result, in HEX, of the first four octets of the 1st round of AES-128 to:
Encrypt one data block of all 1s with a key of all 1s.
Question 10:
.Let M be a 16 octet message of all 1s.
1. What is the value, in HEX, of the first octet of the message checksum of MD2?
2. What is the value, in HEX, of the first octet of the 0th pass of the MD2 digest?
3