CS 772
Network and Systems Security
Fall 2013
Midterm Exam
Time 2 & 1/2 hours
Open Book & Notes
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Each Question is 10 points
2
Question 1:
Which of the following methods of encrypting plain English text is easy to break using
statistical analysis:
1. Generalized Caesar cipher.
2. Mono alphabetic cipher
3. XOR the text with a repeated Key of one character
4. Homophonic Substitution cipher
5. Rail Fence cipher
6. Vignere cipher
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Question 2:
A. The following cipher message was produced using the Rail Fence algorithm with 2 rails:
“cioeytar gp”
What is the corresponding plain text?
B. Use the Rail Fence algorithm with 3 rails to encode the following plain message
"old dominion university"
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Question 3:
Use the Vignere Square algorithm to encode the message:
“old dominion university”
using the key “oducsc”
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Question 4:
A. In DES assume the input data block is:
FF 81 81 81 81 81 81 FF
What is the value of IP?
B. In DES assume that both the input data block and the encryption key are all 1s.
What is the output of the Mangler function of the first round?
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Question 5:
If both the input data block and the encryption key are all 0s,
which of the following encryption algorithms produces all zeros cipher block?
1. DES
2. IDEA
3. AES
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Question 6:
All block chaining techniques generates a random block called IV which is sent with the cipher
blocks to the receiver.
Assume Trudy replaces the IV with another IV and did not change the transmitted cipher blocks.
How many plain text blocks get garbled at the receiver in each of the following?
1. Cipher Block Chaining (CBC)
2. Output Feedback Mode (OFB)
3. Cipher Feedback Mode (CFB)
4. Counter Mode (CTR)
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Question 7:
A. Assume we are using EDE with the 3 keys: K1 and K2, K3.
Evaluate the cryptographic strength of following schemes:
1. Using 1 keys:
m>>>> E >>>> D >>>> E >>>>c
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K1
K1
K1
2. Using 2 keys:
m>>>> E >>>> D >>>> E >>>>c
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K1
K2
K2
3. Using 2 keys:
m>>>> E >>>> D >>>> E >>>>c
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K1
K2
K1
4. Using 3 keys:
m>>>> E >>>> D >>>> E >>>>c
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K1
K2
K3
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Question 8:
In submitting Assignment #1 you have used the following 6 keys.
Briefly explain the need for each of these keys:
1. Your own Public Key
2. Your own Private Key
3. A chosen symmetric password P
4. The grader Public Key
5. The grader Private Key
6. The grader password (oducsc)
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Question 9:
The following are the basic ingredients used for almost all the symmetric cryptographic
algorithms:
XOR, Substitutions, Permutations & Rounds.
Specify for each of the following Algorithms which ingredients are NOT used:
1. DES
2. IDEA
3. AES
4. RC4
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Question 10:
Show the result of the 1st round of AES-128, assume:
The bits of the encryption key are all 0s and the input block octets are all 7d.
Show your final answer to HEX.
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