CS 772
Network and Systems Security
Fall 2013
Midterm Exam
Time 2 & 1/2 hours
Open Book & Notes
Name:
Login: wahab
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. Easy
2. Mono alphabetic cipher Easy
3. XOR the text with a repeated Key of one character No
4. Homophonic Substitution cipher No
5. Rail Fence cipher No
6. Vignere cipher No
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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?
Since it is 2, divide string int0 2:
cioeyt
ar gp
the answer is : cairo egypt
B. Use the Rail Fence algorithm with 3 rails to encode the following plain message
"old dominion university"
odn vi | l oiinuiest | dmnonry
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Question 3:
Use the Vignere Square algorithm to encode the message:
“old dominion university”
using the key “oducsc”
Message=
Key=
Cipher=
old dominion university
oducstvwxyz abefghijkl
boxbvggdjfmm vrnalzasdj
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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?
81 81 81 FF FF 81 81 81
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?
It is the 000000 of each of the 8 S-Box:
E FA 72 C4 D
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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 no
2. IDEA all zeroa
3. AES no
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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) First
2. Output Feedback Mode (OFB) ALL
3. Cipher Feedback Mode (CFB) First
4. Counter Mode (CTR) ALL
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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
|
|
|
K1
K1
K1
weak
2. Using 2 keys:
m>>>> E >>>> D >>>> E >>>>c
|
|
|
K1
K2
K2
weak
3. Using 2 keys:
m>>>> E >>>> D >>>> E >>>>c
|
|
|
K1
K2
K1
strong
4. Using 3 keys:
m>>>> E >>>> D >>>> E >>>>c
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|
|
K1
K2
K3
strong
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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: to verify my signature
2. Your own Private Key: to sign
3. A chosen symmetric password P: to encrypt using DES
4. The grader Public Key: to encrypt P
5. The grader Private Key: to get the value of P
6. The grader password (oducsc): to be able to use the grader private key to get P
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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 ALL
2. IDEA XOR & ROUNDS
3. AES ALL
4. RC4 XOR
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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.
7d  ff
Ff  e5
ff
ff
1a
XOR columns produce
ff
ff
ff
ff
so the answer is ALL 1s
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