CS 472
Network and Systems ecurity
Fall 2005
Midterm Exam
Time 2 & 1/2 hours
Open Book & Notes
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1. A typical combination lock (A) requires three turns between 1 and 40.
Assume that we have a special combination lock (B) that requires five
turns between 1 and 10.
Which lock (A or B) is more secure? Explain?
2. Why it is the norm to publish the encryption algorithms and do not
keep them secret?
3. Why AES is more secure than DES?
4. Assume we have the following encryption algorithm:
Caesar-based cipher: Divide the string into 4 bit long blocks and
replace each block with another block K positions away with wrap
around.
If you have a cipher text C, how many trials are needed (in the worst
case) to find the plain text
5. Repeat the above question if we use the following encryption
algorithm:
Monoalphabetic-based cipher: arbitrary map one 4 bit block
with another block.
6. Describe how Bob can authenticate himself to Alice by sending her
only one message using:
1. Secrete Key Cryptography.
2. Public Key Cryptography.
3. A Message Digest Algorithm.
7. Explain why UNIX requires its users to authenticate themselves by
sending their password in plain text?
8. Explain how to use a MD algorithm as a symmetric encryption
algorithm?
9. Dr. Wahab used the simple crypt program of assignment #1 to encrypt
his grade sheet for this class. Assume he keeps the grade sheet sorted
alphabetically by last name and he uses a 4-character key. Describe
how some of you may be able to decipher the grade sheet.
10.In Generating RSA keys we use the command:
> openssl genrsa -out rsaprivatekey.pem -des3 1024
What is 1024?
What is –des3?
11.In DES, assume the we have the following data block:
FF00 0000 0000 0000
What is the result of the initial permutation in HEX?
12.In DES, assume the key is: FF00 0000 0000 0000
what is the value of C0 in binary.
13.In DES, assume that input to one round is all 0s and the key of that
round is all 1s.
What is the value of the first 40 bits (in HEX) of the output of that
round?
14.In AES-128: What the result of applying MixColumn to that column
of all 1s?
15.Consider the MD2 Algorithm. Let M be a 15 octet message of all
ones. What is the value of the first octet of the message checksum?