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CS 472
Network and Systems Security
Fall 2009
Midterm Exam
Time 2 & 1/2 hours
Open Book & Notes
Name:
Unix
Login:
Each Question is 10 points
Question 1:
The crypto program of assignment #1 is used to encrypt files.
The syntax of this program is:
% crypto key plainfile
cipherfile.
Assume you got hold of a cipherfile
and find out that
the content of the file is all 0s.
How you interpret this?
Key
and Plainfile are identical
Question 2:
XOR is a
very important ingredient in symmetric encryption techniques.
Why is
that?
It
is Reversible: K (+) P = C à K (+) C = P
Give two examples
of symmetric encryption techniques that use XOR and refer to the specific steps of these techniques
where the XOR is actually used.
DES:
Inside Rounds: Left (+)
Righ (after Mangler
Function)
Inside Mangler
Function: Input Date (+) Round Key chunks.
AES:
Mix column step.
Between Rounds: Input Data after Mix Column
(+) Round Key.
Inside
Key Expansion steps.
Question 3:
The traditional
combination lock, T, requires three turns between 1 and 40.
Assume that you have
the option of buying new weaker combination locks, W that requires two turns
between numbers 0 and 9.
W is weaker than T by X%.
What is the value of
X?
T
40.40.40 = 64000
W
10.10 = 100
W is weaker that T by 640 %
Question
4:
Almost all
encryption algorithms use the concepts of substitution..
How many bits are required
to store the substitution table to map 8-bit blocks to other arbitrary 8-bit
blocks?
2^8 * 8 = 256 * 8 = 2048 bits.
Question
5:
Assume that Bob and Alice share a secret K . (e.g., by meeting at
Starbucks in Chicago J)
Describe two methods by which Alice can authenticate Bob
using the shared secret K.
Alice authenticating Bob:
Alice
Bob
challenge:
r
>>>>>>> r
response:
K{r}
<<<<<<< K{r}
r is a random number,
K{r} is the secret key
encryption of r using shared key K.
Alice
authenticating Bob:
Alice
Bob
challenge:
r
>>>>>>> r
response:
d <<<<<<< d
=MD{K|r}
r
is a random number,
D{K|r} is the message
digest of K concatenated with r.
Alice computes MD{K|r}
and if = d, then Bob must know K.
Question
6:
UNIX stores the
hash of a random salt and the user password.
What is the security value of this salt?
To
prevent dictionary attach.
Question 7:
Consider In the following openssl commands:
% openssl genrsa -out
rsaprivatekey.pem -des3
1024
What is the meaning of the option
–des3 ?
Encryption
used to save the private password.
What is the value of
1024 represents?
Lenght of the private key
When using this command
in class, what is the specific password that Dr. wahab have used?
*****
% openssl rsautl
-encrypt -pubin -inkey rsapublickey.pem -in file1 -out file2
What is the size limit imposed on file1 and why?
< 1024
Since
it does not use symmetric key encryption.
Question 8:
Show the result, in HEX, of the 1st
octet out of the S-Boxes of the 1st round of DES to encrypt
one data block of all 0s using a key
of all 0s.
EF
Question 9:
Show the result, in HEX, of the 1st octet of
the 1st round of AES-128 to encrypt
one data block of all 1s using a key
of all 0s.
16
Question 10:
Here
is a description of a Silly Encryption
Algorithm (SEA):
· The algorithm has a single round.
· The input data block size and the encryption key lengths are one octet.
· Just XOR the input data block with the encryption key.
Use the SEA
algorithm to encrypt the message: “F0F0” using the CBC chaining.
Assume the IV is “FF” and the encryption key is “FF”.
F0FF