Network Security
Problem Set 1
To be submitted until Dec. 18 by email to reuvenaviv@gmail.com
Question 1.
Consider the Cipher Block Chaining (CBC) mode of encryption. Suppose that during
transmission the transmitted block C1 got an error (say, some bits were inverted). None of the
other transmitted blocks had errors.
(i)
(ii)
Show that the decryption at the receiver of blocks P1 and P2 will be incorrect.
Will the decryption of block P5 be correct or not?. Explain
Question 2:
(i)
Learn about One Time Pad encryption method. How does it work? What are its strengths
and weaknesses?
(ii)
Download the program One Time Pad (I found version 2.3). How it works?
(iii)
Learn how to use it. Generate a key from the character Z, and use it to encrypt the letter
A. What is the result?
(iv)
Repeat previous part three times – in all cases using same Z to create key, and encrypting
A. The results are not the same. Why not?
Question 3:
In part 1A we construct a 3 letters (3 bytes) message, X, and then construct a secret Message
Authentication Code, that are sent together. In part 1B we work as an attacker who does not know
the secret, that captured the sent message: we replace the message with a false message Y that
has the same secret Message Authentication code as the original message.
1A.
Here we work as a sender
(i)
The message is (in Hex): X = 59 49 53. What is the textual representation of this
message (3 letters)?
(ii)
Write down the binary representation of the message. Explain
(iii)
Create a one byte hash of this message, H(X), using the Simple Hash (bitwise XOR)
discussed in class. Explain what you do.
(iv)
Create your 1 byte secret key, K: this is the binary code of the second letter of your
family name. Write this key in Hex and in binary forms.
(v)
Create a Message Authentication Code MAC(X) by bitwise XOR of the hash with your
secret key.
The sent message is X|MAC(X). Write this in Hex form
1B.
Now we work as an attacker
(vi)
(vii)
(viii)
(ix)
Create the new first two bytes of the false message: Y’ = 4E 4F. What is the textual
representation of this message (2 letters)?
Calculate the one byte hash of this H(Y’), using the same Simple Hash (bitwise XOR).
Write this in Hex.
Calculate a third byte for the false message so that the hash of the false message H(Y)
(where Y is the concatenation of the first two bytes Y’ and the third byte) is equal to the
hash of the original message: H(Y) = H(X). Write down the third byte in Hex. Explain
what you have done.
Write down the original message and the false message in textual forms (3 letters each).
Explain why the Message Authentication Code of Y is now equal to the message
authentication code of X: MAC(Y) = MAC(X).