Question
Consider a simple symmetric encryption algorithm as follows:
1- Input text T and 64 bit key K
2- Divide T into 64 bit size blocks B1...Bn
3- Get first block B1 and perform bit-wise XOR with K to
produce encrypted block E1
4- Repeat
a. Get next block Bi and perform bit-wise XOR with Ei-1
to produce encrypted block Ei
5- Until End of Text
The algorithm is very simple, and naturally, we do not expect the ciphertext to be cryptographically strong. Given the above algorithm answer the following:
i. Is it a problem if the first block of input happens to be the same as the key? Explain why?
ii. Modify the algorithm to rectify the above problem.
Question
User A and B use Diffie-Hellman algorithm to exchange a shared key and generate
public keys of their own. Consider a common prime number q=71 and α (a primitive root of q)=7. Determine the followings:
i. If user A has private key=5, what is A's public key?
ii. If user B has private key=12, what is B's public key?
iii. What is the shared key?
Show the full working process of your work in at least three steps in both
encryption and decryption.
Question
Identify and discuss one of the challenges of security and three vulnerabilities associated with the use of a wireless network in the following scenario:
i. A private or home-based wireless network.
ii. A public wireless network.
Question
Alice and Bob agree to communicate privately via email using a scheme based on RC4, but they want to avoid using a new secret key for each transmission. Alice and Bob privately agree on a 128-bit key k. To encrypt a message m consisting of a string
of bits, the following procedure is used:
i. Choose a random 80-bit value v
ii. Generate the ciphertext c=RC4(v?k)⊕?m
iii. Send the bit string (v?c)
Where (a?b) is the concatenation of a and b bits; and
a⊕?b= a XOR b;
Answer the following:
a. Suppose Alice uses this procedure to send a message to Bob. Describe how Bob can recover the message m from (v?c) using k.
b. If an adversary observes several values of (v1?c1), (v2?c2), ... transmitted between Alice and Bob, how can he/she determine when the same key stream has been used to encrypt two messages?
c. If Alice and Bob agree to use 16-bit Cipher Feedback (CFB) mode instead of RC4, and a bit error occurs in the transmission of a ciphertext; how far does
the error propagate?