1. Consider the following key exchange protocol which is a variant of the Diffie-Hellman protocol. Each user P has a secret key sk_P=x_P and public key pk= gx_P mod p.
1: A B: gr_A mod p
2: B A: gr_B mod p
In the protocol, r_A and r_B are randomly chosen in each session by A and B. The shared key for communication is defined as K = gr_Ax_B + gr_Bx_A mod p.
a) Show the key derivation formulas of User A and User B (i.e., how does each
user compute the shared key?)
c) Does the protocol provide Forward Secrecy? Justify your answer.
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2. Two questions related to the A1. For each question, you should use one sentence to highly summarize the problem (1 mark) and then use 1-3 sentences to explain it.
a)Briefly describe the issue when the adversary is able to know the secret key sk_s.
b)Briefly describe the problem when Alice’s algorithm always uses the same r for secure communications.
3. Consider the following certificate mechanism.
The CA generates the signature but the message to be signed excludes “Refer to Key’s Algorithm”. Describe how the adversary can launch the MITM attack on TLS protocol in the real world. You should start with the application scenario that user Alice wants to access the web resources manager by the web server Bob. You should use at least three steps to highly describe this kind of attack.
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