Question 1: Alice and bob agree to communicate privately via e-mail 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 128-bit key k. To encrypt a message m consisting of a string of bits, the following procedure is used.
1. Choose a random 80- bit value v
2. Generate the ciphertext c = RC4(v||k)⊕m
3. Send the bit string(v||c)
a. Suppose Alice uses this procedure to send a message m to Bob. Describe how Bob can recover the message m from (v ll c) using k.
b. If an adversary observes several values (V1 ||C1), (V2, C2).......transmitted be-tween Alice and Bob. how can he or she determine when the same key stream has been used to encrypt two messages?
Question 2: With the ECB mode. if there is an error in a block of the transmitted ciphertext, only the corresponding plaintext block is affected. However, in the CBC mode, this error propagates. For example. an error in the transmitted C1 (Figure 2.9) obviously cor¬rupts P1 and P2.
a. Are any blocks beyond P2 affected?
b. Suppose that there is a bit error in the source version of P1. Through how many ciphertext blocks is this error propagated? What is the effect at the receiver?