Reference no: EM13657173

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?