21/1 2.11 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 a 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|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?