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 7 k) m 3. Send the bit string (v 7 c) a. Suppose Alice uses this procedure to send a message m to Bob. Describe how Bob can recover the message m from (v 7 c) using k. b. If an adversary observes several values (v1 7 c1), (v2 7 c2), . . . transmitted between Alice and Bob, how can he/she determine when the same key stream has been used to encrypt two messages? | |
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