V 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
abato noisgnarof bits, the following procedure is used.
16
1.
one2.
3.
Choose a random 80-bit value v
Generate the ciphertext c = RC4(v || k) + m
Send the bit string (v || c)
a. Suppose Alice uses this procedure to send a message m to Bob. Describe how
ford sleBob 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?
Fig: 1