We explore what happens when the basic CBC-MAC construction is used with messages of different lengths. Say the sender and receiver do not agree 0n the message length? in advance (and so Vrfy_k (m, t) = 1 iff t = ? MaC_k (m), regardless of the length of m), but the sender is careful to only authenticate messages of length 2n. Show that an adversary can forge a valid tag on a message of length 4n. Say the receiver only accepts 3-block messages (so Vrfy_k (m, t) = 1 only if m has length 3n and t = ? MaC_k (m)), but the sender authenticates messages of any length a multiple of n. Show that an adversary can forge a valid tag 0n a new message. We explore what happens when the basic CBC-MAC construction is used with messages of different lengths. Say the sender and receiver do not agree 0n the message length? in advance (and so Vrfy_k (m, t) = 1 iff t = ? MaC_k (m), regardless of the length of m), but the sender is careful to only authenticate messages of length 2n. Show that an adversary can forge a valid tag on a message of length 4n. Say the receiver only accepts 3-block messages (so Vrfy_k (m, t) = 1 only if m has length 3n and t = ? MaC_k (m)), but the sender authenticates messages of any length a multiple of n. Show that an adversary can forge a valid tag 0n a new message