For hash functions it is crucial to have a sufficiently large number of output bits, with, e.g., 160 bits, in order to thwart attacks based on the birthday paradox. Why are much shorter output lengths of, e.g., 80 bits, sufficient for MACs? For your answer, assume a message x that is sent in clear together with its MAC over the channel: (x,MACk(x)). Exactly clarify what Oscar has to do to attack this system.