Keyed hash functions (X, Y,K, H)is a keyed hash family, where X is a set of possible messages y is the set of possible message digests, or authentication tags K is the keyspace h is a set of hash functions For each key K there is a hash function hx: X,Y H Assume x>=Y(even better, 2X>=YD5 Keyed hash functions (X,Y,K,H) is a keyed hash family, where • X is a set of possible messages • Y is the set of possible message digests, or authentication tags • K is the keyspace • H is a set of hash functions • For each key K there is a hash function hK: X → Y in H • Assume |X| >= |Y| (even better, 2|X| >= |Y|)