 Furthermore, given some hash value, it is typically infeasible to find some input data (other than the one given) that will yield the same hash value.If an attacker can change not only the message but also the hash value, then a "keyed hash" or message authentication code (MAC) can be used for additional security.Without knowing the key, it is not possible for the attacker to easily or conveniently calculate the correct keyed hash value for a modified message.