Suppose that you know the output of an HMAC is X and the key is K, but you do not know the message M. Can you construct a message M' that has its HMAC equal to X, using the key K? If so, give an...

Suppose that you know the output of an HMAC is X and the key is K, but you do not know the message M. Can you construct a message M' that has its HMAC equal to X, using the key K? If so, give an algorithm for constructing such a message. If not, why not? Note that we are assuming that you know the key K, and the same key is used for both HMAC computations. (It may be instructive to compare this problem to Problem 43 of Chapter 3.)