7. (10pt) Suppose that you know a MAC value X and the key K that was used to compute the MAC, but you do not know the original message. (a) (8pt) Show that you can construct a message M that also has its MAC equal to X. Note that we are assuming that you know the key K and the same key is used for both MAC computations. (b) (2pt) How much of the message M are you free to choose? 7. (10pt) Suppose that you know a MAC value X and the key K that was used to compute the MAC, but you do not know the original message. (a) (8pt) Show that you can construct a message M that also has its MAC equal to X. Note that we are assuming that you know the key K and the same key is used for both MAC computations. (b) (2pt) How much of the message M are you free to choose