Suppose ?1 := (KeyGen1 ,Tag1 , Verify1 ), and ?2 := (KeyGen2 ,Tag2 , Verify2 ) are two message authentication code schemes.

1) ?4 := (KeyGen4 ,Tag4 , Verify4 ) is defined as follows: KeyGen4 is identical to KeyGen1 , and it outputs a key k. Tag4 [[k,(m1, m2)] first runs Tag1 (k, m1) and obtains ?1 and it runs Tag2 (k, m2) and obtains ?2. The final tag ? := ?1 ? ?2. Is ?4 a secure MAC? Briefly explain why.