. Alice developed a message authentication code (MAC) based on DES. Her algorithm works as follows: For a given input message M (assume that M is formed by multiple blocks of 64 bits), represent M as M = (X1 || X2 || ... || Xm), where Xi is a 64-bit block and || represents concatenation. Compute Delta(M) = X1 ^ X2 ^ ... ^ Xm, where ^ represents bit-wise XOR. Then the MAC for M is computed as CK(M) = EK(Delta(M)), where E is DES encryption algorithm and K is the secret key. Unfortunately, this scheme is vulnerable. Describe an attack against it.