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 $=($X$1||$ X$2||...||$ Xm$),$ where Xi is a $64-$bit block and $||$

represents concatenation. Compute Delta$($M$)=$ X$1^$ X$2^...^$ 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$.$