Let CBC$-$MAC$$ be a variation on CBC$-$MAC intended to allow one to MAC messages

of arbitrary length, and defined as follows:

CBC$-$MAC$($M$)=$ CBC$-$MAC$($M$)\backslash |$ H$($M$)$

where H is a publicly$-$known, secure hash function H : $\{0,1\}$

$$

$7->\{0,1\}$

n

$.$ Prove that

this variant is not UF$-$CMA secure$.$