$(5$ points$)$ Let Fk : $\{0,1\}$n $\{0,1\}$ be a secure PRF from n bits to $$ bits. Consider

a PRF G : $\{0,1\}2$n $\{0,1\}2$ bits defined as follows. The key space of G is same as

that of F $,$ while the function is defined as Gk$($x$,$ y$)=$ Fk$($x$)||($Fk$($x$)$ Fk$($y$)).($The

inputs x and y are n bits each and $||$ denotes concatenation.$)$ Show an attack in

the PRF security game; that is$,$ explicitly define a polynomial time attacker A and

compute the winning probability of A$.$