$5.($Chap$.6,10$ points$)$

Show that a $2-$round keyed Feistel cipher cannot be a secure pseudorandom permutation $($PRP$),$ no matter secure PRF $\backslash ($ f $\backslash )$ is used.

Your attack should work without knowing the round keys $\backslash (\backslash$left$($k$\_\{1\},$ k$\_\{2\}\backslash$right$)\backslash ),$ and it should work even with different $($independent$)$ round keys. $($Hint: A successful attack requires two queries of two "related" inputs.$)$