(20 points) (Brute-force key search.) Let F:K x {0,1}" {0,1}" be a keyed function, and suppose that the key space K is small enough that it can be feasibly enumerated. Use this fact to prove that F cannot be a PRF, by formally describing a feasible distinguisher and analyzing its advantage. (20 points) (Brute-force key search.) Let F:K x {0,1}" {0,1}" be a keyed function, and suppose that the key space K is small enough that it can be feasibly enumerated. Use this fact to prove that F cannot be a PRF, by formally describing a feasible distinguisher and analyzing its advantage