Let Fl: {0,1}" x {0,1}2n + {0,1}" be defined as follows, and prove that F is NOT a secure PRF: F (21||22) = F(21) 12 (Note: n is the key length, and 2n is the input length.) Let Fl: {0,1}" x {0,1}2n + {0,1}" be defined as follows, and prove that F is NOT a secure PRF: F (21||22) = F(21) 12 (Note: n is the key length, and 2n is the input length.)