Message Modification All the adversary does is compute C' = C XOR , and transmit (v,C') to the receiver. What does the receiver do? Decrypt the (modified) packet to find = C' XOR RC4(v,k) = C XOR XOR RC4(v,k) = XOR = = = Check that c' = c(M'), which it does (they're both c(F)) Accept the message F as the one transmitted The adversary has succeeded!