Message Injection

Here we will show how to inject a fake message F of the adversary's choice into the wireless net so that it will be accepted by a receiver as genuine.
The adversary just needs to know a single plaintext, and its corresponding encrypted packet.
Again, pings or spam can provide this easily
The encrypted packet is (v,C), and the plaintext is <M,c(M)>, so the adversary can compute 
     RC4(v,k) = C XOR <M,c(M)>.
Now he can take his fake message F, compute c(F), and compute C' = <F,c(F)> XOR RC4(v,k).
He then transmits (v,C').