Message Injection

What does the receiver do?
C' = <F,c(F)> XOR RC4(v,k)
C' is a correct encryption of the message F, so he has to accept it.
The adversary has succeeded!


Note that we only used the fact that the CRC does not depend on the key in order to perform this attack.  It would work just as well if the CRC were replaced by, say, SHA-1.