RC4 arguments question

[airmong]

Hello,

In every WEP documentation it is explained that the Cypherstream is calculated by:


RC4(k||IV). Where || is the concatenation operator.


But RC4 is a function which takes as arguments a plaintext and a key. RC4(M,k).

So does anybody know how is it done exactly in WEP? RC4(k||IV, k)?

Thanks so much!
Carlos

[Jason Cooper]

RC4 is a stream cipher and not a block cipher so RC4(k||IV) produces a stream of bits which is then xor'd with the bits in the message. So in WEP you would have

RC4(k||IV) = 10011011001........
Message    = 11100101101........
  XOR
Output     = 01111110100........

Sometimes you encounter RC4 functions in languages which do the xor'ing process for you, that is why they want the key and the message.