Lancom Systems LCOS 3.50 - page 216

LANCOM Reference Manual LCOS 3.50  Chapter 11: Wireless LAN – WLAN
216
Wireless LAN – WLAN
data packet—a double application of the XOR operation with the same
values cancels out.
The advantage of RC4 is that the operations
generation of the byte sequence from the key
XOR operation on the data stream
on the sending and receiving sides are identical—so the hardware need only
be built into the WLAN card once, and then can be used for both transmission
and receiving. Since the data in the WLAN are transmitted half-duplex only,
simultaneous transmission and receiving will never occur. However, RC4 has
one serious disadvantage: one may only use a particular RC4 key once for a
single packet! If the same RC4 key is used for two different data packets, then
a potential eavesdropper is able to take the two packets and XOR them
together. This operation doesn't result in clear text, but the pseudo-random
sequence, and thus the encryption, cancels out, and one has the XOR
+
Figure 1: WEP process
Data
Data + CRC
CRC
WEP key
Current IV
+1
Generator XOR
Data + CRC (encrypted)
IV
RC4