 With certain IVs, an attacker knowing the first byte of the keystream and the first "m" bytes of the key can derive the ("m" + 1)th byte of the key due to a weakness in the PRNG used to generate the keystream.Because the first byte of the plaintext comes from the WEP SNAP header, an attacker can assume he can derive the first byte of the keystream from "B" ⊕ 0x"AA" (the SNAP header is almost always 0xAA).From there, he only needs an IV in the form ("a" + 3, "n" − 1, "x") for key index "a" origin 0, element value space "n" (256 since 8 bits make a byte), and any "X".