- bit ▁initialization ▁vector ▁( IV ) ▁in ▁each ▁message ▁packet . ▁The ▁RC 4 ▁key ▁for ▁that ▁packet ▁is ▁the ▁IV ▁concaten ated ▁with ▁the ▁W EP ▁key . ▁W EP ▁keys ▁have ▁to ▁be ▁changed ▁manually ▁and ▁this ▁typically ▁happens ▁inf re qu ently . ▁An ▁attack er ▁therefore ▁can ▁assume ▁that ▁all ▁the ▁keys ▁used ▁to ▁encrypt ▁packets ▁share ▁a ▁single ▁W EP ▁key . ▁This ▁fact ▁opened ▁up ▁W EP ▁to ▁a ▁series ▁of ▁attacks ▁which ▁proved ▁dev ast ating . ▁The ▁simplest ▁to ▁understand ▁uses ▁the ▁fact ▁that ▁the ▁ 2 4 - bit ▁IV ▁only ▁allows ▁a ▁little ▁under ▁ 1 7 ▁million ▁possibilities . ▁Because ▁of ▁the ▁birth day ▁par ado x , ▁it ▁is ▁likely ▁that ▁for ▁every ▁ 4 0 9 6 ▁packets , ▁two ▁will ▁share ▁the ▁same ▁IV ▁and ▁hence ▁the ▁same ▁RC 4 ▁key , ▁allowing ▁the ▁packets ▁to ▁be ▁attacked . ▁More ▁dev ast ating ▁attacks ▁take ▁advantage ▁of ▁certain ▁weak ▁keys ▁in ▁RC 4 ▁and ▁eventually ▁allow ▁the ▁W EP ▁key ▁itself ▁to ▁be ▁recovered . ▁In ▁ 2 0 0 5 , ▁agents ▁from ▁the ▁U . S . ▁Federal ▁Bureau ▁of ▁Investig ation ▁public ly ▁demonstrated ▁the ▁ability ▁to ▁do ▁this ▁with ▁widely ▁available ▁software ▁tools ▁in ▁about ▁three ▁minutes . ▁ ▁Pre vent ing ▁related - key ▁attacks ▁One ▁approach ▁to ▁prevent ing ▁related - key ▁attacks ▁is ▁to ▁design ▁protocol s ▁and ▁applications ▁so ▁that ▁encryption ▁keys ▁will ▁never ▁have ▁a ▁simple ▁relationship ▁with ▁each