▁the ▁Ge ffe ▁generator ▁is ▁a ▁very ▁weak ▁generator ▁and ▁should ▁never ▁be ▁used ▁to ▁generate ▁stream ▁ci pher ▁ke yst ream s . ▁ ▁Note ▁from ▁the ▁table ▁above ▁that ▁ ▁agre es ▁with ▁the ▁generator ▁output ▁ 4 ▁times ▁out ▁of ▁ 8 ▁- ▁a ▁ 5 0 % ▁correlation . ▁ ▁We ▁cannot ▁use ▁this ▁to ▁br ute ▁force ▁L FS R - 1 ▁independently ▁of ▁the ▁others : ▁ ▁the ▁correct ▁key ▁will ▁yield ▁output ▁which ▁agre es ▁with ▁the ▁generator ▁output ▁ 5 0 % ▁of ▁the ▁time , ▁but ▁on ▁average ▁so ▁will ▁an ▁incorrect ▁key . ▁ ▁This ▁represents ▁the ▁ideal ▁situation ▁from ▁a ▁security ▁perspective ▁- ▁the ▁combining ▁function ▁ ▁should ▁be ▁chosen ▁so ▁that ▁the ▁correlation ▁between ▁each ▁variable ▁and ▁the ▁combining ▁function ' s ▁output ▁is ▁as ▁close ▁as ▁possible ▁to ▁ 5 0 %. ▁ ▁In ▁practice ▁it ▁may ▁be ▁difficult ▁to ▁find ▁a ▁function ▁which ▁ach ieves ▁this ▁without ▁sacrific ing ▁other ▁design ▁criteria , ▁e . g . ▁period ▁length , ▁so ▁a ▁comprom ise ▁may ▁be ▁necessary . ▁ ▁Clar ifying ▁the ▁statistical ▁nature ▁of ▁the ▁attack ▁ ▁While ▁the ▁above ▁example ▁illustr ates ▁well ▁the ▁relatively ▁simple ▁concepts ▁behind ▁correlation ▁attacks , ▁it ▁perhaps ▁simpl ifies ▁the ▁explanation ▁of ▁precisely ▁how ▁the ▁br ute ▁forcing ▁of ▁individual ▁L FS R s ▁proceed s . ▁ ▁We ▁make ▁the ▁statement ▁that ▁incorrectly ▁guess ed ▁keys ▁will ▁generate ▁L FS R ▁output ▁which ▁agre es ▁with ▁the ▁generator ▁output ▁roughly