▁weak ness ▁that ▁ar ises ▁from ▁a ▁poor ▁choice ▁of ▁the ▁Boolean ▁function ▁– ▁it ▁is ▁possible ▁to ▁select ▁a ▁function ▁which ▁avoid s ▁correlation ▁attacks , ▁so ▁this ▁type ▁of ▁ci pher ▁is ▁not ▁inher ently ▁in secure . ▁ ▁It ▁is ▁simply ▁essential ▁to ▁consider ▁sus cept ibility ▁to ▁correlation ▁attacks ▁when ▁design ing ▁stream ▁ci ph ers ▁of ▁this ▁type . ▁ ▁Ex plan ation ▁ ▁Cor relation ▁attacks ▁are ▁possible ▁when ▁there ▁is ▁a ▁significant ▁correlation ▁between ▁the ▁output ▁state ▁of ▁one ▁individual ▁L FS R ▁in ▁the ▁ke yst ream ▁generator ▁and ▁the ▁output ▁of ▁the ▁Boolean ▁function ▁that ▁comb ines ▁the ▁output ▁state ▁of ▁all ▁of ▁the ▁L FS R s . ▁ ▁Com b ined ▁with ▁partial ▁knowledge ▁of ▁the ▁ke yst ream ▁( which ▁is ▁easily ▁derived ▁from ▁partial ▁knowledge ▁of ▁the ▁plain text , ▁as ▁the ▁two ▁are ▁simply ▁X OR ed ▁together ), ▁this ▁allows ▁an ▁attack er ▁to ▁br ute - force ▁the ▁key ▁for ▁that ▁individual ▁L FS R ▁and ▁the ▁rest ▁of ▁the ▁system ▁separately . ▁ ▁For ▁instance , ▁if , ▁in ▁a ▁ke yst ream ▁generator ▁in ▁which ▁four ▁ 8 - bit ▁L FS R s ▁are ▁combined ▁to ▁produce ▁the ▁ke yst ream , ▁and ▁one ▁of ▁the ▁registers ▁is ▁correl ated ▁to ▁the ▁Boolean ▁function ▁output , ▁we ▁may ▁br ute ▁force ▁it ▁first ▁and ▁then ▁the ▁remaining ▁three , ▁for ▁a ▁total ▁attack ▁complexity ▁of ▁ 2 8 ▁+ ▁ 2 2 4 .