▁for ▁each ▁of ▁them ▁is ▁shown ▁in ▁the ▁table ▁below : ▁ ▁Consider ▁the ▁output ▁of ▁the ▁third ▁register , ▁. ▁ ▁The ▁table ▁above ▁makes ▁it ▁clear ▁that ▁of ▁the ▁ 8 ▁possible ▁outputs ▁of ▁. ▁ 6 ▁of ▁them ▁are ▁equal ▁to ▁the ▁corresponding ▁value ▁of ▁the ▁generator ▁output , ▁, ▁i . e . ▁ ▁in ▁ 7 5 % ▁of ▁all ▁possible ▁cases . ▁ ▁Thus ▁we ▁say ▁that ▁L FS R - 3 ▁is ▁correl ated ▁with ▁the ▁generator . ▁ ▁This ▁is ▁a ▁weak ness ▁we ▁may ▁explo it ▁as ▁follows : ▁ ▁Suppose ▁we ▁intercept ▁the ▁ci pher text ▁ ▁of ▁a ▁plain text ▁ ▁which ▁has ▁been ▁encrypted ▁by ▁a ▁stream ▁ci pher ▁using ▁a ▁Ge ffe ▁generator ▁as ▁its ▁ke yst ream ▁generator , ▁i . e . ▁ ▁for ▁, ▁where ▁ ▁is ▁the ▁output ▁of ▁L FS R - 1 ▁at ▁time ▁, ▁etc . ▁ ▁Suppose ▁further ▁that ▁we ▁know ▁some ▁part ▁of ▁the ▁plain text , ▁e . g . ▁we ▁know ▁, ▁the ▁first ▁ 3 2 ▁bits ▁of ▁the ▁plain text ▁( cor respond ing ▁to ▁ 4 ▁ASCII ▁characters ▁of ▁text ). ▁ ▁This ▁is ▁not ▁as ▁impro b able ▁as ▁it ▁may ▁seem : ▁if ▁we ▁know ▁the ▁plain text ▁is ▁a ▁valid ▁XML ▁file , ▁for ▁instance , ▁we ▁know ▁that ▁the ▁first ▁ 4 ▁ASCII ▁characters ▁must ▁be ▁"< xml ". ▁ ▁Similar ▁to ▁this , ▁many ▁file ▁formats ▁or ▁network ▁protocol s ▁have