▁only ▁a ▁hand ful ▁of ▁different ▁al ph ab ets ▁could ▁be ▁used ; ▁anything ▁more ▁complex ▁would ▁be ▁imp ract ical . ▁However , ▁using ▁only ▁a ▁few ▁al ph ab ets ▁left ▁the ▁ci ph ers ▁vulner able ▁to ▁attack . ▁The ▁in vention ▁of ▁rot or ▁machines ▁mechan ised ▁pol yal ph ab etic ▁encryption , ▁providing ▁a ▁practical ▁way ▁to ▁use ▁a ▁much ▁larger ▁number ▁of ▁al ph ab ets . ▁ ▁The ▁earliest ▁crypt anal yt ic ▁technique ▁was ▁frequency ▁analysis , ▁in ▁which ▁letter ▁patterns ▁unique ▁to ▁every ▁language ▁could ▁be ▁used ▁to ▁discover ▁information ▁about ▁the ▁substitution ▁alphabet ( s ) ▁in ▁use ▁in ▁a ▁mon o - alph ab etic ▁substitution ▁ci pher . ▁For ▁instance , ▁in ▁English , ▁the ▁plain text ▁letters ▁E , ▁T , ▁A , ▁O , ▁I , ▁N ▁and ▁S , ▁are ▁usually ▁easy ▁to ▁identify ▁in ▁ci pher text ▁on ▁the ▁basis ▁that ▁since ▁they ▁are ▁very ▁frequent ▁( see ▁E TA OIN ▁S HR DL U ), ▁their ▁corresponding ▁ci pher text ▁letters ▁will ▁also ▁be ▁as ▁frequent . ▁In ▁addition , ▁big ram ▁combinations ▁like ▁N G , ▁ST ▁and ▁others ▁are ▁also ▁very ▁frequent , ▁while ▁others ▁are ▁rare ▁indeed ▁( Q ▁followed ▁by ▁anything ▁other ▁than ▁U ▁for ▁instance ). ▁The ▁simplest ▁frequency ▁analysis ▁re lies ▁on ▁one ▁ci pher text ▁letter ▁always ▁being ▁subst it uted ▁for ▁a ▁plain text ▁letter ▁in ▁the ▁ci pher : ▁if ▁this ▁is ▁not ▁the ▁case