, ▁de ci ph ering ▁the ▁message ▁is ▁more ▁difficult . ▁For ▁many ▁years , ▁crypt ograph ers ▁attempted ▁to ▁hide ▁the ▁tell t ale ▁frequencies ▁by ▁using ▁several ▁different ▁substitution s ▁for ▁common ▁letters , ▁but ▁this ▁technique ▁was ▁unable ▁to ▁fully ▁hide ▁patterns ▁in ▁the ▁substitution s ▁for ▁plain text ▁letters . ▁Such ▁schemes ▁were ▁being ▁widely ▁broken ▁by ▁the ▁ 1 6 th ▁century . ▁ ▁In ▁the ▁mid - 1 5 th ▁century , ▁a ▁new ▁technique ▁was ▁invent ed ▁by ▁Al ber ti , ▁now ▁known ▁generally ▁as ▁pol yal ph ab etic ▁ci ph ers , ▁which ▁recogn ised ▁the ▁virtue ▁of ▁using ▁more ▁than ▁a ▁single ▁substitution ▁alphabet ; ▁he ▁also ▁invent ed ▁a ▁simple ▁technique ▁for ▁" cre ating " ▁a ▁mult itude ▁of ▁substitution ▁patterns ▁for ▁use ▁in ▁a ▁message . ▁Two ▁parties ▁ex changed ▁a ▁small ▁amount ▁of ▁information ▁( re ferred ▁to ▁as ▁the ▁key ) ▁and ▁used ▁it ▁to ▁create ▁many ▁substitution ▁al ph ab ets , ▁and ▁so ▁many ▁different ▁substitution s ▁for ▁each ▁plain text ▁letter ▁over ▁the ▁course ▁of ▁a ▁single ▁plain text . ▁The ▁idea ▁is ▁simple ▁and ▁effective , ▁but ▁proved ▁more ▁difficult ▁to ▁use ▁than ▁might ▁have ▁been ▁expected . ▁Many ▁ci ph ers ▁were ▁only ▁partial ▁implementations ▁of ▁Al ber ti ' s , ▁and ▁so ▁were ▁easier ▁to ▁break ▁than ▁they ▁might ▁have ▁been ▁( e . g . ▁the ▁V igen ère ▁ci pher ). ▁ ▁Not ▁until ▁the ▁ 1 8