a . ▁ ▁The ▁tab ula ▁rect a ▁uses ▁a ▁letter ▁square ▁with ▁the ▁ 2 6 ▁letters ▁of ▁the ▁alphabet ▁followed ▁by ▁ 2 6 ▁rows ▁of ▁additional ▁letters , ▁each ▁shift ed ▁once ▁to ▁the ▁left ▁from ▁the ▁one ▁above ▁it . ▁This , ▁in ▁ess ence , ▁creates ▁ 2 6 ▁different ▁Ca esar ▁ci ph ers . ▁ ▁The ▁resulting ▁ci pher text ▁appears ▁as ▁a ▁random ▁string ▁or ▁block ▁of ▁data . ▁Due ▁to ▁the ▁variable ▁sh ifting , ▁natural ▁letter ▁frequencies ▁are ▁hidden . ▁However , ▁if ▁a ▁code bre aker ▁is ▁aware ▁that ▁this ▁method ▁has ▁been ▁used , ▁it ▁becomes ▁easy ▁to ▁break . ▁The ▁ci pher ▁is ▁vulner able ▁to ▁attack ▁because ▁it ▁la cks ▁a ▁key , ▁thus ▁viol ating ▁Ker ck hoff s ' s ▁principle ▁of ▁crypt ology . ▁ ▁Im prov ements ▁ ▁In ▁ 1 5 5 3 , ▁an ▁important ▁extension ▁to ▁Tr ith em ius ' s ▁method ▁was ▁developed ▁by ▁Gi ovan ▁Batt ista ▁Bell aso , ▁now ▁called ▁the ▁V igen ère ▁ci pher . ▁Bell aso ▁added ▁a ▁key , ▁which ▁is ▁used ▁to ▁dict ate ▁the ▁switching ▁of ▁ci pher ▁al ph ab ets ▁with ▁each ▁letter . ▁This ▁method ▁was ▁mis att ributed ▁to ▁Bla ise ▁de ▁V igen ère , ▁who ▁published ▁a ▁similar ▁auto key ▁ci pher ▁in ▁ 1 5 8 6 . ▁ ▁The ▁classic ▁Tr ith em ius ▁ci pher ▁( using ▁a ▁shift ▁of ▁one )