▁almost ▁nothing ▁about ▁the ▁message ▁being ▁sent . ▁The ▁goal ▁here ▁is ▁to ▁hide ▁all ▁functions ▁of ▁the ▁plain text ▁rather ▁than ▁all ▁information ▁about ▁it . ▁ ▁Quant um ▁crypt ography ▁is ▁largely ▁part ▁of ▁information - the or etic ▁crypt ography . ▁ ▁Security ▁levels ▁ ▁Per fect ▁security ▁is ▁a ▁special ▁case ▁of ▁information - the or etic ▁security . ▁For ▁an ▁encryption ▁algorithm , ▁if ▁there ▁is ▁ci pher text ▁produced ▁that ▁uses ▁it , ▁no ▁information ▁about ▁the ▁plain text ▁is ▁provided ▁without ▁knowledge ▁of ▁the ▁key . ▁If ▁E ▁is ▁a ▁perfectly ▁secure ▁encryption ▁function , ▁for ▁any ▁fixed ▁message ▁m , ▁there ▁must ▁be , ▁for ▁each ▁ci pher text ▁c , ▁at ▁least ▁one ▁key ▁k ▁such ▁that ▁. ▁Mathemat ically , ▁let ▁m ▁and ▁c ▁be ▁the ▁random ▁variables ▁representing ▁the ▁plain text ▁and ▁ci pher text ▁messages , ▁respectively ; ▁then , ▁we ▁have ▁that ▁ ▁where ▁ ▁is ▁the ▁mut ual ▁information ▁between ▁m ▁and ▁c . ▁In ▁other ▁words , ▁the ▁plain text ▁message ▁is ▁independent ▁of ▁the ▁transm itted ▁ci pher text ▁if ▁we ▁do ▁not ▁have ▁access ▁to ▁the ▁key . ▁It ▁has ▁been ▁proved ▁that ▁any ▁ci pher ▁with ▁the ▁perfect ▁secre cy ▁property ▁must ▁use ▁keys ▁with ▁effectively ▁the ▁same ▁requirements ▁as ▁one - time ▁pad ▁keys . ▁ ▁It ▁is ▁common ▁for ▁a ▁crypt os ystem ▁to ▁leak ▁some ▁information ▁but ▁never theless ▁maintain ▁its ▁security ▁properties ▁even ▁against ▁an ▁advers ary ▁that ▁has ▁un limited