▁point ▁P , ▁and ▁with ▁the ▁second ▁agent ▁of ▁Alice ▁sending ▁a ▁secret ▁message ▁y ▁to ▁the ▁second ▁agent ▁of ▁Bob ▁at ▁the ▁spac etime ▁point ▁Q , ▁with ▁P ▁and ▁Q ▁spac eli ke ▁separated , ▁then ▁Bob ▁can ▁be ▁guaranteed ▁that ▁the ▁message ▁y ▁received ▁from ▁Alice ▁was ▁chosen ▁independently ▁of ▁the ▁message ▁x ▁that ▁he ▁gave ▁Alice , ▁and ▁vice ▁vers a . ▁This ▁is ▁a ▁useful ▁mathematical ▁property ▁that ▁is ▁explo ited ▁to ▁prove ▁the ▁security ▁of ▁crypt ographic ▁protocol s ▁in ▁relativ istic ▁crypt ography . ▁ ▁The ▁setting ▁It ▁is ▁a ▁fundamental ▁requirement ▁in ▁relativ istic ▁crypt ography ▁that ▁the ▁parties ▁implementing ▁the ▁crypt ographic ▁task ▁have ▁a ▁good ▁description ▁of ▁spac etime , ▁at ▁least ▁within ▁the ▁region ▁of ▁spac etime ▁where ▁the ▁task ▁is ▁implemented . ▁For ▁example , ▁in ▁protocol s ▁implemented ▁near ▁the ▁Earth ▁surface , ▁it ▁can ▁be ▁assumed ▁that ▁spac etime ▁is ▁close ▁to ▁M ink owski . ▁Import antly , ▁this ▁means ▁that , ▁near ▁the ▁Earth ▁surface , ▁physical ▁systems ▁and ▁information ▁cannot ▁travel ▁faster ▁than ▁the ▁speed ▁of ▁light ▁through ▁vac u um , ▁which ▁is ▁approximately ▁ 3 0 0 , 0 0 0 ▁km / s . ▁In ▁principle , ▁relativ istic ▁crypt ography ▁can ▁be ▁applied ▁with ▁more ▁general ▁spac et imes , ▁as ▁long ▁as ▁the ▁parties ▁can ▁guarantee ▁that ▁there ▁are ▁no ▁mechan isms ▁allowing ▁instant ▁communication , ▁like ▁w orm hol es . ▁Another ▁requirement ▁is ▁that ▁the ▁parties ▁have ▁access ▁to