▁by ▁Bob ▁independently ▁of ▁the ▁bit ▁ ▁chosen ▁by ▁her . ▁Since ▁Alice ▁cho oses ▁ ▁randomly , ▁and ▁since ▁b ▁is ▁independent ▁of ▁, ▁Alice ▁is ▁guaranteed ▁that ▁the ▁bit ▁ ▁is ▁random . ▁With ▁similar ▁arguments , ▁Bob ▁is ▁also ▁guaranteed ▁that ▁the ▁bit ▁d ▁is ▁random . ▁ ▁Vari ations ▁of ▁coin ▁toss ing ▁have ▁been ▁investig ated ▁in ▁relativ istic ▁crypt ography ▁by ▁Col beck ▁and ▁Kent . ▁ ▁O bl iv ious ▁transfer ▁and ▁secure ▁comput ations ▁Lo ▁showed ▁that ▁ob liv ious ▁transfer ▁and ▁other ▁secure ▁comput ations ▁cannot ▁be ▁achieved ▁with ▁un cond itional ▁security ▁based ▁only ▁on ▁the ▁laws ▁of ▁quantum ▁physics . ▁This ▁impos s ibility ▁result ▁by ▁Lo ▁extends ▁to ▁the ▁more ▁general ▁setting ▁of ▁relativ istic ▁quantum ▁crypt ography . ▁Col beck ▁showed ▁that ▁various ▁secure ▁comput ations ▁are ▁impossible ▁to ▁achieve ▁with ▁un cond itional ▁security ▁in ▁relativ istic ▁quantum ▁crypt ography . ▁ ▁Position - based ▁quantum ▁crypt ography ▁Position - based ▁quantum ▁crypt ography ▁consists ▁in ▁crypt ographic ▁tasks ▁whose ▁security ▁explo it ▁the ▁location ▁of ▁a ▁party , ▁the ▁principle ▁of ▁no - super l um inal ▁sign alling ▁and ▁the ▁laws ▁of ▁quantum ▁physics . ▁For ▁example , ▁in ▁the ▁problem ▁of ▁quantum ▁location ▁authentication , ▁a ▁pro ver ▁wants ▁to ▁demonstrate ▁his ▁location ▁L ▁to ▁a ▁set ▁of ▁ver ifiers ▁using ▁quantum ▁systems . ▁A ▁protocol ▁for ▁quantum ▁location ▁authentication ▁works ▁as ▁follows . ▁A ▁set ▁of ▁ver ifiers ▁at ▁various ▁locations ▁that ▁surr ound ▁the