Digital contents are typically sold online through centralized and custodian
marketplaces, which requires the trading partners to trust a central entity. We
present FileBounty, a fair protocol which, assuming the cryptographic hash of
the file of interest is known to the buyer, is trust-free and lets a buyer
purchase data for a previously agreed monetary amount, while guaranteeing the
integrity of the contents. To prevent misbehavior, FileBounty guarantees that
any deviation from the expected participants’ behavior results in a negative
financial payoff; i.e. we show that honest behavior corresponds to a subgame
perfect Nash equilibrium. Our novel deposit refunding scheme is resistant to
extortion attacks under rational adversaries. If buyer and seller behave
honestly, FileBounty’s execution requires only three on-chain transactions,
while the actual data is exchanged off-chain in an efficient and
privacy-preserving manner. We moreover show how FileBounty enables a flexible
peer-to-peer setting where multiple parties fairly sell a file to a buyer.

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