Optimistic Fair Priced Oblivious Transfer

Priced oblivious transfer (POT) is a two-party protocol between a vendor and a buyer in which the buyer purchases digital goods without the vendor learning what is bought. Although privacy properties are guaranteed, current schemes do not offer fair exchange. A malicious vendor can, e.g., prevent the buyer from retrieving the goods after receiving the payment, and a malicious buyer can also accuse an honest vendor of misbehavior without the vendor being able to prove this untrue. In order to address these problems, we define the concept of optimistic fair priced oblivious transfer and propose a generic construction that extends secure POT schemes to realize this functionality. Our construction, based on verifiably encrypted signatures, employs a neutral adjudicator that is only involved in case of dispute, and shows that disputes can be resolved without the buyer losing her privacy, i.e., the buyer does not need to disclose which digital goods she is interested in. We show that our construction can be instantiated with an existing universally composable POT scheme, and furthermore we propose a novel full-simulation secure POT scheme that is much more efficient.