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Privacy-preserving optimistic fair exchange (P2OFE) is a kind of protocols which aim to solve the fairness problem in the exchange of digital signatures between two parties and in the meanwhile protect their privacy. In P2OFE, no one else including the semi-trusted third party in charge of arbitration can confirm an exchange even after resolving a dispute. In this paper we present a black-box construction of P2OFE based on a tag-based public key encryption scheme and a standard digital signature scheme. Our construction follows the ‘sign-then-encrypt’ paradigm, and is secure in the standard model. Our construction is generic and admits more instantiations of P2OFE.