On the User Privacy and Ambiguity of Optimistic Fair Exchange

Optimistic Fair Exchange (OFE) is a cryptographic protocol which allows two parties to exchange electronic items such as digital signatures in an all-or-nothing manner, that is, either having both parties complete the exchange or having none of them get the exchanging item. There exists a trusted third party which is called in only when there is a dispute. In almost all the existing OFE protocols, after obtaining a partial signature from one party called the signer, the other party called the verifier can present it to others and show that the signer has indeed committed itself to something corresponding to the partial signature prior to the completion of the protocol. The lack of partial signature ambiguity is undesirable in practice and may be harmful to the signer. In addition, existing OFE protocols do not provide identity protection to the communicating parties. Motivated by these problems, in this project, we propose to investigate the following two notions, namelyAmbiguous Optimistic Fair Exchange (A-OFE); andGroup-Oriented Optimistic Fair Exchange (G-OFE).These notions are targeted to solve the partial signature ambiguity problem and enhance the user privacy of the communicating parties. We will investigate the security models, in particular, targeting on capturing unforgeability, fairness, anonymity and partial signature ambiguity under the multi-party setting and chosen-key model. We will also propose new techniques for constructing efficient A/G-OFE protocols.