Fair Exchange with a Semi-Trusted Third Party

We present new protocols for two parties to exchange documents with fairness, i.e., such that no party can gain an advantage by quitting prematurely or otherwise misbehaving. We use a third party that is L‘semi-trusted”, in the sense that it may misbehave on its own but will not conspire with either of the main parties. In our solutions, disruption by any one of the three parties will not allow the disrupter gain any useful new information about the documents. Our solutions are efficient and can be based on any of several cryptographic assumptions (e.g., factoring, discrete log, graph isomorpbism). We also discuss the application of our techniques to electronic commerce protocols to achieve fair payment.