Efficient abuse-free fair contract-signing protocol based on an ordinary crisp commitment scheme

A mathematical framework for conventional commitment schemes is proposed. Digital contract-signing protocols represent an important application of the proposed framework, where usually two mistrusted parties wish to exchange their commitments in a fair way. Building on a variant of the proposed framework, an efficient contract-signing protocol over the Internet is developed. The latter protocol is `optimistic, fairness and abuse-free´. Detailed security and performance analyses are provided. The performance analysis reveals an important computational aspect of the proposed protocol which requires only `four rounds´ to complete unlike existing protocols. Moreover, the communication and computational costs are relatively small. Given these theoretical and practical features, the proposed contract-signing protocol is not only of theoretical interest, but it also enjoys practical merits which make it very suitable for electronic transactions requiring online signature exchange.