Abstract
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.