A Multi-Party Optimistic Certified Email Protocol Using Verifiably Encrypted Signature Scheme For Line Topology

摘要

While many two-party fair exchange protocols have been proposed, more than three-party fair exchange protocol is required depending on a type of applications. Asokan et al. have proposed a multi-party fair exchange protocol for mesh topology. This scheme can be adapted to all kinds of topologies but requires much communication costs, which is 4n(n -1) passes in the all n-participators honest case and 8n~2 -n -10 passes in the worst case. In previous works, more efficient and multi-functional schemes specialized for a kind of topologies have been proposed but most of these are for ring topology and star topology. Zhou et al. have proposed an efficient multi-party contract signing protocol for line topology. It is shown the two protocols such that a simple protocol which is completed with 4(n -1) passes in the all honest case and 5n -3 passes in the worst case and an optimized protocol which is completed with 3(n -1) passes in the all honest case and 4n -2 passes in the worst. This result means that a fair exchange protocol for line topology is efficient in a number of communications nevertheless a certified email protocol specialized for line topology has not been proposed. That is, a certified email protocol run on line topology is only a protocol for mesh proposed by Asokan et al. adapted to line topology, therefore there is large room for reducing a number of communications by specializing for a certified email protocol. In this paper, we propose a multi-party certified email protocol for fine topology. The proposed protocol has such basic properties of certified email as fairness, non-repudiation, trusted third party invisibility and timeliness, as same as the previous works. Our scheme is completed with 4(n -1) passes in the all n-participators honest case and 5n -3 passes in the worst. The communication cost is less than the Asokan et al.'s scheme with 4n(n -1) passes even if the all n-participators are honest.