Abstract: Multi-party non-repudiation protocol needs to meet three main security goals of non-repudiation,fairness and timeliness,but the existing formal analysis methods for multi-party non-repudiation proctocol are just simple extensions of those applied to two-party protocols.At the same time,each method can not cover all three security goals and has limited ability to analyze one goal with result unreliable.In this paper,the existing analysis methods were compared and the SVO logic was chosen for further study.Time factor was introduced in the logic system with relevant syntax definition and deduction axioms brought in explicitly.Then,the semantic model of the improved logic was stated and the soundness of logic system was proved,causing that the improved logic system can support the analysis of all three secu-rity goals of multi-party non-repudiation protocol.In the end,a typical multi-party non-repudiation protocol was analyzed with the improved logic and the defects of timeliness and fairness were found with corresponding attacks stated.Among the defects,the defect of fairness was discovered for the first time.

Key words: Fairness, Multi-party non-repudiation protocol, SVO logic, Timeliness