Based on the study of existing fair exchange protocols, this paper sets up an accurate formal model by stepwise refinement. In the process of refinement an unreliable channel is employed to simulate an attack behavior. The model provides a novel formal definition of exchanged items, and presents the formal goals for fairness, accountability, etc., reflecting the inherent requirements for fair exchange protocols across-the-board. In order to check, prove, and design fair exchange protocols effectively and efficiently, the model puts forward a novel property of abuse-freeness which applies to all fair exchange protocols, gives a formal definition for trust strand of the third party, and presents general criteria of designing a secure and effective fair exchange protocol. Taking a typical fair exchange protocol as an example, this paper presents the analysis steps of fair exchange protocols appealing to our model. An unknown attack is uncovered. The analysis reveals the process of a complete attack, discovering deeper reasons for causing an attack. Finally, we modify the flawed protocol and the revised protocol ensures the desirable properties.
QING Sihan1,2,3 & LI Gaicheng1,2,3 1. Engineering Research Center for Information Security Technology, Institute of Software, Chinese Aca- demy of Sciences, Beijing 100080, China
授权约束是基于角色的访问控制模型中一个非常重要的部分.已经有方法来对约束进行形式化的描述,但主要集中在有关约束的静态特性的描述上.提出一套形式化地描述时间特性的模型,使之能够描述带时间特性的授权约束.对已有的角色授权约束模型,即约束描述语言进行了深入研究,并进行了相应的扩展.这种扩展后的语言称为RCLT(role-based constraints language with time-character).同时,对RCLT的时间效率的优化以及在RCLT规则被违反时如何恢复合法状态的问题进行了形式化的讨论.RLCT语言被证明可以很好地表示出对授权约束的时间特性的需求,但在效率优化和状态恢复方面要找到实用、高效的通用算法还需要作进一步的工作,这也是将来研究工作的主要目标.
