Affiliations: Department of Computer Science and Engineering, Zhangzhou Normal University, Fujian, 363000, P. R. China, e-mail: zjhuang@fjzs.edu.cn | Department of Computer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, P. R. China
Note: [] This work is supported by the National Natural Science Foundation of China under Grant No.60673078, the China Postdoctoral Science Foundation under Grant No. 20060390152, the Natural Science Foundation of Fujian Province of China under Grant No.2006J0045 and the Innovation Foundation for Young Technological Talents of Fujian Province of China under Grant No.2005J055.
Abstract: The notion of concurrent signatures was introduced by Chen, Kudla and Paterson in their seminal paper in Eurocrypt 2004. In concurrent signature schemes, two entities can produce two signatures that are not binding, until an extra piece of information (namely the keystone) is released by one of the parties. Upon release of the keystone, both signatures become binding to their true signers concurrently. In ICICS 2005, two identity-based perfect concurrent signature schemes were proposed by Chow and Susilo. In this paper, we show that these two schemes are unfair. In which the initial signer can cheat the matching signer. We present a formal definition of ID-based concurrent signatures which redress the flaw of Chow et al.'s definition and then propose two simple but significant improvements to fix our attacks.
