Improved tripartite concurrent signature

The concrete tripartite concurrent signature scheme designed by Susilo et al. is found being unambiguity. In other words, any fourth party can identify who is the real signer of the publicly available valid signatures before the keystone is released. In order to overcome this flaw, we proposed an improved concrete tripartite concurrent signature (iCTCS) scheme based on the ring signature. Every user can choose his keystone to guarantee the fairness in the proposed scheme. However, the performance analysis results show that the iCTCS scheme is existentially unforgeable under a chosen message attack in the random oracle, and satisfies correctness, ambiguity and fairness.