Cryptanalysis and improvement of a concurrent signature scheme based on identity

Concurrent signatures were introduced as an alternative approach to solving the problem of fair exchange of signatures by Chen et al.[1], in which two entities can produce two signatures that are not binding, until the keystone is released by one of the parties. Recently, Huang et al.[10] proposed a more ambiguity and stronger fairness concurrent signature scheme based on identity. However, we will show that their scheme did not satisfy the strong unforgeability properties, and was vulnerable to forgery attack. Then, we propose an improved scheme to prevent such attack.