Extending concurrent signature to multiple parties

Since the introduction of concurrent signature, improved results have been obtained on constructing schemes with enhanced ambiguity, refined security models and better efficiency, while extending concurrent signature to multiple users, that is, allowing n   parties (where n≥2n≥2) to perform fair exchange of signatures concurrently, is still one of the most challenging problems that remain unsolved. In the literature, there is a three-party concurrent signature scheme which achieves a weaker form of ambiguity, that an ambiguous signature can either be generated by the real signer or jointly by the other two parties, but not by any single party of the rest. There are also two other multi-party concurrent signature schemes. However, both of them have been found insecure, that they could not achieve unforgeability, ambiguity, and fairness simultaneously. Furthermore, there is no formal security model available for Multi-party Concurrent Signature (MCS). In this paper, we propose an efficient MCS construction and show its security in the random oracle model under our newly proposed security model for MCS. The scheme is also comparable in efficiency to the best existing two-party concurrent signature schemes.