Cryptanalysis of Some Group-Oriented Proxy Signature Schemes

A proxy signature scheme allows an entity to delegate its signing power to another entity. Since the notion of proxy signatures was first introduced, many proxy signature schemes and various extensions have been considered. As an example, the notion of threshold proxy signature or proxy multi-signature was introduced as a group-oriented variant. In this paper, we show that some threshold proxy signature schemes and proxy multi-signature schemes are all insecure against the malicious original singer(s). Our result provides a simple example that the way to put the secret parts together should be carefully considered.