Invisibility and Anonymity of Undeniable and Confirmer Signatures

Abstract

Traditionally, the strongest notion of security for undeniable and confirmer signatures is invisibility under adaptive attacks. This security property was promoted by Camenisch and Michels and they provided schemes with this property. Gennaro, Krawczyk and Rabin (GKR) developed an RSA-based scheme which is much more efficient than the schemes of Camenisch and Michels, but it does not have invisibility. We give an RSA-based scheme which is as efficient as the GKR scheme, and which has invisibility. We suggest that anonymity is the most relevant security property for undeniable and confirmer signatures. We give a precise definition of anonymity for undeniable and confirmer signatures in the multi-user setting and show that anonymity and invisibility are closely related. Finally, we show that anonymity can be achieved even when the parties use completely different cryptographic primitives.