Paper 2002/051

Fully Distributed Proxy Signature Schemes

Javier Herranz and Germán Sáez

Abstract

In a proxy signature scheme, a potential signer delegates his signing capability to a proxy entity, who signs a message on behalf of the original signer. All the proposals of proxy signature schemes made until now have been based on Schnorr's signature scheme. Threshold versions of these schemes have also been proposed, in which the power of the proxy signer is distributed among a group of players, in such a way that any subset with a minimum number (threshold) of players can sign a message on behalf of the original signer. We consider a model that is fully distributed, because we want to distribute not only the power of the proxy signer, but also the original signer ability to delegate his signing capability. Furthermore, we consider general structures, instead of only the threshold ones, for both the tolerated subsets of dishonest players and the subsets of honest players authorized to execute a valid instance of the protocol, and in both the original and the proxy signer entities. We find sufficient combinatorial conditions that these structures must satisfy in order to design a fully distributed, secure and robust proxy signature scheme for this general scenario. We propose such a scheme for this setting. It is also based on Schnorr's signature scheme.

Note: A revised version of this paper, entitled 'Verifiable Secret Sharing for General Access Structures, with Application to Fully Distributed Proxy Signatures', will be published by LNCS in the proceedings of FC'03.