Gradually Convertible Undeniable Signatures (Michels-Petersen-Horster Convertible Undeniable Signatures Revisited)

convertible undeniable signatures which limit the self-authenticating prop-erty of digital signatures but can be converted by the signer to ordi-nary signatures. Michels, Petersen and Horster presented, in 1996, an attack on the El Gamal-based seminal scheme of Boyar et al. and pro-posed a repaired version without formal security analysis. In this paper, we modify their protocol so that it becomes a generic one and it pro-vides an advanced feature which permits the signer to universally convert achronously all signatures pertaining to a specific time period. We sup-ply a formal security treatment of the modified scheme: we prove, in the generic group model, that the protocol is existentially unforgeable and anonymous under chosen message attacks, assuming new assumptions (though reasonable) on the underlying hash function. 1 Introduction. In 1996, Michels, Petersen and Horster [16] proposed an efficient convertible undeniable signature protocol whose security relies on the difficulty of the dis