Strongly Unforgeable Proxy Re-Signature Schemes in the Standard model

Proxy re-signatures are generally used for the delegation of signatures to a semi-trusted proxy which transforms the signatures between the users of the system. Its a handy primitive for network security and automated delegations in hierarchical organizations. Though proxy re-signature schemes that are secure in the standard model are available, none of them have addressed the security notion of strong existential unforgeability, where the adversary will not be able to forge even on messages for which signatures are already available. In this paper, we define the security model for strong unforgeability of proxy re-signature schemes. We propose two concrete strong unforgeable proxy re-signature schemes, where we induce the strong unforgeability in the scheme by embedding the transformation techniques carefully in the sign and resign algorithms. The second scheme proposed also possesses a tight security reduction thereby strengthening our security argument. The security of both the schemes is related to the hardness of solving Computational Diffie-Hellman (CDH) problem.