Paper 2012/080

Strongly Unforgeable Proxy Re-Signatures in the Standard Model

S. Sree Vivek, S. Sharmila Deva Selvi, Guhan Balasubramanian, and C. Pandu Rangan

Abstract

Proxy re-signatures are generally used for the delegation of signing rights of a user (delegator) to a semi- trusted proxy and a delegatee. The proxy can convert the signature of one user on a message into the signature of another user on the same message by using the delegation information (rekey) provided by the delegator. This is 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. This is an important property for applications which involve the delegation of authentication on sensitive data. 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 security of both the schemes is related to the hardness of solving Computational Diffie-Hellman (CDH) problem.

Note: The second scheme and proof are modified to address a few security flaws.