Paper 2007/200

ProSiBIR: Proactive Signer-Base Intrusion Resilient Signatures

Philip Atzemoglou and Tal Malkin

Abstract

The notion of Signer-Base Intrusion-Resilient (SiBIR) signatures was introduced in [IR02] as a scheme that can withstand an arbitrary number of key-exposures, as long as both of its modules are not compromised simultaneously. This was achieved by dividing time into predefined time periods, each corresponding to a different time-evolving secret key, while maintaining a constant public key. The two modules of this scheme consist of a signer that can generate signatures on its own, and a base that is used to update the signer's key as it evolves through time. The purpose of this paper is to provide a model for multi-signer, multi-base intrusion-resilient signatures. This proactive SiBIR scheme essentially breaks the preexisting notions of signer and base, to an arbitrary number of signer and base modules. This tends to implementations where multiple parties need to agree for a document to be signed. An attacker needs to break into all the signers at the same time in order to forge a signature for that period. Moreover, he needs to break into all the bases as well, at that same time period, in order to "break" the scheme and generate future signatures. Thereby, by assuming a large number of bases, the risk of our scheme being compromised becomes arbitrarily small. We provide an implementation that's provably secure in the random oracle model, based on the strong RSA assumption. We also yield a modest improvement in the upperbound of our scheme's insecurity function, as opposed to the one presented in [IR02].