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Abstract

Proactive robust threshold schemes are presented for general 'homomorphic-type' public key systems, as well as optimized systems for the RSA function. Proactive security employs dynamic memory refreshing and enables us to tolerate a 'mobile adversary' that dynamically corrupts the components of the systems (perhaps all of them) as long as the number of corruptions (faults) is bounded within a time period. The systems are optimal-resilience. Namely they withstand any corruption of minority of servers at any time-period by an active (malicious) adversary (i.e., any subset less than half. Also disclosed are general optimal-resilience public key systems which are 'robust threshold' schemes (against stationary adversary), and are extended to 'proactive' systems (against the mobile one). The added advantage of proactivization in practical situations is the fact that, in a long-lived threshold system, an adversary has a long time (e.g., years) to break into any t out of the l servers. In contrast, the adversary in a proactive systems has only a short period of time (e.g., a week) to break into any t servers. The model of mobile adversary seems to be crucial to such 'long-lived' systems that are expected to span the secure network and electronic commerce infrastructure.

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