Theoretical Foundations for Mobile Target Defense: Proactive Secret Sharing and Secure Multiparty Computation

One option to instantiate Mobile Target Defense (MTD) [27] strategies in distributed storage and computing systems is to design such systems from the ground up using cryptographic techniques such as secret sharing (SS) and secure multiparty computation (MPC). In standard SS a dealer shares a secret s among n parties such that an adversary corrupting no more than t parties does not learn s, while any \(t+1\) parties can efficiently recover s. MPC protocols based on secret sharing allow one to perform computations on such secret shared data without requiring reconstructing the data at a central location. MPC thus enables a set of distrusting parties to perform computation on their secret shared data while guaranteeing secrecy of their inputs and outputs, and correctness of the computation, also as long as no more than t parties are corrupted. Over a long period of time all parties may be corrupted and the threshold t may be violated, which is accounted for in proactively secure protocols such as Proactive Secret Sharing (PSS) and Proactive MPC (PMPC). Proactive security is an example of a cryptographically grounded and theoretically well-studied approach to realize MTD. PSS retains confidentiality even when a mobile adversary corrupts all parties over the lifetime of the secret, but no more than a threshold t during a certain window of time, called the refresh period. As an example of a proactively secure protocol that realizes an MTD strategy we overview the first PSS scheme secure in the presence of a dishonest majority (developed recently in [15]). The PSS scheme is robust and secure against \(t<n-2\) passive adversaries when there are no active corruptions, and secure but non-robust (but with identifiable aborts) against \(t<n/2-1\) active adversaries when there are no additional passive corruptions. The scheme is also secure (with identifiable aborts) against mixed adversaries controlling a combination of passively and actively corrupted parties such that if there are k active corruptions there are less than \(n-k-2\) total corruptions.