COBRA: Dynamic Proactive Secret Sharing for Confidential BFT Services

Byzantine Fault-Tolerant (BFT) State Machine Replication (SMR) is a classical paradigm for implementing trustworthy services that has received renewed interest with the emergence of blockchains and decentralized infrastructures. A fundamental limitation of BFT SMR is that it provides integrity and availability despite a fraction of the replicas being controlled by an active adversary, but does not offer any confidentiality protection. Previous works addressed this issue by integrating secret sharing with the consensus-based framework of BFT SMR, but without providing all features required by practical systems, which include replica recovery, group reconfiguration, and acceptable performance when dealing with a large number of secrets. We present COBRA, a new protocol stack for Dynamic Proactive Secret Sharing that allows implementing confidentiality in practical BFT SMR systems. COBRA exhibits the best asymptotic communication complexity and optimal storage overhead, being able to renew 100k shares in a group of ten replicas $5 \times$ faster than the current state of the art.