Private Proof-of-Stake Blockchains using Differentially-Private Stake Distortion

Safety, liveness, and privacy are three critical properties for any private proof-of-stake (PoS) blockchain. However, prior work (SP'21) has shown that to obtain safety and liveness, a PoS blockchain must in theory forgo privacy. Specifically, to ensure safety and liveness, PoS blockchains elect parties based on stake proportion, potentially exposing a party's stake even with private transaction processing. In this work, we make two key contributions. First, we present the first stake inference attack applicable to both deterministic and randomized PoS with exponentially less running time in comparison with SOTAdesigns. Second, we use differentially private stake distortion to achieve privacy in PoS blockchains, and design two stake distortion mechanisms that any PoS protocol can use. We further evaluate our proposed methods using Ethereum 2.0, a widely-recognized PoS blockchain in operation. Results demonstrate effective stake inference risk mitigation, reasonable privacy, and preservation of essential safety and liveness properties.