  Reconfiguration of long-lived blockchain and Byzantine fault-tolerant (BFT)
systems poses fundamental security challenges. In case of state-of-the-art
Proof-of-Stake (PoS) blockchains, stake reconfiguration enables so-called
long-range attacks, which can lead to forks. Similarly, permissioned blockchain
systems, typically based on BFT, reconfigure internally, which makes them
susceptible to a similar "I still work here" attack.
  In this work, we propose BMS (Blockchain/BFT Membership Service) offering a
secure and dynamic reconfiguration service for BFT and blockchain systems,
preventing long-range and similar attacks. In particular: (1) we propose a root
BMS for permissioned blockchains, implemented as an Ethereum smart contract and
evaluate it reconfiguring the recently proposed Mir-BFT protocol, (2) we
discuss how our BMS extends to PoS blockchains and how it can reduce PoS stake
unbonding time from weeks/months to the order of minutes, and (3) we discuss
possible extensions of BMS to hierarchical deployments as well as to multiple
root BMSs.
