Correctness and Fairness of Tendermint-core Blockchains
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Tendermint-core blockchains offer strong consistency (no forks) in an open system relying on two ingredients (i) a set of validators that generate blocks via a variant of Practical Byzantine Fault Tolerant (PBFT) consensus protocol and (ii) a rewarding mechanism that dynamically selects nodes to be validators for the next block via proof-of-stake, a non-energy consuming alternative of proof-of-work. It is well-known that in those open systems the main threat is the tragedy of commons that may yield the system to collapse if the rewarding mechanism is not adequate. At minima the rewarding mechanism must be f air, i.e. distributing the rewards in proportion to the merit of participants. The contribution of this paper is twofold. First, we provide a formal description of Tendermint-core protocol and we prove that in eventual synchronous systems (i) it verifies a variant of one-shot consensus for the validation of one single block and (ii) a variant of the repeated consensus problem for multiple blocks. Our second contribution relates to the fairness of Tendermint rewarding mechanism. We prove that Tendermint rewarding is not fair. However, a small twist in the protocol makes it eventually fair. Additionally, we prove that there exists an (eventual) fair rewarding mechanism in repeated consensus-based blockchains if and only if the system is (eventually) synchronous.
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