Tortoise and Hares Consensus: The Meshcash Framework for Incentive-Compatible, Scalable Cryptocurrencies

We propose Meshcash, a protocol for implementing a permissionless ledger (blockchain) via proofs of work, suitable for use as the underlying consensus mechanism of a cryptocurrency. Unlike most existing proof-of-work based consensus protocols, Meshcash does not rely on leader-election (e.g., the single miner who managed to extend the longest chain). Rather, we use ideas from traditional (permissioned) Byzantine agreement protocols in a novel way to guarantee convergence to a consensus from any starting state. Our construction combines a local “hare” protocol that guarantees fast consensus on recent blocks (but doesn’t, by itself, imply irreversibility) with a global “tortoise” protocol that guarantees irreversibility. Our global protocol also allows the ledger to “self-heal” from arbitrary violations of the security assumptions, reconverging to consensus after the assumptions hold again.