Ledger Combiners for Fast Settlement

Blockchain protocols based on variations of the longest-chain rule—whether following the proofof-work paradigm or one of its alternatives—su er from a fundamental latency barrier. This arises from the need to collect a su cient number of blocks on top of a transaction-bearing block to guarantee the transaction’s stability while limiting the rate at which blocks can be created in order to prevent security-threatening forks. Our main result is a black-box security-amplifying combiner based on parallel composition ofm blockchains that achievesΘ(m)-fold security ampli cation or, equivalently,Θ(m)-fold reduction in latency. Our construction breaks the latency barrier to achieve, for the rst time, a worst-case constant-time-settlement ledger based purely on Nakamoto longest-chain consensus: Transaction settlement can be accelerated to a constant multiple of block propagation time with negligible error. Operationally, our construction shows how to view any family of blockchains as a uni ed, virtual ledger without requiring any coordination among the chains or any new protocol metadata. Users of the system have the option to inject a transaction into a single constituent blockchain or—if they desire accelerated settlement—all of the constituent blockchains. Our presentation and proofs introduce a new formalism for reasoning about blockchains, the dynamic ledger, and articulate our constructions as transformations of dynamic ledgers that amplify security. We additionally illustrate the versatility of this formalism by presenting a class of robustcombiner constructions for blockchains that can protect against complete adversarial control of a minority of a family of blockchains.