The Bitcoin cryptocurrency system relies on a novel distributed consensus mechanism relying on economic incentives. It is often argued that Bitcoin is “incentive-compatible” in simplified models; that is, that the scenario in which all miners follow the Bitcoin protocol is a stable Nash Equilibrium in which no miner has any incentive to defect. We introduce the notion of a bribery attack in which an attacker can purchase mining power (perhaps at a cost premium) for a short duration, using it to profit by double-spending. Explaining the lack of such attacks in practice requires significant additional modeling assumptions, demonstrating the inadequacy of current models of Bitcoin consensus.
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