41 Pages Posted: Date Written: February 15, 2024 Abstract Much has been hypothesized and feared about 51% attacks on Bitcoin and 34% attacks on Ethereum. However, the costs and benefits associated with perpetrating these attacks remain a mystery. In this paper, we present a novel model to quantify the costs to breach Byzantine fault tolerance thresholds in Bitcoin and Ethereum. We introduce a new metric called Total Cost to Attack (TCA) which encompasses the operational and capital expenditures ...