  Proof-of-work (PoW) cryptocurrency blockchains like Bitcoin secure vast
amounts of money. Their operators, called miners, expend resources to generate
blocks and receive monetary rewards for their effort. Blockchains are, in
principle, attractive targets for Denial-of-Service (DoS) attacks: There is
fierce competition among coins, as well as potential gains from short selling.
Classical DoS attacks, however, typically target a few servers and cannot scale
to systems with many nodes. There have been no successful DoS attacks to date
against prominent cryptocurrencies. We present Blockchain DoS (BDoS), the first
incentive-based DoS attack that targets PoW cryptocurrencies. Unlike classical
DoS, BDoS targets the system's mechanism design: It exploits the reward
mechanism to discourage miner participation. Previous DoS attacks against PoW
blockchains require an adversary's mining power to match that of all other
miners. In contrast, BDoS can cause a blockchain to grind to a halt with
significantly fewer resources, e.g., 21% as of March 2020 in Bitcoin, according
to our empirical study. We find that Bitcoin's vulnerability to BDoS increases
rapidly as the mining industry matures and profitability drops. BDoS differs
from known attacks like Selfish Mining in its aim not to increase an
adversary's revenue, but to disrupt the system. Although it bears some
algorithmic similarity to those attacks, it introduces a new adversarial model,
goals, algorithm, and game-theoretic analysis. Beyond its direct implications
for operational blockchains, BDoS introduces the novel idea that an adversary
can manipulate miners' incentives by proving the existence of blocks without
actually publishing them.
