The Economic Limits of Bitcoin and the Blockchain [pdf]
The paper outlines two attacks that a hash rate majority can undertake: (1) double-spending; and (2) sabotage (force a decline in exchange rate).<p>But there is a far more benign and profitable attack: collect <i>all</i> of the block reward. No need to tell anybody, and no need to upset users by double-spending. Business as usual.<p>In other words, establish a benevolent mining monopoly:<p><a href="https://bitzuma.com/posts/bitcoins-end-game-the-benevolent-mining-monopoly/" rel="nofollow">https://bitzuma.com/posts/bitcoins-end-game-the-benevolent-m...</a><p>The hash rate majority becomes an effective monopoly by censoring blocks from outside the cartel. The incentive is equal to half the block reward. If done properly and gradually, few users would notice. Nor would many care. Those who did would simply leave.<p>I suspect there's even a version of this attack in which a cartel makes public threats to censor blocks it disapproves of. This is, after all, what BIP-9 does:<p><a href="https://github.com/bitcoin/bips/blob/master/bip-0009.mediawiki" rel="nofollow">https://github.com/bitcoin/bips/blob/master/bip-0009.mediawi...</a><p>Using this approach, a cartel may be able to leverage a hash rate below 50% to 100%.<p>There's some precedent for this with UASF. The hash rate of the group threatening to censor blocks there was quite small (certainly below 50%). Depending on who you talk to, the threat of block censorship by this group was enough to shut down segwit2x.