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+ FWIW, 51% of the mining power only allows you to double-spend, not arbitrarily modify the existing ledger.

+ Since Libra uses a HotStuff variant, you need to control 2/3+ of the voting power to violate safety, i.e., double-spend. In exchange, however, you only need to control 1/3+ to halt progress (liveness).

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If you can halt progress, you can make any particular person's holdings impossible to spend (if you can identify them), by refusing to include any blocks that include transactions from them. That isn't quite as lucrative as simply confiscating someone's holdings, but it's close; you can demand a ransom of a sufficient fraction of their holdings, perhaps 5% to 50%, depending on human nature.

I wouldn't say it is quite the same threat model, but certainly a powerful attack vector if you happen to control 1/3+ of the voting power. In the unlikely event that someone does compromise 1/3+ of the voting power, the remaining validators can always hard fork to a new quorum, though this is an expensive and highly synchronized affair.

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