An N block reorg on Bitcoin with more proof-of-work appears out of nowhere with a double spend. How big does N have to be before you choose not to accept the reorg?
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... since whoever capable of taking over majority hash rate & caused a deep reorg like that can easily overwhelm your minority chain if they need to. You can also change the hash algo with the HF, which'd brick all mining hardware- might not be a safe position to be in either.

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Although it would also depend on how much hash rate is behind the reorg
If it’s a clear majority, e.g. 90% chain A vs. 10% chain B, hardforking away while reusing the same hash algo is not a long term solution. But if its 55% vs 45%, better shot at survivability.
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Regardless of whether you consider the entity malicious or not, forking (even if coordinated) is not an option. Doing so is conceding that the consensus mechanism has failed, and the new coin is unsound.
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I'm inclined to agree. The point of having consensus codified in the protocol is that you can avoid stuff like this. Even if the fork works, it would set a precedent & reduce the value proposition of Bitcoin. Otoh, deep reorgs might also indicate a failed experiment... Catch-22.
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Yes, agreed w/ both of you. I actually see this attack (eg NK govt secretly invests in ASICs) as a major vuln of PoW/motivation for PoS.
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(Caveats: 1. "Major" ≠ "likely" (it's not) - just "one of the likeliest" 2. O/c no one's denying PoS has other risks; just not this one.)
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In particular, the knowledge that this more-or-less-game-over attack is a remote possibility, will always hang over PoW chains.
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The PoS equivalent - buying a majority of stakers - can be reverted by social consensus, which disincentivizes it. See "spawn camping" etc
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While I ack. that occurrences of deep reorgs is problematic for PoW, I disagree that PoS is safer in this regard
Let's face it: an entity who's able to overcome high stock cost barriers in PoW, will have zero problem achieving controlling stake in PoS.https://medium.com/@hugonguyen/work-is-timeless-stake-is-not-554c4450ce18 …
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