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I'm not looking for a number, but rather for a way to make an estimate taking things like market capitalization and the fact that mining uses ASICs into account. (I assume ASICs are relevant since their usage implies that an attacker can't get very far with general-purpose hardware from a botnet or public cloud.)

My motivation for asking this question is as follows. I assume the current rate at which energy is expended on Bitcoin mining is primarily the result of high rewards. As the 21-million supply limit is approached, mining will most likely become less profitable unless transaction fees increase immensely. At the time I'm writing this question, transaction fees only account for a small fraction of miners' profits, while minting new coins is the primary driver of profits. At the supply limit all minting stops, so transaction fees are bound to increasingly become the major (and eventually only) mining reward. It seems inevitable that mining activity will decrease as a result.

Given that mining activity is likely to decrease significantly in the medium- to long-term, how much is needed for Bitcoin to remain reasonably secure?

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  • We have a few related topics like bitcoin.stackexchange.com/q/3111/5406, bitcoin.stackexchange.com/q/876/5406, bitcoin.stackexchange.com/q/5406/5406. Does any of these cover your question?
    – Murch
    Dec 1, 2021 at 19:35
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    Those are all relevant to this question and seem to come from similar lines of thought. I think none of them specifically tries to get at where Bitcoin would become fragile if mining activity goes down, though. bitcoin.stackexchange.com/q/876 comes closest (the top-and-accepted answer from 2011 seems to boil down to "Bitcoin will collapse"), but I think this question is meaningfully different.
    – meribold
    Dec 2, 2021 at 7:54
  • Isn't the answer simply that the amount of mining activity in hash/sec must be greater than the additional amount a consortium of attackers could feasibly deploy without prior warning signs? Dec 2, 2021 at 12:35
  • That sounds like a good start. But how much could they feasibly deploy without warning signs? Additionally, maybe an existing group of mining pools could go rogue under some circumstances. I also read that a 51% attack could potentially be dealt with (bitcoin.stackexchange.com/a/1062), which would make security easier.
    – meribold
    Dec 4, 2021 at 11:37
  • Currently fees make up < 5% of mining revenue, but this has not always been the case, fees have ballooned to the point that they approached 20%, and some specific blocks crossed 50% in the past. Ultimately, as the block reward decreases this chart will, of course, balloon up to 50%, 70% and eventually 100%. Not exactly your question, but just some perspective. bitinfocharts.com/comparison/…
    – Dan
    Dec 8, 2021 at 5:57

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