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A not unthinkable scenario is that the Bitcoin price decreases to, say, 10-20% of today's value. This would make mining less profitable, and the difficulty should adapt accordingly.

This should lead to a large amount of unused hash power, that could be made available to some majority attacker.

What are the implications of just knowing that someone (although unlikely) could be covertly using this hash power to mine a parallel and longer blockchain before difficulty adjusts, causing all sorts of trouble?

Can this scenario be remedied somehow by altering the protocol?

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I grasp your question, mining becomes unprofitable so mining hardware is turned off and the difficulty reduces. Then, an attacker coerces miners to turn on their hardware in a bid to attack the network.

This would be difficult to achieve. First, you would have to identify where the mining hardware that is turned off is, identify who it is that operated it and, make contact with them. Then, you would need to convince them to go along with your attack.

As mining is largely anonymous, at best you could potentially source some of the IP's that miners are connecting through. Nothing says that the IP a miner is connecting through is their IP and even less identifies the mining operator working behind a particular IP, and still convince enough to go along with your attack to make up 51% of the hash power. This attack is infeasible.

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This is a tautology. The hardware used to build a longer chain (a 51% attack) is hardware that is profitable to mine with. Thus you cannot use unprofitable hardware.

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  • Well, yes, but it might be profitable for the wrong reasons, such as being funded by a government that wants to undermine bitcoin.
    – Daniel R
    Commented Dec 5, 2017 at 13:16
  • That happens in a 51% attack in the sense that that hardware could be used to mine and make a profit but instead it is funded by someone else with the scope of attacking the blockchain. (You cannot build a 51% attack with obsolete hardware)
    – Emil R
    Commented Dec 5, 2017 at 13:18
  • Yes, but isn't the case today that it is hard to amass the amount of hash power that you need for a 51 % attack? If difficulty goes down drastically, hash power is there, somewhere, and can be powered up easily for malicious reasons.
    – Daniel R
    Commented Dec 5, 2017 at 13:23
  • If that were to happen that would be a 51% attack and the solution to that is a pow change.
    – Emil R
    Commented Dec 5, 2017 at 13:32
  • Do you mean a pow change such as a change of hashing algorithm rendering all mining hardware useless?
    – Daniel R
    Commented Dec 5, 2017 at 13:33

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