What you describe is an example of a majority attack ("51% attack"). No
technical mechanism in Bitcoin prevents this. However in practice, miners
are not incentivized to perform this attack as it would destroy confidence
in Bitcoin, and would ultimately impact their revenues.

-Marc

On Mon, Nov 6, 2017, 22:32 Robert Taylor via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Forgive me if this has been asked elsewhere before, but I am trying to
> understand a potential failure mode of Bitcoin mining.
>
> A majority of miners can decide which valid blocks extend the chain. But
> what would happen if a majority of miners, in the form of a cartel decided
> to validly orphan any blocks made by miners outside of their group? For
> example, they could soft fork a new rule where the block number is signed
> by set of keys known only to the cartel, and that signature placed in the
> coinbase. Miners outside of the cartel would not be able to extend the
> chain.
>
> It would be immediately obvious but still valid under the consensus rules.
> What are the disincentives for such behavior and what countermeasures could
> be done to stop it and ensure mining remained permissionless? I think this
> is a valid concern because while it may not be feasible for one actor to
> gain a majority of hash alone, it is certainly possible with collusion.
>
> Robert
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>