<p dir="ltr">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.</p>
<p dir="ltr">-Marc</p>
<br><div class="gmail_quote"><div dir="ltr">On Mon, Nov 6, 2017, 22:32 Robert Taylor via bitcoin-dev <<a href="mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-dev@lists.linuxfoundation.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Forgive me if this has been asked elsewhere before, but I am trying to understand a potential failure mode of Bitcoin mining.<div><br>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.<br><br>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.</div><div><br></div><div>Robert</div></div>
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