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I am having a question regarding forks. We had a discussion about attacks on bitcoin in class recently. One of the attacks was about how an attacker gains some advantage if he is able to mine a block and hold it back. He would release the block only if another miner finds a block in order to create a fork.

After that we had been discussing if it would be possible to create a fork somewhere in the past. Let's say an attacker decides now I would like to fork the blockchain 2 days ago. He starts to mine blocks from the time in the past and tries to create a longer chain. As soon he has a longer chain, he releases it and hope that the chain forks.

My question is now: Is that actually possible? Is it possible to fork the blockchain somewhere in the past if we are able to create a longer chain and release it later?

I tried to find some information but all I get is hard fork information. Thanks in advance

  • The answer by Pieter Wuille below is correct. I only want to say that we usually use the word "fork" when it changes consensus rules. Reorganizing the chain is called 51-attack (when somebody gains profit) or just reorganizing the mainchain :) – amaclin Jul 11 '17 at 9:34
  • I understand your comment but what exactly do you mean with consensus rules? If 2 miners create a block in the same time, there would be a fork. Is this a change of consensus rules? – Donut Jul 11 '17 at 12:57
  • yes, we can call this situation as a fork. – amaclin Jul 11 '17 at 14:41
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It is absolutely possible - but not necessarily feasible - for an attack to branch off in the past, and create a new majority chain from that.

However, it effectively means that you or the miners who are participating in this attack need to mine faster than the rest of the network combined. Otherwise, you'll never be able to 'outrun' them, and the further back you start, the less likely it is you'll ever catch up.

For this reason, this is also called a 51% attack. For anything further back than a few blocks, you almost certainly need a majority of the hashrate to succeed.

  • I know the 51% attack but you would call even this situation a 51% attack? – Donut Jul 11 '17 at 8:15
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    Yes, absolutely. It's a majority hashrate attack. – Pieter Wuille Jul 11 '17 at 8:22

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