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I often read that checkpoints protect the network from a 51% attack because an attacker cannot reverse transactions made before the last checkpoint.

How exactly does this checkpoint mechanism work? And who creates the checkpoints?

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The checkpoints are hard coded into the standard client. The concept is, that the standard client will accept all transactions up to the checkpoint as valid and irreversible. If anyone tries to fork the blockchain starting from a block before the checkpoint, the client will not accept the fork. This makes those blocks "set in stone".

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    So a checkpoint is usually created everytime the client is updated? – nmat Nov 2 '11 at 1:05
  • As far as I know, yes, but it is all in the hands of the developers. – ThePiachu Nov 2 '11 at 1:12
  • Would there be any way to make this behavior official? Something like "If the block was published to the network over a year ago, it is set in stone" which does not rely on the developers? This seem to be one of the few places where Bitcoin developers have a lot of authority, and it seems they generally work to keep that authority minimal. – Daniel H Aug 21 '12 at 22:02
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    @DanielH I don't see why some automatic algorithm couldn't be implemented that would be choosing suitable checkpoints during each code update. I guess you can propose such a feature on the forum or the github project page. – ThePiachu Aug 22 '12 at 7:02
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    We really plan to get rid of checkpoints, or at least significantly reduce their power. They don't actually add any security (if they would actually affect the chain, the system is already broken). – Pieter Wuille Sep 14 '14 at 20:47
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Update on this as of time of writing, just to clarify more specifically upon the other answer: dependence on checkpoints in the security model has been significantly reduced, they are only used in one very specific case now. That case is just to ignore forks from the chain early on, before the most recently seen checkpoint. When a node has seen a block it recognises as a checkpoint, any further blocks received below that height will be ignored.

You can see this here: https://github.com/bitcoin/bitcoin/blob/0dfc25f82a01d9fec26380d95915df31e1fe2c02/src/validation.cpp#L3107

It is a long term goal of removing the checkpoints entirely, because they are a source of confusion over the security model and power the developers have. But currently the role they serve is to prevent low difficulty header flooding attacks, and there has been no alternative solution proposed yet (that I know of).

0

Bitcoin consensus algo is probabilistic (rather than deterministic) so in theory there is always the possibility of reversibility. The checkpoint gets around that by setting how far back it with even consider looking for blocks to verify.

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