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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4 Answers
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".
answered Oct 29, 2011 at 20:18
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1So a checkpoint is usually created everytime the client is updated?– nmatCommented Nov 2, 2011 at 1:05
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1Would 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 HCommented Aug 21, 2012 at 22:02
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1@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. Commented Aug 22, 2012 at 7:02
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11We 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). Commented Sep 14, 2014 at 20:47
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1@PieterWuille@ThePiachu: I didn't find any checkpoints hardcoded in main.cpp, checkpoints.h, checkpoints.cpp...Can you tell where the first checkpoint is placed? Commented May 31, 2015 at 21:05
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).
answered Feb 14, 2018 at 23:41
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so every node maintains a list of checkpoints? who decides which blocks get added to the list of checkpoints?– fdzsfhaSCommented Mar 14, 2018 at 2:55
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3@Auburn the list of checkpoints is hardcoded here: github.com/bitcoin/bitcoin/blob/master/src/… No new checkpoints are ever added anymore, the last was added on Jul 16, 2014 Commented Mar 14, 2018 at 5:55
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2> there has been no alternative solution proposed yet (that I know of). Sure there have been. E.g. increasing the minimum difficulty for any alternative chain. Or changing the logic used to fetch headers to be able to explorer a competing fork without saving it. Commented Oct 5, 2021 at 7:24
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52022 update: an alternative technique to prevent low difficulty header flooding that does not rely on checkpoints (only on the even weaker minchainwork value) has been merged: github.com/bitcoin/bitcoin/pull/25717 Commented Feb 1, 2023 at 22:33
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.
AIK there‘s one big use case so far for checkpoints and there‘s actually no golden path to achieve that otherwise.
Let‘s say one has a really really lot of computing power and intentionally or accidentally starts mining at a very low difficulty. Then all nodes were „forced by the rules of consensus“ to check that blocks for validity even there is praktically low to now chance they catch up the entire blockchain. On a time critical node that kind of behaviour could at least create disturbance.
Checkpoints are a tool against that. The node will only check higher blocks than the last checkpoint anf hence „focus on real (trans)actions on the net“.
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1This hasn't been the case since the introduction of headers-first synchronization in 2014 (github.com/bitcoin/bitcoin/pull/4468), which meant that blocks won't even be fully downloaded in case they're not on a chain of headers that leads to a tip with a promising amount of work. This did leave open a weaker version of that (header spam), but that has been addressed since the introduction of headers presync (github.com/bitcoin/bitcoin/pull/25717) last year. Commented Feb 1, 2023 at 21:20
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Thanks for the deeper insights @PieterWuille it is hard to keep pace with the developments. I appreciate to get deeper understanding. Well, checkpoints are now obsolete. Are they already removed? The Go implementation btcsuite/btcd still supports it. Commented Feb 3, 2023 at 0:08
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1They're not removed no; there is little harm in leaving them around, but they'll likely be removed from Bitcoin Core at some point. I also don't think btcd has anything like Core's header presync logic, but whether particular software implements and/or keeps something like these checkpoints is their own decision. Commented Feb 3, 2023 at 1:41