<p>Checkpoints are features of certain clients, not inherent to the blockchain itself. So, if an attacker did re-write the chain all the way back to a popular checkpoint, he could maybe get the network to disagree. Some would go with the 'chain with most work wins' approach, and some would go with the 'chain that meets my checkpoints with the most work' rule. </p>

<p>Checkpoints do not inherently help prevent a 51% attack though. In fact, their usefulness is predicated on the assumption that a 51% attack all the way back to a checkpoint will never happen. For example, some clients skip ECDSA verification for anything older than a certain checkpoint, so it's useful to have these checkpoints because initial-block-download syncing will be faster.</p>

CC BY-SA 3.0
2015-09-16T01:20:08.223Z
Sorry! for beeing a bit harsh in my comment - thank you for that good answer!
user54512
CC BY-SA 3.0
2015-09-15T19:07:19.923Z
2015-09-15T19:07:19.923Z
morsecoder
<p>I have read sometimes in the web, that in the blockchain are checkpoints, so that an 51% could not the blockchain before that checkpoint even with over 51%. Is this true?</p>

<p>In this answer this is missing completely:
<a href="https://bitcoin.stackexchange.com/questions/38202/can-an-attacker-with-51-of-hash-power-change-old-blocks">Can an attacker with 51% of hash power change old blocks?</a></p>

<p><strong>For this question i would assume that the 51% attacker would change the client he uses any way that would help to do this attack</strong></p>

<p>Cheers!</p>

CC BY-SA 3.0
2015-09-15T19:05:03.460Z
Which question do you mean by 'this question'? If you would like to suggest an edit on the other answer, you may do so.
morsecoder
CC BY-SA 3.0
2015-09-16T01:18:58.767Z
Is it really that hard to spot? There is only one question in the post besides the hyperlink...
user54512
CC BY-SA 3.0
2015-09-16T01:53:00.933Z
user54512, I just wasn't sure if you were trying to add context for what you are looking for in the answer to 'this' question (i.e. 'Do checkpoints in the block chain...'). So, by 'this' I wasn't sure if you meant the question right here, or the question you gave a link to. Sounds like the latter.
morsecoder
<checkpoints>
Do checkpoints in the block chain help to avoid a 51% attack that rewrites the entire blockchain?
user54512
# Do checkpoints in the block chain help to avoid a 51% attack that rewrites the entire blockchain?

(Asked by: username_0 on 2015-09-15)

I have read sometimes in the web, that in the blockchain are checkpoints, so that an 51% could not the blockchain before that checkpoint even with over 51%. Is this true?

In this answer this is missing completely:
[Can an attacker with 51% of hash power change old blocks?](https://bitcoin.stackexchange.com/questions/38202/can-an-attacker-with-51-of-hash-power-change-old-blocks)

**For this question i would assume that the 51% attacker would change the client he uses any way that would help to do this attack**

Cheers!

**Tags:** checkpoints

> **Comments:**
> **username_1**: Which question do you mean by 'this question'? If you would like to suggest an edit on the other answer, you may do so.  
> **username_0**: Is it really that hard to spot? There is only one question in the post besides the hyperlink...  
> **username_1**: username_0, I just wasn't sure if you were trying to add context for what you are looking for in the answer to 'this' question (i.e. 'Do checkpoints in the block chain...'). So, by 'this' I wasn't sure if you meant the question right here, or the question you gave a link to. Sounds like the latter.  

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## 1 answers

### Answer by username_1 on 2015-09-15. Score: 4 | Accepted answer

Checkpoints are features of certain clients, not inherent to the blockchain itself. So, if an attacker did re-write the chain all the way back to a popular checkpoint, he could maybe get the network to disagree. Some would go with the 'chain with most work wins' approach, and some would go with the 'chain that meets my checkpoints with the most work' rule.

Checkpoints do not inherently help prevent a 51% attack though. In fact, their usefulness is predicated on the assumption that a 51% attack all the way back to a checkpoint will never happen. For example, some clients skip ECDSA verification for anything older than a certain checkpoint, so it's useful to have these checkpoints because initial-block-download syncing will be faster.

> **Comments:**
> **username_0**: Sorry! for beeing a bit harsh in my comment - thank you for that good answer!  

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