<h2>51% attack</h2>
<p>My understanding is that the 51% attack only requires more than 50% of the total hashpower. It doesn't require control over a majority of nodes.</p>
<p>Nodes each choose the chain with most work as their active chain. If one miner can produce more work than the rest of the miners combined, they can produce a chain with more work than any alternative chain.</p>
<h2>Attack detection and reaction</h2>
<p>So far as I know, the broad ecosystem of wallet software is not able to explicitly detect an attack of this sort and automatically apply countermeasures consistently.</p>
<p>the human operator of the software may be able to apply some measures such as block invalidation, but I believe every person operating a wallet or other node would have to do this manually. It is likely that people who choose a wallet for ease of use do not actively monitor bitcoin news and would not react in a timely manner. Since there is no established system of coordination between ordinary users, the network would presumably fragment. At least initially. It seems likely this would develop into a major issue with significant long term technical, community and reputational consequences.</p>
<p>The countermeasure to the 51% attack is not a reactive one but more of a preemptive one - it is considered very difficult for one person or organisation to suddenly amass 51% of the network hashpower.</p>
<p>I guess if the more vigilant members of the Bitcoin community detect some gradual centralisation of hashpower rising towards 50%, the response is possibly to engage more people in diversifying hashpower and perhaps develop new measures that aren't currently automatic.</p>

CC BY-SA 4.0
2022-06-13T17:11:23.030Z
and even if nodes know that one party is attacking it they choose the longest chain? Can't they choose a smaller chain in order to prevent the attack? In other words, choosing the longest chain is a part of the protocol? or just a sign to check which chain has the chance to more healthy?
Amir reza Riahi
CC BY-SA 4.0
2022-06-13T17:13:50.900Z
Another point, adding a block to the block chain at the final step is nodes' job, is this ok? so if nodes detect that one party has gained 51 percentage of the hashpower they can reject its blocks, so their chain would not become the longest one. Is this conclusion right?
Amir reza Riahi
CC BY-SA 4.0
2022-06-13T17:22:26.167Z
@AmirrezaRiahi Yes, yes they can. But do they want to? The goal of the protocol was to introduce an economic incentive to avoid misbehaving parties. If that incentive doesn't work (exemplified by the occurrence of an attack), it means the assumptions underlying the system were wrong, and maybe the conclusion is that PoW in general just doesn't work. You can't patch it up by replacing it with the consensus of users overriding the hashrate - if that was ok in the first, why were you using PoW in the first place, as its goal is specifically avoiding the need for trusted decision makers.
Pieter Wuille
CC BY-SA 4.0
2022-06-13T17:24:27.440Z
I think the possibility of the users of the system deciding to reject a hashrate majority consensus-valid chain does factor in to the equation, but only to the extent that it is hardly more than a mutually assured destruction mechanism. Yes, if absolutely need be, the users can decided to reject the PoW-decided chain, but it likely be a very catastrophic event (only slightly less bad than the alternative of the attack going through), so much that it becomes undesirable for able parties to even attempt the attack.
Pieter Wuille
CC BY-SA 4.0
2022-06-13T17:45:52.370Z
@PieterWuille I think in this situation the attack would be much harder and more costly. Because it have to control both nodes and miners other than only miners. Think of PoS which removes miners and has only nodes (I'm not very familiar with PoS in high detail but I just know the concept) and the attack only needs to control 51 percent of stakers. I think this is why PoW is much safer than Pos.
Amir reza Riahi
CC BY-SA 4.0
2022-06-13T17:48:26.933Z
In PoS stakers and nodes are still separate groups. The alternative with just nodes is traditional consensus algorithms like PBFT - but you can't compare these algorithms, as they operate in a very different context (needs trusted parties!).
Pieter Wuille
CC BY-SA 4.0
2022-06-13T17:52:56.837Z
@PieterWuille why this would be catastrophic? Because there is no secondary chain to consensus?
Amir reza Riahi
CC BY-SA 4.0
2022-06-13T17:53:40.343Z
@PieterWuille I'm not familiar to PoS, would explain why it needs trusted party or a link for explanation?
Amir reza Riahi
CC BY-SA 4.0
2022-06-13T18:04:30.513Z
The Bitcoin ecosystem (humans) needing to come to consensus to discard the result of PoW seems catastrophic to me. Yes, it may be needed in order to avoid a 51% attacker preventing progress to the chain for example, but it's making it evident that PoW just doesn't work, as its entire point of existence is avoiding the need for humans to decide which version of history is the correct one. That combined with the fact that it inevitably requires political consensus about this decision to be made, which would raise questions about whether it can happen again.
Pieter Wuille
CC BY-SA 4.0
2022-06-13T18:10:45.160Z
So to be clear my point here is: when we're talking about Bitcoin nodes, we're assuming nodes which faithfully follow the most-work valid chain, even if that chain is produced by an attacker in some form. While yes, indeed, node operators could jointly decide to discard that result and switch over to another chain, and functionality for doing so even exists in Bitcoin Core (the `invalidateblock` RPC), I'd argue that if you're talking about a system where this mechanism actually needs to be used and is relied upon, it's not anything like Bitcoin as we know it anymore.
Pieter Wuille
CC BY-SA 4.0
2022-06-13T17:01:20.947Z
2022-06-13T19:36:46.320Z
2022-06-13T19:36:46.320Z
RedGrittyBrick
<p>Is this true that miners alone cannot do 51 attack even one or a group of miners posses &gt;51% of hashrate of the network?</p>
<p>As far as I know, every block should be accepted by nodes to be added to the blockchain. In other words, nodes are responsible for maintaining the whole blockchain, so they can choose which blockchain is the best one.</p>
<p>If a group of miners decide to perform an attack such as double spending a coin, nodes can block those miners and so their blocks would not be a part of the blockchain. So any party who owns more than 51% of hashrate can not do any malicious thing, unless it controls 51% of both nodes and miners. Is this claim correct?</p>

CC BY-SA 4.0
2023-02-26T13:01:43.610Z
I think nodes without economic relevance (or with redundant economic relevance) don't really cause 51% attacks, but eclipse attacks.
Mercedes
<nodes><hashpower><majority-attack>
Can miners single handedly perform 51 attack toward the network?
Amir reza Riahi
# Can miners single handedly perform 51 attack toward the network?

(Asked by: username_0 on 2022-06-13)

Is this true that miners alone cannot do 51 attack even one or a group of miners posses >51% of hashrate of the network?

As far as I know, every block should be accepted by nodes to be added to the blockchain. In other words, nodes are responsible for maintaining the whole blockchain, so they can choose which blockchain is the best one.

If a group of miners decide to perform an attack such as double spending a coin, nodes can block those miners and so their blocks would not be a part of the blockchain. So any party who owns more than 51% of hashrate can not do any malicious thing, unless it controls 51% of both nodes and miners. Is this claim correct?

**Tags:** nodes, hashpower, majority-attack

> **Comments:**
> **username_1**: I think nodes without economic relevance (or with redundant economic relevance) don't really cause 51% attacks, but eclipse attacks.  

---

## 1 answers

### Answer by username_2 on 2022-06-13. Score: 2 | Accepted answer

## 51% attack

My understanding is that the 51% attack only requires more than 50% of the total hashpower. It doesn't require control over a majority of nodes.

Nodes each choose the chain with most work as their active chain. If one miner can produce more work than the rest of the miners combined, they can produce a chain with more work than any alternative chain.

## Attack detection and reaction

So far as I know, the broad ecosystem of wallet software is not able to explicitly detect an attack of this sort and automatically apply countermeasures consistently.

the human operator of the software may be able to apply some measures such as block invalidation, but I believe every person operating a wallet or other node would have to do this manually. It is likely that people who choose a wallet for ease of use do not actively monitor bitcoin news and would not react in a timely manner. Since there is no established system of coordination between ordinary users, the network would presumably fragment. At least initially. It seems likely this would develop into a major issue with significant long term technical, community and reputational consequences.

The countermeasure to the 51% attack is not a reactive one but more of a preemptive one - it is considered very difficult for one person or organisation to suddenly amass 51% of the network hashpower.

I guess if the more vigilant members of the Bitcoin community detect some gradual centralisation of hashpower rising towards 50%, the response is possibly to engage more people in diversifying hashpower and perhaps develop new measures that aren't currently automatic.

> **Comments:**
> **username_0**: and even if nodes know that one party is attacking it they choose the longest chain? Can't they choose a smaller chain in order to prevent the attack? In other words, choosing the longest chain is a part of the protocol? or just a sign to check which chain has the chance to more healthy?  
> **username_0**: Another point, adding a block to the block chain at the final step is nodes' job, is this ok? so if nodes detect that one party has gained 51 percentage of the hashpower they can reject its blocks, so their chain would not become the longest one. Is this conclusion right?  
> **username_3**: @username_0 Yes, yes they can. But do they want to? The goal of the protocol was to introduce an economic incentive to avoid misbehaving parties. If that incentive doesn't work (exemplified by the occurrence of an attack), it means the assumptions underlying the system were wrong, and maybe the conclusion is that PoW in general just doesn't work. You can't patch it up by replacing it with the consensus of users overriding the hashrate - if that was ok in the first, why were you using PoW in the first place, as its goal is specifically avoiding the need for trusted decision makers.  
> **username_3**: I think the possibility of the users of the system deciding to reject a hashrate majority consensus-valid chain does factor in to the equation, but only to the extent that it is hardly more than a mutually assured destruction mechanism. Yes, if absolutely need be, the users can decided to reject the PoW-decided chain, but it likely be a very catastrophic event (only slightly less bad than the alternative of the attack going through), so much that it becomes undesirable for able parties to even attempt the attack.  
> **username_0**: @username_3 I think in this situation the attack would be much harder and more costly. Because it have to control both nodes and miners other than only miners. Think of PoS which removes miners and has only nodes (I'm not very familiar with PoS in high detail but I just know the concept) and the attack only needs to control 51 percent of stakers. I think this is why PoW is much safer than Pos.  
> **username_3**: In PoS stakers and nodes are still separate groups. The alternative with just nodes is traditional consensus algorithms like PBFT - but you can't compare these algorithms, as they operate in a very different context (needs trusted parties!).  
> **username_0**: @username_3 why this would be catastrophic? Because there is no secondary chain to consensus?  
> **username_0**: @username_3 I'm not familiar to PoS, would explain why it needs trusted party or a link for explanation?  
> **username_3**: The Bitcoin ecosystem (humans) needing to come to consensus to discard the result of PoW seems catastrophic to me. Yes, it may be needed in order to avoid a 51% attacker preventing progress to the chain for example, but it's making it evident that PoW just doesn't work, as its entire point of existence is avoiding the need for humans to decide which version of history is the correct one. That combined with the fact that it inevitably requires political consensus about this decision to be made, which would raise questions about whether it can happen again.  
> **username_3**: So to be clear my point here is: when we're talking about Bitcoin nodes, we're assuming nodes which faithfully follow the most-work valid chain, even if that chain is produced by an attacker in some form. While yes, indeed, node operators could jointly decide to discard that result and switch over to another chain, and functionality for doing so even exists in Bitcoin Core (the `invalidateblock` RPC), I'd argue that if you're talking about a system where this mechanism actually needs to be used and is relied upon, it's not anything like Bitcoin as we know it anymore.  

---

