What's a 51% attack?

[Pokapoka124]

Hey guys! I'd like to learn more about what a 51% attack means in the crypto space. Etherum classic had three 51% attack this year and nobody seem to budge. Is the blockchain network really safe from this sort of attacks?

https://www.coindesk.com/markets/2020/09/08/crypto-investors-have-ignored-three-straight-51-attacks-on-etc/
link

[odolvlobo]

In a 51% attack, the attacker must gain more than 50% of the hashing power. With this majority, the attacker has the ability to do two things:

1. Add and remove transactions from recent blocks, effectively rewriting history. The main purpose is to invalidate and nullify past transactions that have already been confirmed.
2. Control which transactions can be confirmed, which includes a DOS attack preventing any transactions from being confirmed.

[btc_angela]

You can take a lot on the below threads:

[1] 51% Attack
[2] 51% attack on Bitcoin
[3] How exactly would a 51% attack work?
[4] Bitmain 51% Attack? Would they do such if they get the hash? [POLL]

Please take note though that some of the threads are as old as 2011, meaning this topic has been discussed many times in the community.

[OcTradism]

You can get some cost estimates from https://www.crypto51.app/. These estimates are estimated from network total hashrates and Nicehash cost for 1 hour. Cost to attack is big for one hour and to get a successful attack, it can require more than 1 hour.

There are times that some big mining pools that were doubt to be controlled by same company (Bitman) with net-hashrates higher than 51% but they did not do any attack on the network. Bitman decides to split their pools. I think the attack (on bitcoin network) can only be done by pool owners but I don't see how they will take risk and do so.

Bitcoin network is decentralized and many full nodes will detect the attack and the community, exchange will react quickly to stop it. The probability to run successful attacks for long time is zero. It's impossible.
Coinbase’s perspective on the recent Ethereum Classic (ETC) double spend incidents

During the first attack, Parity (OpenEthereum) nodes running in pruned mode ignored the attacking blocks, while the rest of the network accepted the attacking blocks as valid, resulting in a partitioned network of pruned Parity nodes vs the rest of the network

[pooya87]

Etherum classic had three 51% attack this year and nobody seem to budge.

That is because nobody cares about altcoins at all. They all want to make profit from trading altcoins, in fact majority of people in crypto space don't even own any altcoins! They have custodial accounts that may hold some coins on their behalf (ie holding on exchanges) so that they can dump it at a moment's notice.
They don't provide any utility either so nobody cares if the coin is insecure or its blockchain mutable, they care about how much pump and dump is going to happen.

[PrimeNumber7]

Etherum classic had three 51% attack this year and nobody seem to budge.

That is because nobody cares about altcoins at all. They all want to make profit from trading altcoins, in fact majority of people in crypto space don't even own any altcoins! They have custodial accounts that may hold some coins on their behalf (ie holding on exchanges) so that they can dump it at a moment's notice.

Altcoins are vulnerable to 51% attacks because they share mining algos with other altcoins (or with bitcoin), and the hashrate being utilized towards the altcoin is less than a majority of the hashrate for that algo.

In order to perform a 51% attack on bitcoin, an attacker would need to invest in mining equipment with mining capacity equal to >50% of the network hashrate. Once a 51% attacker executes his attack against bitcoin, the value of his mining equipment would decline substantially. This is not true for most altcoins.

They don't provide any utility either so nobody cares if the coin is insecure or its blockchain mutable, they care about how much pump and dump is going to happen.

While this is often true for altcoins, this has nothing to do with their vulnerability to a 51% attack.

[pooya87]

They don't provide any utility either so nobody cares if the coin is insecure or its blockchain mutable, they care about how much pump and dump is going to happen.

While this is often true for altcoins, this has nothing to do with their vulnerability to a 51% attack.

I was pointing out the fact the reason why there sometimes isn't any market reaction to such attacks and why people don't care. But it kind of has a lot to do with 51% attacks, lack of utility means lack of interest hence lower price which translates into low profit in mining it hence lower hashrate which is the reason for vulnerability to such attacks.
If they were useful, they would have had higher price and more miners and higher hashrate which would have made the attack a lot harder.

Another major reason is the incompetency of their developers. As you pointed out majority of them are copy coins and those who copy another project have very little understanding of what a 51% attack is let alone be capable of thinking about solutions.

[Oshosondy]

There are many altcoins like bitcoin gold and ethereum classic that have suffered 51% attack before, you can read the links given above to understand what 51% attack is, but to just make it brief, it is an attack carried out by hackers to compromise the blockchain of a particular coin, they have their own miners that is powerful and make it centralized in a way to steal coins from people, in a 51% attack like ethereum classic attack (the same way it happens on other coins blockchain too), you can have ethereum classic on you wallet but all gone because of the attack.

Is the blockchain network really safe from this sort of attacks?

Did you mean the bitcoin blockchain? Bitcoin blockchian is safe from 51% attack. When the blockchain was not so strong like this before, no attack has occured on bitcoin blockchain before not to talk of when bitcoin blockchain is stronger and getting stronger.

[ranochigo]

they have their own miners that is powerful and make it centralized in a way to steal coins from people, in a 51% attack like ethereum classic attack (the same way it happens on other coins blockchain too), you can have ethereum classic on you wallet but all gone because of the attack.

51% attacks cannot steal other people's coins. The most they could do is to prevent their transactions from confirming. Other than that, double spending requires the attacker to have the ability to create an alternative transaction which would require the private keys to create the transaction.

Did you mean the bitcoin blockchain? Bitcoin blockchian is safe from 51% attack. When the blockchain was not so strong like this before, no attack has occured on bitcoin blockchain before not to talk of when bitcoin blockchain is stronger and getting stronger.

It's not safe. The reason why Bitcoin isn't attacked is just simply a result of the cost-benefit involved. The cost is way too high for most to execute and for those that could, the profits gained from such attack wouldn't outweigh the cost. The blockchain didn't get stronger per se, it just makes such attacks less economical.

[CryptocurencyKing]

Obviously i need more education on hashrate and some related calculations but, from the answers provided in this link:

[1] 51% Attack

, a 51% attack is a could have been a big deal but then, it isn't, at least not much due to the fact that some measures has been put in place without even looking at it as a measure in the first place. That is,

In order to get 51%, you would need roughly 2 million s17, taking into account that while you set them up some miners will drop because of the reward getting smaller, maybe you can do it with 1,2 - 1,5 million, but I doubt Bitmain or Cannan or all the manufacturer together would be able to produce that in a short period of time.

The fact that Bitman cant produce as many as 1.5 - 2.5 million ASICs within a short time to aid this anti bitcoin and hackers from harming the network is a very nice big step. This is to tell us how sable bitcoin really is and its good news. else having anti bitcoin users or bodies in charge of mining power would have been a big blow to the system but, it makes no sense for a government to coordinate attacks on that which empowers its citizens. Bitcoin and altcoins actually helps more than they think. with it not in place simply means the once self employed citizens turning all their attention towards the government to provide all needed amenities.
Should bitcoin not be in place, wont other altcoins come up to fill in the gap?
And why aren't government turning their attention towards that which is not stable and more harmful like ponzi schemes?

[OgNasty]

I keep reading in this thread that you need >50% of a network’s hashrate to perform a 51% attack. This is not true. Having >50% of the network’s hashrate only increases your odds of being successful. A bit of good luck could reduce the needed hashrate greatly.

[kakonhat]

Hey guys! I'd like to learn more about what a 51% attack means in the crypto space. Etherum classic had three 51% attack this year and nobody seem to budge. Is the blockchain network really safe from this sort of attacks?

If you would like to learn about a 51% attack then you should learn what is PoS and PoW. Even you must learn how to work mining in PoS and PoW. After getting those network idea then you will easily understand what is a 51% attack. To learn those network you should do google or Youtube.

[Welsh]

I keep reading in this thread that you need >50% of a network’s hashrate to perform a 51% attack. This is not true. Having >50% of the network’s hashrate only increases your odds of being successful. A bit of good luck could reduce the needed hashrate greatly.

Yeah, you could deem having less than 50% of the networks hashrate a success if you simply wanted to disrupt the network, its probably fairly achievable for a lot of people out there, but as discussed several times over by now, its simply not worth it, especially on a prolonged basis.

51% attacks cannot steal other people's coins. The most they could do is to prevent their transactions from confirming. Other than that, double spending requires the attacker to have the ability to create an alternative transaction which would require the private keys to create the transaction.

Pretty much this. There is no great benefit of doing it on a prolonged basis, and all it ends up is the attacker losing money, while not benefiting other than maybe bragging rights of disrupting the network? As soon as the attack stops, the network can get back to "normal", and get through the backlog which might have been created.

[PrimeNumber7]

they have their own miners that is powerful and make it centralized in a way to steal coins from people, in a 51% attack like ethereum classic attack (the same way it happens on other coins blockchain too), you can have ethereum classic on you wallet but all gone because of the attack.

51% attacks cannot steal other people's coins. The most they could do is to prevent their transactions from confirming. Other than that, double spending requires the attacker to have the ability to create an alternative transaction which would require the private keys to create the transaction.

An attacker could send coin to someone, receive something of value in return, wait n confirmations, and double-spend the transaction via an alternate blockchain. I would consider this to be stealing.

I keep reading in this thread that you need >50% of a network’s hashrate to perform a 51% attack. This is not true. Having >50% of the network’s hashrate only increases your odds of being successful. A bit of good luck could reduce the needed hashrate greatly.

Yeah, you could deem having less than 50% of the networks hashrate a success if you simply wanted to disrupt the network, its probably fairly achievable for a lot of people out there, but as discussed several times over by now, its simply not worth it, especially on a prolonged basis.

Depending on how severe the 51% attack is, an attacker could direct their own equipment towards a pool they control and get miners to mine on their pool via higher actual mining earnings. The pool would have higher mining earnings because it wouldn't have its blocks orphaned.

[Cryptoababe]

In a 51% attack, the attacker must gain more than 50% of the hashing power. With this majority, the attacker has the ability to do two things:

1. Add and remove transactions from recent blocks, effectively rewriting history. The main purpose is to invalidate and nullify past transactions that have already been confirmed.
2. Control which transactions can be confirmed, which includes a DOS attack preventing any transactions from being confirmed.

This means hackers can't have access to any funds but transaction history right? Or will hacker be able to take funds too in this 51% attack?

[ranochigo]

An attacker could send coin to someone, receive something of value in return, wait n confirmations, and double-spend the transaction via an alternate blockchain. I would consider this to be stealing.

Yes. But you have to have control of the inputs that were used in the transaction to double spend. Quote appears to say that you can just randomly siphon funds from other addresses.

Depending on how severe the 51% attack is, an attacker could direct their own equipment towards a pool they control and get miners to mine on their pool via higher actual mining earnings. The pool would have higher mining earnings because it wouldn't have its blocks orphaned.

At the end of the day, miners probably won't support the attacker. After all, the miners would be losing money after any 51% attack. The rational thing miners would do is to try to mitigate the 51% attack or to switch off their equipment. Helping the attacker would just be burning through your cash. 

Honestly, if someone were to 51% attack Bitcoin, I doubt they will be honest enough to let me get the block rewards.

[pooya87]

I keep reading in this thread that you need >50% of a network’s hashrate to perform a 51% attack. This is not true. Having >50% of the network’s hashrate only increases your odds of being successful. A bit of good luck could reduce the needed hashrate greatly.

51% is used because this type of attack is not just about finding and replacing a single block, the attacker has to be able to make their own chain longer than the rest and it is increasingly difficult to do the less hashrate the attacker has. In other words they may get lucky and find one block fast but the second block isn't going to be as easy and the third could be near impossible to find faster.

[PrimeNumber7]

Depending on how severe the 51% attack is, an attacker could direct their own equipment towards a pool they control and get miners to mine on their pool via higher actual mining earnings. The pool would have higher mining earnings because it wouldn't have its blocks orphaned.

At the end of the day, miners probably won't support the attacker. After all, the miners would be losing money after any 51% attack. The rational thing miners would do is to try to mitigate the 51% attack or to switch off their equipment. Helping the attacker would just be burning through your cash. 

I was referring to something along the lines of the attacker would occasionally orphan a block they don't like or refuse to confirm transactions that don't meet certain criteria.

I could see the miners revolting against a 51% attack that double spends transactions on any large scale, or orphans a chain of many blocks.

[ranochigo]

I was referring to something along the lines of the attacker would occasionally orphan a block they don't like or refuse to confirm transactions that don't meet certain criteria. 

I think that's quite a stretch to say that miners will support deliberate censorship and undermine the foundation that Bitcoin is built on. The last time Ghash exceeded 51% threshold, the price dipped briefly and there was a huge discussion about the whole fiasco. If that's was the consequence of them having 51% of the network without doing anything malicious, I would think that miners that care about their own returns would take more proactive steps to prevent this from becoming a threat.

If the pool is able to offer more than what other pools (even those with very low/no fees), then I would think there is some motives behind it as well.

[PrimeNumber7]

I was referring to something along the lines of the attacker would occasionally orphan a block they don't like or refuse to confirm transactions that don't meet certain criteria. 

I think that's quite a stretch to say that miners will support deliberate censorship and undermine the foundation that Bitcoin is built on. The last time Ghash exceeded 51% threshold, the price dipped briefly and there was a huge discussion about the whole fiasco. If that's was the consequence of them having 51% of the network without doing anything malicious, I would think that miners that care about their own returns would take more proactive steps to prevent this from becoming a threat.

It may not be a single pool that is controlled by the same entity. It could be 5 pools that are controlled by a single entity (secretly), that always end up on the correct chain when a block gets orphaned. If the pools are paying out actual earnings, those that have a higher block orphan rate will end up paying out less.