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How to Destroy Bitcoin with 51%

78 points by tonyztan 9y ago

deleted 9y ago
[deleted]
gpm 9y ago
Messing with exchange accounts and buying BTC seems unnecessary. Just take your 51% and mine a dozen blocks privately with no transactions repeatedly and replace the head of the blockchain with them. Suddenly everyone else does the double spending for you, which apart from making it less work, will do a lot more damage to trust. To ruin trust even more effectively sell double spending as a service. Let people pay you to not include transactions in your split chain.
amluto 9y ago
Wow, what an uncreative attack. Let's say you control decently over 51% of mining power and want to destroy bitcoin. What do you do? Keep in mind that this gives you two significant powers. First, you can more or less unilaterally dictate the new consensus blockchain, subject to the usual constraints that you can't create transactions that weren't appropriately signed. That is, you can't spend other people's money or directly destroy it. Second, you have a limited ability to replace the consensus with something else. The first power, by itself, allows you to block certain transactions from ever occurring. You could pick some subset of coins and prevent them from being spent. This could be targetted against exchanges, random users, or maybe even be based on geography. The goal (according to the article) is presumably to undermine confidence. The second power can be used a lot more insidiously than just double-spending your own coins. How about tracking all inbound transfers to some exchange over some period of time and then undoing them? Now that exchange is out a lot of coins. For extra fun, could use your consensus-choosing power to effectively lock down the coins that you just stole from the exchange so their original owners don't have them either. There are probably even more interesting things you could do, too.
- w-ll 9y ago
  Anyone can track Bitcoin transactions; That IS the blockchain. Undoing them will just result in a chain that they are still in the original wallets keys, you STILL cant steal them. And the window for doing this is likely an hour or 2 at tops, but still very unlikely. An exchange might credit you on a few confirmed blocks, but after 6 (an hour) its pretty much confirmed, and any withdraws on the same chain will fail once the 51% attack is over.
  - gpm 9y ago
    You didn't steal them for you, you stole them for the original owner who tried to transfer them to the exchange. In theory with 51% hashing power you could undo exchanges going back as long as you want, as long as you dedicated your entire hashing power to doing so from the point you are trying to revert to. You will get the longer chain so once you publish it it will take over. In practice I'm not sure if there are controls against very old forks from coming in and becoming the 'right one'.
    - deleted 9y ago
      [deleted]
    - w-ll 9y ago
      No, it is the exchanges who's coins are "stolen", but still not really. 51% hashing doesn't allow you to `undo exchanges going back as long as you want`. There is a small time frame with exchanges/merches that accept <6 block confirmations that a transaction could be reversed. And if a fork in the chain is happens (it does more often than not that 2 blocks get made and 1 is orphaned,) all other miners switch to the longer chain as soon as a fork is detected.
      - amluto 9y ago
        You seem to be assuming that, once 6 confirmations have happened, a transaction is set in stone. This isn't true at all. It's true that, once 6 confirmations have happened, a transaction is highly unlikely to disappear in the absence of the 51% attack, but the whole point here is that we're assuming that a 51% attack is occurring.
ploggingdev 9y ago
There's an easier way to destroy the value of Bitcoin (temporarily) if China wanted to : pass a law banning mining. I don't think China would risk trying to carry out a 51% attack though, what would the economic fallout of that be long term? Would any country or individual trust China after such a public attack? Worth noting that a lot of US based companies are built around Bitcoin (exchanges, payment processors etc) and the US government won't sit around watching billions of dollars being destroyed. Compared to the US, China is not known for being an aggressor in matters beyond it's borders so this behaviour would be pretty shocking to the rest of the world and will affect the perception of the country. The cons outweigh whatever little benefit destroying Bitcoin would bring, so that's not going to happen.
tristanj 9y ago
It's worth mentioning there's no need for 51+% control of hashrate to execute this double spend attack. It's very possible to execute one with minority control however success is not guaranteed. With 20% control of hashrate an attacker would have a 2.33% chance of executing a double spend attack. Even at 10% control, the attack is still possible (0.059% success rate). At 40% control, the attacker has a near coinflip for success (49.30%). Probabilities come from this paper which has a table about double spending on page 10. https://bitcoil.co.il/Doublespend.pdf https://bitcoil.co.il/Doublespend.pdf
tlrobinson 9y ago
The Bitcoin community seems to be doing a pretty good job of destroying itself right now, and I say that as a long time (cautiously optimistic) believer in Bitcoin. The division, infighting, toxic and dogmatic rhetoric, etc make me wonder if there’s an external force attempting to disrupt Bitcoin. Or perhaps it’s just the inevitable outcome of a leaderless/decentralized project with billions of dollars on the line.
- matt_wulfeck 9y ago
  > The division, infighting, toxic and dogmatic rhetoric, etc make me wonder if there’s an external force attempting to disrupt Bitcoin. I would say the force has a name, and it’s “Pride”.
  - deevolution 9y ago
    Actually, I think the force is called natural selection. The blockchain is kind of like DNA and these branches represent evolutions.
- QML 9y ago
  Decentralization only works when everyone is in accordance with the mission and with the implementation. Bitcoin community is together in the former, divided in the latter.
  - tlrobinson 9y ago
    I'm not even sure they are together on the former. Some want a secure store of value, some want a scalable medium of exchange. I think we can have both, but only with efficient layer 2 protocols to accomplish scaling without compromising decentralization.
matt_wulfeck 9y ago
IIRC, there was a time when just a few large pools were crossing over into the 51% threshold, thus control of the network if they got wise to it. Does anyone know what happened to that? What are the largest pool control stakes now?
deleted 9y ago
[deleted]
stale2002 9y ago
Uhhhhh, so this guys "brilliant" attack vector that he came up with was a 51% attack? The same attack that is literally spelled out in the bitcoin whitepaper and has been known about from day one? He didn't even add anything interesting on top of it. He just said "get 51% of miners, and then double spend". Nor did he even discuss the ways of mitigating a 51% attack. If the network was being attacked, what I would do is wait for more than 6 Transactions to go on the network, before considering them "final". In a worse case scenario of a sustained 51% attack, the users could simply change the Proof Of Work algorithm (which is a hard fork). It wouldn't be 'great' but it allows the currency to survive. Now the government has to spend more and more money going on the attack. Bitcoin works because of this economic game theory. Yes, it is possible to attack the network. This is well known. The reason why it doesn't happen is because attacking the network is EXPENSIVE, and can't last forever.
- viraptor 9y ago
  > the users could simply change the Proof Of Work algorithm "Simply"? You're saying that an attacker having >51% hashing power on current system wouldn't benefit further from: a) user base splitting between forks (if block structure was changed), or b) all current ASICs dropping off the network (if the using method was changed). > Now the government has to spend more and more money going on the attack. They would have direct access to chip fabs and money to throw at it. Or just laws available to stop production (mining is illegal now).
  - stale2002 9y ago
    > They would have direct access to chip fabs and money to throw at it. And every time they make chips and attack the network, the users change the POW or move to a different altcoin. There are even crytocurrencies that don't use A Proof Of Work algorithm, and use something called proof of stake (which means that the current holders of the coin get to vote on what happens.). Hows the government going to stop THAT? Buy up all the votes? And THEN buy up all the new Proof of Stake coins that ever get created? > Or just laws available to stop production (mining is illegal now). What people need to understand that it is no enough to 'just' destroy the current version of bitcoin. The government would need to destroy EVERY version of bitcoin, and EVERY altcoin, all at once, now and forever. And then ever single other NEW crytocurrency that people create. And not only that, but the government would have to do it EVERYWHERE. It is not enough for 'just' china to ban mining. Every country in the world would have to ban it, because if there is a single country that doesn't ban it, or is ineffective, then everyone would just trust THAT altcoin. The government can't even stop online piracy. Hows it going to convince every government, all around the world, to stop something that most people believe is perfectly OK? Like, fine, if the world governments somehow convince 90% of the population that Crytocurrencies are somehow as bad as child porn, then MAYBE they have a slim hope of stopping them. But I Think that this goal is extremely unlikely to be successful.
AlexCoventry 9y ago
Perhaps you could inflict more mayhem by shutting down the miners under your control for two weeks, waiting for the difficulty to drop, and then undoing all transactions from the prior month.
- w-ll 9y ago
  1728 Bitcoin are minted a day right now (next halving in 2020 ), lets say you have 50% mining so ~900 btc a day. Thats ~4.5M at a 5k BTC. Sure you double the transaction confirmation time but the transactions won't be undone. You might have a chance to mine blocks twice as quick after the difficulty adjusts, but still no long term effects.
juskrey 9y ago
China can just firewall all its bitcoin nodes for a day or two and return them back. Do this several times and Western nodes will be very very tired rolling back everything..
- banku_brougham 9y ago
  i think this is the prize winning strategy. elegantly simple, almost amusing like a child turning a light switch on and off, with deadly purpose however. Well actually, a lot depends on what the miners do when they are locked out.
- homakov 9y ago
  Great idea
matt4077 9y ago
The bitcoinbros really have to get over their fixation on "the government". Here's the real pocket guide for governments: "If they get annoying, make it illegal" "Bitcoin enthusiasts" are completely delusional, holding both an inferiority complex (constant fear) as well as a superiority complex vis-a-vis "governments". In reality, governments mostly just don't care. Then, of far lesser importance, they're mildly intrigued by the concept. Meanwhile, some interns are doing a few things to handle some stuff that came up. Like pensioners giving their savings to ICOs.

78 points by tonyztan 9y ago

deleted 9y ago

[deleted]

gpm 9y ago

Messing with exchange accounts and buying BTC seems unnecessary. Just take your 51% and mine a dozen blocks privately with no transactions repeatedly and replace the head of the blockchain with them. Suddenly everyone else does the double spending for you, which apart from making it less work, will do a lot more damage to trust. To ruin trust even more effectively sell double spending as a service. Let people pay you to not include transactions in your split chain.

amluto 9y ago

Wow, what an uncreative attack. Let's say you control decently over 51% of mining power and want to destroy bitcoin. What do you do? Keep in mind that this gives you two significant powers. First, you can more or less unilaterally dictate the new consensus blockchain, subject to the usual constraints that you can't create transactions that weren't appropriately signed. That is, you can't spend other people's money or directly destroy it. Second, you have a limited ability to replace the consensus with something else. The first power, by itself, allows you to block certain transactions from ever occurring. You could pick some subset of coins and prevent them from being spent. This could be targetted against exchanges, random users, or maybe even be based on geography. The goal (according to the article) is presumably to undermine confidence. The second power can be used a lot more insidiously than just double-spending your own coins. How about tracking all inbound transfers to some exchange over some period of time and then undoing them? Now that exchange is out a lot of coins. For extra fun, could use your consensus-choosing power to effectively lock down the coins that you just stole from the exchange so their original owners don't have them either. There are probably even more interesting things you could do, too.

w-ll 9y ago
Anyone can track Bitcoin transactions; That IS the blockchain. Undoing them will just result in a chain that they are still in the original wallets keys, you STILL cant steal them. And the window for doing this is likely an hour or 2 at tops, but still very unlikely. An exchange might credit you on a few confirmed blocks, but after 6 (an hour) its pretty much confirmed, and any withdraws on the same chain will fail once the 51% attack is over.
- gpm 9y ago
  You didn't steal them for you, you stole them for the original owner who tried to transfer them to the exchange. In theory with 51% hashing power you could undo exchanges going back as long as you want, as long as you dedicated your entire hashing power to doing so from the point you are trying to revert to. You will get the longer chain so once you publish it it will take over. In practice I'm not sure if there are controls against very old forks from coming in and becoming the 'right one'.
  - deleted 9y ago
    [deleted]
  - w-ll 9y ago
    No, it is the exchanges who's coins are "stolen", but still not really. 51% hashing doesn't allow you to `undo exchanges going back as long as you want`. There is a small time frame with exchanges/merches that accept <6 block confirmations that a transaction could be reversed. And if a fork in the chain is happens (it does more often than not that 2 blocks get made and 1 is orphaned,) all other miners switch to the longer chain as soon as a fork is detected.
    - amluto 9y ago
      You seem to be assuming that, once 6 confirmations have happened, a transaction is set in stone. This isn't true at all. It's true that, once 6 confirmations have happened, a transaction is highly unlikely to disappear in the absence of the 51% attack, but the whole point here is that we're assuming that a 51% attack is occurring.

ploggingdev 9y ago

There's an easier way to destroy the value of Bitcoin (temporarily) if China wanted to : pass a law banning mining. I don't think China would risk trying to carry out a 51% attack though, what would the economic fallout of that be long term? Would any country or individual trust China after such a public attack? Worth noting that a lot of US based companies are built around Bitcoin (exchanges, payment processors etc) and the US government won't sit around watching billions of dollars being destroyed. Compared to the US, China is not known for being an aggressor in matters beyond it's borders so this behaviour would be pretty shocking to the rest of the world and will affect the perception of the country. The cons outweigh whatever little benefit destroying Bitcoin would bring, so that's not going to happen.

tristanj 9y ago

It's worth mentioning there's no need for 51+% control of hashrate to execute this double spend attack. It's very possible to execute one with minority control however success is not guaranteed. With 20% control of hashrate an attacker would have a 2.33% chance of executing a double spend attack. Even at 10% control, the attack is still possible (0.059% success rate). At 40% control, the attacker has a near coinflip for success (49.30%). Probabilities come from this paper which has a table about double spending on page 10. https://bitcoil.co.il/Doublespend.pdf https://bitcoil.co.il/Doublespend.pdf

tlrobinson 9y ago

The Bitcoin community seems to be doing a pretty good job of destroying itself right now, and I say that as a long time (cautiously optimistic) believer in Bitcoin. The division, infighting, toxic and dogmatic rhetoric, etc make me wonder if there’s an external force attempting to disrupt Bitcoin. Or perhaps it’s just the inevitable outcome of a leaderless/decentralized project with billions of dollars on the line.

matt_wulfeck 9y ago
> The division, infighting, toxic and dogmatic rhetoric, etc make me wonder if there’s an external force attempting to disrupt Bitcoin. I would say the force has a name, and it’s “Pride”.
- deevolution 9y ago
  Actually, I think the force is called natural selection. The blockchain is kind of like DNA and these branches represent evolutions.
QML 9y ago
Decentralization only works when everyone is in accordance with the mission and with the implementation. Bitcoin community is together in the former, divided in the latter.
- tlrobinson 9y ago
  I'm not even sure they are together on the former. Some want a secure store of value, some want a scalable medium of exchange. I think we can have both, but only with efficient layer 2 protocols to accomplish scaling without compromising decentralization.

matt_wulfeck 9y ago

IIRC, there was a time when just a few large pools were crossing over into the 51% threshold, thus control of the network if they got wise to it. Does anyone know what happened to that? What are the largest pool control stakes now?

deleted 9y ago

[deleted]

stale2002 9y ago

Uhhhhh, so this guys "brilliant" attack vector that he came up with was a 51% attack? The same attack that is literally spelled out in the bitcoin whitepaper and has been known about from day one? He didn't even add anything interesting on top of it. He just said "get 51% of miners, and then double spend". Nor did he even discuss the ways of mitigating a 51% attack. If the network was being attacked, what I would do is wait for more than 6 Transactions to go on the network, before considering them "final". In a worse case scenario of a sustained 51% attack, the users could simply change the Proof Of Work algorithm (which is a hard fork). It wouldn't be 'great' but it allows the currency to survive. Now the government has to spend more and more money going on the attack. Bitcoin works because of this economic game theory. Yes, it is possible to attack the network. This is well known. The reason why it doesn't happen is because attacking the network is EXPENSIVE, and can't last forever.

viraptor 9y ago
> the users could simply change the Proof Of Work algorithm "Simply"? You're saying that an attacker having >51% hashing power on current system wouldn't benefit further from: a) user base splitting between forks (if block structure was changed), or b) all current ASICs dropping off the network (if the using method was changed). > Now the government has to spend more and more money going on the attack. They would have direct access to chip fabs and money to throw at it. Or just laws available to stop production (mining is illegal now).
- stale2002 9y ago
  > They would have direct access to chip fabs and money to throw at it. And every time they make chips and attack the network, the users change the POW or move to a different altcoin. There are even crytocurrencies that don't use A Proof Of Work algorithm, and use something called proof of stake (which means that the current holders of the coin get to vote on what happens.). Hows the government going to stop THAT? Buy up all the votes? And THEN buy up all the new Proof of Stake coins that ever get created? > Or just laws available to stop production (mining is illegal now). What people need to understand that it is no enough to 'just' destroy the current version of bitcoin. The government would need to destroy EVERY version of bitcoin, and EVERY altcoin, all at once, now and forever. And then ever single other NEW crytocurrency that people create. And not only that, but the government would have to do it EVERYWHERE. It is not enough for 'just' china to ban mining. Every country in the world would have to ban it, because if there is a single country that doesn't ban it, or is ineffective, then everyone would just trust THAT altcoin. The government can't even stop online piracy. Hows it going to convince every government, all around the world, to stop something that most people believe is perfectly OK? Like, fine, if the world governments somehow convince 90% of the population that Crytocurrencies are somehow as bad as child porn, then MAYBE they have a slim hope of stopping them. But I Think that this goal is extremely unlikely to be successful.

AlexCoventry 9y ago

Perhaps you could inflict more mayhem by shutting down the miners under your control for two weeks, waiting for the difficulty to drop, and then undoing all transactions from the prior month.

w-ll 9y ago
1728 Bitcoin are minted a day right now (next halving in 2020 ), lets say you have 50% mining so ~900 btc a day. Thats ~4.5M at a 5k BTC. Sure you double the transaction confirmation time but the transactions won't be undone. You might have a chance to mine blocks twice as quick after the difficulty adjusts, but still no long term effects.

juskrey 9y ago

China can just firewall all its bitcoin nodes for a day or two and return them back. Do this several times and Western nodes will be very very tired rolling back everything..

banku_brougham 9y ago
i think this is the prize winning strategy. elegantly simple, almost amusing like a child turning a light switch on and off, with deadly purpose however. Well actually, a lot depends on what the miners do when they are locked out.
homakov 9y ago
Great idea

matt4077 9y ago

The bitcoinbros really have to get over their fixation on "the government". Here's the real pocket guide for governments: "If they get annoying, make it illegal" "Bitcoin enthusiasts" are completely delusional, holding both an inferiority complex (constant fear) as well as a superiority complex vis-a-vis "governments". In reality, governments mostly just don't care. Then, of far lesser importance, they're mildly intrigued by the concept. Meanwhile, some interns are doing a few things to handle some stuff that came up. Like pensioners giving their savings to ICOs.