8 ms·

A 51% attack only allows you to (1) censor transactions for a short time, and (2) double-spend. The first part is a mild nuisance. The second one can be retalia

by toomim 9y ago

A 51% attack only allows you to (1) censor transactions for a short time, and (2) double-spend. The first part is a mild nuisance. The second one can be retaliated against off-chain, which puts the attacker (who must be a large entity) at great risk in the real world.

VMG 9y ago
> The second one can be retaliated against off-chain. What do you mean?
- chc 9y ago
  I believe it means that the recipient of the double-spent transaction can sue you or send goons after you just like they would someone who defrauded them without using cryptocurrency.
  - VMG 9y ago
    How would you determine the culprit? You can't even prove that there was an intent to roll back a particular transaction.
    - cperciva 9y ago
      No, but you can show that there was an intention to double-spend coins.
- gizmo686 9y ago
  If you do a double spend attack of any significance, the other party will notice. Unless you are doing something completely anonymous, the other party will also know who you are and be able to pursue other remedies (eg. sue you for fraud).
  - VMG 9y ago
    If double-spend detection rather than prevention was enough, you could build a much cheaper and more efficient payment system than Bitcoin. What makes Bitcoin interesting is the fact that you don't have to resort to the legal system for final arbitration.
    - gizmo686 9y ago
      Bitcoin still succeeds in making double spend attacks expensive. If I am selling pizza's at $10 a piece, I am not going to go after someone for a double spend attack. Attackers know this and so would be more willing to engage in this type of attack, possibly often enough for me to drop the proposed system entirely. If I am engaged in a $100,000 transaction, I would definitely be willing to sue you. Not to mention the fact that a double spend attack still involves a large capital investment, greatly limiting the number of people capable of pulling it off.
  - tree_of_item 9y ago
    How could you possibly sue someone for double spending on a blockchain? I don't get it, isn't the point of this blockchain stuff to be the wild west? What is the grounds for a lawsuit? Why would a 51% attack be illegal? "Code is law", right?
    - gizmo686 9y ago
      Fraud. If I agree to buy $100,000 worth of bitcoin from you, and you take my money without giving me the bitcoins you engaged in fraud and are liable to be sued. The fact that you did this through a double spend attack demonstrates a clear intent to defraud me.
    - Sir_Substance 9y ago
      >I don't get it, isn't the point of this blockchain stuff to be the wild west? I guarantee you no judge in the world is going to accept that interpretation. >"Code is law", right? From a legal perspective, obviously not, and given how well accepted the etherium hard fork was in the wake of the DOA smart contract being exploited, you can be pretty sure that a large percentage of the crypto-user population doesn't think so either.
      - cies 9y ago
        Cardano is speaking out in favour of "code is law". Not sure what they do when faced with an actual catastrophy. There are others also taking this stance to some extend. Anyway, Cardano is cool (Haskell, PoS, lots of papers published)
    - em3rgent0rdr 9y ago
      "isn't the point of this blockchain stuff to be the wild west?" Not necessarily. Even in an "Code is law" society, there could be some code/law specifically addressing 51% double spending attacks.
sillysaurus3 9y ago
Yes. A 51% attack is a popular whipping boy. It's not as dangerous as it seems. The price would tank, for sure. But your money won't be gone. And eventually when the attack dies down you'll get your wealth back. However, it remains a very interesting way for governments to completely destroy Bitcoin. It's the only way, and they have to do it now before centralized, dedicated mining facilities come online. It would disrupt the price enough and scare away enough people that the threat BTC poses will vanish, at least for awhile. And that will discourage miners from setting up facilities, such that the governments can just beat them again if they have to. But it's already almost impossible. The difference is intent. It's not really much of a problem for a miner to have >51% share. They're there to make money. And Satoshi pointed out they'll probably just keep mining rather than devaluing their own wealth. But an organized, worldwide governmental campaign against cryptos would be another matter.
- jancsika 9y ago
  > And eventually when the attack dies down you'll get your wealth back. Here's the attack the author is describing: block 42: exchange sends 1 BCH to foo block 42: in secret, BTC meanies solve a different block 42 block 62: foo is comfortable believing the transaction is confirmed, so is the exchange block 62: in secret, BTC meanies are now solving secret branch block 63, or possibly secret branch block 64/65/etc. depending on how much hashing power they have block 123: foo and many others have made transactions, exchange has made many other transactions. Maybe some users transfered out dollars during this time. block 123: BTC meanies reveal their branch, which is at block 130 or whatever, propagate it, and all the BCH nodes accept it as having the greatest total difficulty because that is the rule for how the protocol works block 150: the once secret branch now has twenty confirmations, some of those blocks holding transactions from bona fide BCH users. Maybe the exchange halted trading at block 124 because something looked screwy. Maybe the exchange eats the cost of sending more BCH to the users whose coins "disappeared." Or perhaps the BTC meanies keep their branch secret for several days. The point is the appearance of the secret branch undermines faith that 20 confirmations is enough to confirm a transaction, and the users don't know how to judge what constitutes the canonical state of the blockchain. Avoiding that kind of confusion was a stated goal of the Bitcoin whitepaper. Hm... it occurs to me that such an attack could be even more insidious. I'll call it the "Robinhood Attack." The BTC meanies could begin hashing their secret branch and include all the transactions they see happening on the main chain. Then for each new secret block, they could send the block reward to a random recipient chosen from the transactions on the previous block. When the BTC meanies finally reveal their alternate chain, it gets picked up and includes all the transactions, except the block rewards have now been moved from the miners to the speculators. :) Edit: typo

by toomim 9y ago

VMG 9y ago

> The second one can be retaliated against off-chain. What do you mean?

chc 9y ago
I believe it means that the recipient of the double-spent transaction can sue you or send goons after you just like they would someone who defrauded them without using cryptocurrency.
- VMG 9y ago
  How would you determine the culprit? You can't even prove that there was an intent to roll back a particular transaction.
  - cperciva 9y ago
    No, but you can show that there was an intention to double-spend coins.
gizmo686 9y ago
If you do a double spend attack of any significance, the other party will notice. Unless you are doing something completely anonymous, the other party will also know who you are and be able to pursue other remedies (eg. sue you for fraud).
- VMG 9y ago
  If double-spend detection rather than prevention was enough, you could build a much cheaper and more efficient payment system than Bitcoin. What makes Bitcoin interesting is the fact that you don't have to resort to the legal system for final arbitration.
  - gizmo686 9y ago
    Bitcoin still succeeds in making double spend attacks expensive. If I am selling pizza's at $10 a piece, I am not going to go after someone for a double spend attack. Attackers know this and so would be more willing to engage in this type of attack, possibly often enough for me to drop the proposed system entirely. If I am engaged in a $100,000 transaction, I would definitely be willing to sue you. Not to mention the fact that a double spend attack still involves a large capital investment, greatly limiting the number of people capable of pulling it off.
- tree_of_item 9y ago
  How could you possibly sue someone for double spending on a blockchain? I don't get it, isn't the point of this blockchain stuff to be the wild west? What is the grounds for a lawsuit? Why would a 51% attack be illegal? "Code is law", right?
  - gizmo686 9y ago
    Fraud. If I agree to buy $100,000 worth of bitcoin from you, and you take my money without giving me the bitcoins you engaged in fraud and are liable to be sued. The fact that you did this through a double spend attack demonstrates a clear intent to defraud me.
  - Sir_Substance 9y ago
    >I don't get it, isn't the point of this blockchain stuff to be the wild west? I guarantee you no judge in the world is going to accept that interpretation. >"Code is law", right? From a legal perspective, obviously not, and given how well accepted the etherium hard fork was in the wake of the DOA smart contract being exploited, you can be pretty sure that a large percentage of the crypto-user population doesn't think so either.
    - cies 9y ago
      Cardano is speaking out in favour of "code is law". Not sure what they do when faced with an actual catastrophy. There are others also taking this stance to some extend. Anyway, Cardano is cool (Haskell, PoS, lots of papers published)
  - em3rgent0rdr 9y ago
    "isn't the point of this blockchain stuff to be the wild west?" Not necessarily. Even in an "Code is law" society, there could be some code/law specifically addressing 51% double spending attacks.

sillysaurus3 9y ago

Yes. A 51% attack is a popular whipping boy. It's not as dangerous as it seems. The price would tank, for sure. But your money won't be gone. And eventually when the attack dies down you'll get your wealth back. However, it remains a very interesting way for governments to completely destroy Bitcoin. It's the only way, and they have to do it now before centralized, dedicated mining facilities come online. It would disrupt the price enough and scare away enough people that the threat BTC poses will vanish, at least for awhile. And that will discourage miners from setting up facilities, such that the governments can just beat them again if they have to. But it's already almost impossible. The difference is intent. It's not really much of a problem for a miner to have >51% share. They're there to make money. And Satoshi pointed out they'll probably just keep mining rather than devaluing their own wealth. But an organized, worldwide governmental campaign against cryptos would be another matter.

jancsika 9y ago
> And eventually when the attack dies down you'll get your wealth back. Here's the attack the author is describing: block 42: exchange sends 1 BCH to foo block 42: in secret, BTC meanies solve a different block 42 block 62: foo is comfortable believing the transaction is confirmed, so is the exchange block 62: in secret, BTC meanies are now solving secret branch block 63, or possibly secret branch block 64/65/etc. depending on how much hashing power they have block 123: foo and many others have made transactions, exchange has made many other transactions. Maybe some users transfered out dollars during this time. block 123: BTC meanies reveal their branch, which is at block 130 or whatever, propagate it, and all the BCH nodes accept it as having the greatest total difficulty because that is the rule for how the protocol works block 150: the once secret branch now has twenty confirmations, some of those blocks holding transactions from bona fide BCH users. Maybe the exchange halted trading at block 124 because something looked screwy. Maybe the exchange eats the cost of sending more BCH to the users whose coins "disappeared." Or perhaps the BTC meanies keep their branch secret for several days. The point is the appearance of the secret branch undermines faith that 20 confirmations is enough to confirm a transaction, and the users don't know how to judge what constitutes the canonical state of the blockchain. Avoiding that kind of confusion was a stated goal of the Bitcoin whitepaper. Hm... it occurs to me that such an attack could be even more insidious. I'll call it the "Robinhood Attack." The BTC meanies could begin hashing their secret branch and include all the transactions they see happening on the main chain. Then for each new secret block, they could send the block reward to a random recipient chosen from the transactions on the previous block. When the BTC meanies finally reveal their alternate chain, it gets picked up and includes all the transactions, except the block rewards have now been moved from the miners to the speculators. :) Edit: typo