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> This is ignoring that if you own 51% of the network... Are you talking about some kind of node sybil attack? Because the only way I can make sense of what yo

> This is ignoring that if you own 51% of the network... Are you talking about some kind of node sybil attack? Because the only way I can make sense of what you've said is if a system had client implementations maintaining zero state - relying completely on unsecured network consensus. But in that scenario hashing power wouldn't enter the equation. I don't know of any cryptocurrency that operates that way, and doubt that one ever has outside of a home lab. Or are you talking about someone trying to hijack a cryptocurrency by creating brand confusion and convincing people to run modified code, like the failed bcash campaign against bitcoin? A 51% attack on a blockchain has no rewrite ability past the point that where an attacker establishes a longer chain. That is why it is a "chain"... where do you think the hashing function inputs come from that are being fed into the adversarial mining hardware? An earlier block - the one where the attack started, you can't go back further than that without having to throw out and rehash your entire attack fork.

zie 5y ago
You are both mostly right, as I understand it. If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". To re-write actual history takes a lot more work, and wouldn't be possible with 51% for 1 transaction.. If I was able to maintain 51% control for a long time, then I can do anything I want for as long as I have 51% control. Though I imagine after that very 1st transaction, the world of BTC would blow up and everyone would stop hashing BTC as there would be zero point as the current chain is now effectively useless. This is the real issue that I see, 1 transaction of 51% power is enough to permanently wipe out all of BTC's worth. As far as I'm aware every crypto currency basically has this same problem.
- XorNot 5y ago
  What I'm saying is that, if enough big players like Coinbase et al. started saying "oh there's a BTC glitch, you need to pull a new state file..." then a huge number of people would do it. Not all, but you don't need all - just enough.
  - zie 5y ago
    Again, if they don't control 51%, their new statefile would be pointless, unless they did a hard-fork, but if I have the resources for a 51% attack on the current fork, chances are I have the same capability on the new fork. The only way what you propose would work would be if they could cobble together enough resources to break my 51% control. So BTC would go 100% bust if someone managed a 51% attack. The question is, can someone with 1 transaction of 51% get enough converted to USD/etc before enough people noticed. Otherwise the financial incentive isn't there to try. I'd guess no matter what, it would be a huge fricking mess and if you did it in a country that didn't like you, it probably wouldn't end well for you years later when whatever govt you live in gets around to ruining your life, even if you managed to extract a few billion. Because you know the exchanges like Coinbase as soon as they noticed would do their best to stop you(as it's in their best interest).
- sennight 5y ago
  > You are both mostly right, as I understand it. > If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". You clearly don't understand it, and it is kind of amazing that you think such a design would survive for as long as bitcoin has (going on 12 years) - by relying on altruistic miners not taking advantage of such a silly flaw. The Satoshi white paper is 9 pages long and written in very plain language... do like everyone use to do back in the day: read the paper and take a peek at the source, it goes a long way in inoculating you from misinformation that you subsequently repeat, accidentally (?) misinforming others.
  - zie 5y ago
    BTC has changed a bit since the original white paper, and I have read the white paper(though it's been a while). Back it up with sources to convince me I'm wrong, don't just say I am with nothing but YOU ARE WRONG, that doesn't accomplish anything. To help, here is my understanding. In a < 51%(i.e. normal transaction), if I win the BTC mining lottery and get to write the next transaction, I can make it say whatever I want, but then 51% of the mining network has to agree that what I said was sane(as defined by the current mining software). If I control 51%, then I can make the transaction say whatever I want AND I can make everyone else accept it as sane, because a majority of the network agrees with me. This is how BTC changes over time, > 51% of the network agree that they will accept X as the new reality, and it then becomes the new reality.
    - sennight 5y ago
      The old question about how one eats an elephant comes to mind... you've expressed such a fundamental misunderstanding that the only sensible correction is: YOU ARE WRONG, START OVER. For example, this: > ..make the current chain say anything I want, like give me all the BTC.. You know that transactions are cryptographically secured by public and private keys, right? That would be like saying "I can h4x0r all the hotmails and rewrite every PGP armored message to say whatever I want!" Do you think that miners, upon building the next block, have the opportunity to ignore all the rules with regard to PKI? > ..and it would become valid.. There is a very straightforward block validation sequence, your attack would impotently collapse against it for any number of reasons: https://en.bitcoin.it/wiki/Protocol_rules https://en.bitcoin.it/wiki/Protocol_rules > I have read the white paper. No you didn't. Sorry to have to put it so bluntly, but you couldn't have read it and still be so comically wrong - like I said, it is written in very plain language.
- ric2b 5y ago
  > If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". No, this is a big misunderstanding of how it works. Having more than 50% doesn't give you superpowers, it just means that you're able to create VALID blocks faster than the rest of the network combined (creating invalid blocks is useless), which allows you to censor transactions (by not including them in your blocks, which will be the chain with more work because you have 50%+) and you can TRY to do double-spends. A double-spend gets exponentially harder and more expensive to pull off the longer your target waits before considering the original transaction final, because you'll have to rebuild all the blocks generated since that transaction was included in the chain, so that your parallel chain that doesn't include that transaction becomes the chain with the most work that everyone follows.

zie 5y ago

You are both mostly right, as I understand it. If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". To re-write actual history takes a lot more work, and wouldn't be possible with 51% for 1 transaction.. If I was able to maintain 51% control for a long time, then I can do anything I want for as long as I have 51% control. Though I imagine after that very 1st transaction, the world of BTC would blow up and everyone would stop hashing BTC as there would be zero point as the current chain is now effectively useless. This is the real issue that I see, 1 transaction of 51% power is enough to permanently wipe out all of BTC's worth. As far as I'm aware every crypto currency basically has this same problem.

XorNot 5y ago
What I'm saying is that, if enough big players like Coinbase et al. started saying "oh there's a BTC glitch, you need to pull a new state file..." then a huge number of people would do it. Not all, but you don't need all - just enough.
- zie 5y ago
  Again, if they don't control 51%, their new statefile would be pointless, unless they did a hard-fork, but if I have the resources for a 51% attack on the current fork, chances are I have the same capability on the new fork. The only way what you propose would work would be if they could cobble together enough resources to break my 51% control. So BTC would go 100% bust if someone managed a 51% attack. The question is, can someone with 1 transaction of 51% get enough converted to USD/etc before enough people noticed. Otherwise the financial incentive isn't there to try. I'd guess no matter what, it would be a huge fricking mess and if you did it in a country that didn't like you, it probably wouldn't end well for you years later when whatever govt you live in gets around to ruining your life, even if you managed to extract a few billion. Because you know the exchanges like Coinbase as soon as they noticed would do their best to stop you(as it's in their best interest).
sennight 5y ago
> You are both mostly right, as I understand it. > If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". You clearly don't understand it, and it is kind of amazing that you think such a design would survive for as long as bitcoin has (going on 12 years) - by relying on altruistic miners not taking advantage of such a silly flaw. The Satoshi white paper is 9 pages long and written in very plain language... do like everyone use to do back in the day: read the paper and take a peek at the source, it goes a long way in inoculating you from misinformation that you subsequently repeat, accidentally (?) misinforming others.
- zie 5y ago
  BTC has changed a bit since the original white paper, and I have read the white paper(though it's been a while). Back it up with sources to convince me I'm wrong, don't just say I am with nothing but YOU ARE WRONG, that doesn't accomplish anything. To help, here is my understanding. In a < 51%(i.e. normal transaction), if I win the BTC mining lottery and get to write the next transaction, I can make it say whatever I want, but then 51% of the mining network has to agree that what I said was sane(as defined by the current mining software). If I control 51%, then I can make the transaction say whatever I want AND I can make everyone else accept it as sane, because a majority of the network agrees with me. This is how BTC changes over time, > 51% of the network agree that they will accept X as the new reality, and it then becomes the new reality.
  - sennight 5y ago
    The old question about how one eats an elephant comes to mind... you've expressed such a fundamental misunderstanding that the only sensible correction is: YOU ARE WRONG, START OVER. For example, this: > ..make the current chain say anything I want, like give me all the BTC.. You know that transactions are cryptographically secured by public and private keys, right? That would be like saying "I can h4x0r all the hotmails and rewrite every PGP armored message to say whatever I want!" Do you think that miners, upon building the next block, have the opportunity to ignore all the rules with regard to PKI? > ..and it would become valid.. There is a very straightforward block validation sequence, your attack would impotently collapse against it for any number of reasons: https://en.bitcoin.it/wiki/Protocol_rules https://en.bitcoin.it/wiki/Protocol_rules > I have read the white paper. No you didn't. Sorry to have to put it so bluntly, but you couldn't have read it and still be so comically wrong - like I said, it is written in very plain language.
ric2b 5y ago
> If I had >50% hashing power of BTC(for exactly 1 transaction), I can make the current chain say anything I want, like give me all the BTC, and it would become valid and "permanent". No, this is a big misunderstanding of how it works. Having more than 50% doesn't give you superpowers, it just means that you're able to create VALID blocks faster than the rest of the network combined (creating invalid blocks is useless), which allows you to censor transactions (by not including them in your blocks, which will be the chain with more work because you have 50%+) and you can TRY to do double-spends. A double-spend gets exponentially harder and more expensive to pull off the longer your target waits before considering the original transaction final, because you'll have to rebuild all the blocks generated since that transaction was included in the chain, so that your parallel chain that doesn't include that transaction becomes the chain with the most work that everyone follows.