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There are wo things you skip over: 1. You can't rewrite everything. Given enough hash power to create a longer chain, you can create a block that a) removes an

There are wo things you skip over: 1. You can't rewrite everything. Given enough hash power to create a longer chain, you can create a block that a) removes any transactions from a block in the original chain and b) contains new valid transactions (must be signed by you so you must be the owner of the Bitcoins used in the tx), allowing you to double spend your coins, but you can't change other people's transactions. 2. With each new block changing a past one becomes harder, while you make it sound as if you could arbitrarily rewrite history. Merchants usually wait several blocks before accepting your on-chain payment. Exchanges wait 6 blocks as that's seen as infeasible to change a block that's buried under 5 other blocks for non-nation state actors. TL;DR: 1) Other people's transactions can at most be removed but not be changed and 2) data on the Bitcoin blockchain is tamper proof after x blocks.

XorNot 5y ago
This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on. Sure, someone, somewhere will know the truth but who cares - it's all about information control, and you control 51% of the network. Now your opponents have to scramble with social countermeasures to try and discredit your chain. A politically savvy attacker will simply buy off the main social nodes favorably. EDIT: The problem with "oh but people will know" argument is that it assumes credibility outside the network, the thing which cryptocurrencies like to claim is unnecessary. In reality, if the network is 51% attacked, your entire recovery strategy is assuming enough people believe you about what's real or should be real. This isn't even theoretical: this is literally what happened to Ethereum.
- sennight 5y ago
  > 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
- ric2b 5y ago
  > This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on. Not exactly, for every block you rewind there is extra work you have to do, to the point where it may take you close to 2 years to rewind 1 year's worth of blocks (because while you're doing that the rest of the network is still creating new blocks). It gets exponentially harder/more expensive the further back you want to go.

XorNot 5y ago

This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on. Sure, someone, somewhere will know the truth but who cares - it's all about information control, and you control 51% of the network. Now your opponents have to scramble with social countermeasures to try and discredit your chain. A politically savvy attacker will simply buy off the main social nodes favorably. EDIT: The problem with "oh but people will know" argument is that it assumes credibility outside the network, the thing which cryptocurrencies like to claim is unnecessary. In reality, if the network is 51% attacked, your entire recovery strategy is assuming enough people believe you about what's real or should be real. This isn't even theoretical: this is literally what happened to Ethereum.

sennight 5y ago
> 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
ric2b 5y ago
> This is ignoring that if you own 51% of the network, you are free to just rewind the chain back to whatever you think is suitable, and rebuild it from that point on. Not exactly, for every block you rewind there is extra work you have to do, to the point where it may take you close to 2 years to rewind 1 year's worth of blocks (because while you're doing that the rest of the network is still creating new blocks). It gets exponentially harder/more expensive the further back you want to go.