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> This is a similar trust assumption to requiring that 51% of the mining power in PoW (Proof of Work) is not malicious. Yeah, but you're glossing over an impor

> This is a similar trust assumption to requiring that 51% of the mining power in PoW (Proof of Work) is not malicious. Yeah, but you're glossing over an important detail: It's not 66% of the stake that has to be good, it's 66% of anyone who has ever staked. In PoW, I only need to trust the miners of today to tell me the truth about what happened today. In PoS, I need that, plus the miners of yesterday, plus the miners of a year ago, plus ..., in perpetuity. > New parties who are syncing for the first time would be vulnerable, however clients could be programmed to have hardcoded 6th month checkpoints or clients could check block explorers and halt if a fork is detected. Right, maybe you can elaborate on this. Is checking block explorers a decentralized or trustless solution to, well, anything?

mattalex 5y ago
> It's not 66% of the stake that has to be good, it's 66% of anyone who has ever staked. In PoW, I only need to trust the miners of today to tell me the truth about what happened today. I don't quite get that. As far as I understand it the "nothing at stake" problem works by a malicious party inducing a fork, one of which they double-spend in. Since it's in the best interest for everyone else to mine both forks, you can force your double-spend fork to become the longest chain by only validating the double-spend fork. This means you have to trust that nobody part of your current chain has double-spent in this way. But isn't this the same as in PoW where you have to trust that nobody has launched a 51% attack to disrupt the network in the past? Also, can't you just prevent people from mining all forks? I.e. for becoming a validator you have to deposit X as a security beforehand and you can only earn at most X via staking (so it is in the history before you can attack with nothing at stake). If it is recognised that you mine on more than one fork at a time, you lose the security deposit you gave before the fork. X goes to the person who found the fork, incentivising that the mallicious fork is identified on all forks (miners on competing forks are incentivised to look at all forks and quickly add the mallicious fork detection for their own benefit). If you want to retrieve your security and money earned, you have to announce this on all forks (you immediatly seize to be a validator). You are only allowed to retrieve the funds, if it is confirmed on all forks, or the forks are sufficiently behind the longest chain. This allows everybody ample time to look for dual-fork work and also incentivizes rapid solution of forks.
- comex 5y ago
  > If it is recognised that you mine on more than one fork at a time, you lose the security deposit you gave before the fork. Yes, modern proof-of-stake algorithms work this way. The caveat is that at some point (on the order of months later) the security deposit is refunded, and at that point you can lie about the past without consequence. But this is a limited attack: you can only successfully lie to someone who has been offline since you were a staker, or else they would already have a record of the real successor chain (which now has a new set of stakers, who themselves still have their security deposit deposited).
  - PeterisP 5y ago
    The original article asserts that this does not work according to their requirements since there's no way to independently verify which is the "real successor chain" - they just have to trust someone's word that chain A is true and chain B is not, and a convincing liar could provide them with opposite data. In schemes like Bitcoin, there's objective validation of the "longest chain" with the most work invested; in your example where would that "record of the real successor chain" come from, and how can it be validated/verified in a decentralized manner in a way that a major ex-staker can't satisfy? Like, this is trivially solved with a central authority (e.g. have some trusted core developer every day publish a signed message saying "this is the real successor chain"), but it does enable that central authority to arbitrarily bless a fake ex-staker's fork.
    - EthanHeilman 5y ago
      > they just have to trust someone's word that chain A is true and chain B is not, and a convincing liar could provide them with opposite data. In schemes like Bitcoin, there's objective validation of the "longest chain" with the most work invested; Note that in Bitcoin you can have a fork in which both chains have equal length. The idea is that eventually the longest chain will be established, but if say 90% of the mining is malicious that malicious miner could ensure that most of the time both chains are of equal length. With a PoS fork you can ask, which fork has the most amount of stake voting for it. An attacker that controls enough stake might be able to balance the total stake vote in the same way as a malicious miner could on Bitcoin. In both cases if the core security assumption of the blockchain is violated, that blockchain should halt until that assumption is made sound again. If someone orphans the last two years of Bitcoin's blockchain something has gone horribly wrong. The fact that Bitcoin now switches to the longest chain doesn't actually address the problem that two years of transactions may have been rendered invalid.

mattalex 5y ago

> It's not 66% of the stake that has to be good, it's 66% of anyone who has ever staked. In PoW, I only need to trust the miners of today to tell me the truth about what happened today. I don't quite get that. As far as I understand it the "nothing at stake" problem works by a malicious party inducing a fork, one of which they double-spend in. Since it's in the best interest for everyone else to mine both forks, you can force your double-spend fork to become the longest chain by only validating the double-spend fork. This means you have to trust that nobody part of your current chain has double-spent in this way. But isn't this the same as in PoW where you have to trust that nobody has launched a 51% attack to disrupt the network in the past? Also, can't you just prevent people from mining all forks? I.e. for becoming a validator you have to deposit X as a security beforehand and you can only earn at most X via staking (so it is in the history before you can attack with nothing at stake). If it is recognised that you mine on more than one fork at a time, you lose the security deposit you gave before the fork. X goes to the person who found the fork, incentivising that the mallicious fork is identified on all forks (miners on competing forks are incentivised to look at all forks and quickly add the mallicious fork detection for their own benefit). If you want to retrieve your security and money earned, you have to announce this on all forks (you immediatly seize to be a validator). You are only allowed to retrieve the funds, if it is confirmed on all forks, or the forks are sufficiently behind the longest chain. This allows everybody ample time to look for dual-fork work and also incentivizes rapid solution of forks.

comex 5y ago
> If it is recognised that you mine on more than one fork at a time, you lose the security deposit you gave before the fork. Yes, modern proof-of-stake algorithms work this way. The caveat is that at some point (on the order of months later) the security deposit is refunded, and at that point you can lie about the past without consequence. But this is a limited attack: you can only successfully lie to someone who has been offline since you were a staker, or else they would already have a record of the real successor chain (which now has a new set of stakers, who themselves still have their security deposit deposited).
- PeterisP 5y ago
  The original article asserts that this does not work according to their requirements since there's no way to independently verify which is the "real successor chain" - they just have to trust someone's word that chain A is true and chain B is not, and a convincing liar could provide them with opposite data. In schemes like Bitcoin, there's objective validation of the "longest chain" with the most work invested; in your example where would that "record of the real successor chain" come from, and how can it be validated/verified in a decentralized manner in a way that a major ex-staker can't satisfy? Like, this is trivially solved with a central authority (e.g. have some trusted core developer every day publish a signed message saying "this is the real successor chain"), but it does enable that central authority to arbitrarily bless a fake ex-staker's fork.
  - EthanHeilman 5y ago
    > they just have to trust someone's word that chain A is true and chain B is not, and a convincing liar could provide them with opposite data. In schemes like Bitcoin, there's objective validation of the "longest chain" with the most work invested; Note that in Bitcoin you can have a fork in which both chains have equal length. The idea is that eventually the longest chain will be established, but if say 90% of the mining is malicious that malicious miner could ensure that most of the time both chains are of equal length. With a PoS fork you can ask, which fork has the most amount of stake voting for it. An attacker that controls enough stake might be able to balance the total stake vote in the same way as a malicious miner could on Bitcoin. In both cases if the core security assumption of the blockchain is violated, that blockchain should halt until that assumption is made sound again. If someone orphans the last two years of Bitcoin's blockchain something has gone horribly wrong. The fact that Bitcoin now switches to the longest chain doesn't actually address the problem that two years of transactions may have been rendered invalid.