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It... has happened. Multiple times. Bitcoin Gold: https://cointelegraph.com/news/bitcoin-gold-blockchain-hit-by-51-attack-leading-to-70k-double-spend https://c

It... has happened. Multiple times. Bitcoin Gold: https://cointelegraph.com/news/bitcoin-gold-blockchain-hit-by-51-attack-leading-to-70k-double-spend https://cointelegraph.com/news/bitcoin-gold-blockchain-hit-b... Vertcoin: https://www.coindesk.com/the-vertcoin-cryptocurrency-just-got-51-attacked-again https://www.coindesk.com/the-vertcoin-cryptocurrency-just-go... Ethereum Classic: https://cointelegraph.com/news/ethereum-classic-51-attack-the-reality-of-proof-of-work https://cointelegraph.com/news/ethereum-classic-51-attack-th... Google is happy to show you more.

trixie_ 6y ago
Right, I guess I feel less at risk of being the victim of a double spend and more at risk with rewriting history. Are there any reports of that? Like coins I've held on the chain for some time being stolen?
- hn_acc_2 6y ago
  The value of your long-held coins will be almost nothing if the network is vulnerable to double spend. The potential losses are huge (especially for exchanges etc.) from double spend attacks, and the value of a cryptocurrency is very much dependent on users' confidence in its resistance to attacks.
  - trixie_ 6y ago
    Good. My long held worthless altcoins will remain worthless and mine regardless.
  - wmf 6y ago
    Weirdly, this doesn't seem to be the case. Coins with no hashrate like BCH are still trading at pretty high prices. https://cointelegraph.com/news/bitcoin-cash-could-face-51-attack-for-10-000-in-rented-hashpower https://cointelegraph.com/news/bitcoin-cash-could-face-51-at...
    - hn_acc_2 6y ago
      You're correct, and the market is always right after all. However I am confident based on previous events that BCH prices will trend strongly downward when news of the next successful 51% attack is released.
    - gridlockd 6y ago
      BCH does have a significant amount of hashrate. A 51% attack being possible is one thing, it being profitable is another. If it's not profitable, it is unlikely to happen. If Hashrate was to drop significantly without commensurate price drops, attacks will happen and then the price will drop to compensate.
- gridlockd 6y ago
  If you're just holding coins you're "safe" in the sense that you will get to keep your account balance. However, if the network is unstable, then that balance will lose value in real terms or even become worthless.
  - erik_seaberg 6y ago
    If you sell something in return for coins, and the network could agree on a fork of history in which you never received those coins, not all your balance is safe. Either you trust the network operator(s) not to accept a fork, or you trust the implementation to make a winning fork infeasibly expensive (51% of all miners).
    - gridlockd 6y ago
      The question was "Can a bad actor steal the coins I am just holding, not spending or receiving?" and the answer is generally: No.
- AgentME 6y ago
  A double spend is generally what people mean by "rewriting history". A 51% attack only lets people steal money by taking back recent transactions and sending them elsewhere. Transactions that have been received a long time ago are generally safe. (An attacker can reach further back in time if they run the 51% attack longer or with even more hashpower.)
  - brian_cloutier 6y ago
    This is not quite right. 51% lets you rewrite arbitrarily-deep history, limited only by how much time you're willing to spend on the attack. You have more hash strength than the rest of the network combined! You pick some ancient block and start building blocks off of it, and the very definition of a 51% attack is that your new fork will eventually outgrow the "real" fork. Double spends are possible well before 51%. Even a 10% attacker has the ability to (sometimes) double-spend. A 10% attacker can try to double-spend transactions and has a certain chance of being able to. The chance of success decreases exponentially, the deeper in the chain the target transaction is. However, for a high-enough value transaction, attempting to double spend it can have a higher EV than the mining revenue you lose out on by mining honestly.
Drakar1903 6y ago
I keep wondering what pushes people to stay in the ecosystem of the forked chain that quote unquote lost the fight.

trixie_ 6y ago

Right, I guess I feel less at risk of being the victim of a double spend and more at risk with rewriting history. Are there any reports of that? Like coins I've held on the chain for some time being stolen?

hn_acc_2 6y ago
The value of your long-held coins will be almost nothing if the network is vulnerable to double spend. The potential losses are huge (especially for exchanges etc.) from double spend attacks, and the value of a cryptocurrency is very much dependent on users' confidence in its resistance to attacks.
- trixie_ 6y ago
  Good. My long held worthless altcoins will remain worthless and mine regardless.
- wmf 6y ago
  Weirdly, this doesn't seem to be the case. Coins with no hashrate like BCH are still trading at pretty high prices. https://cointelegraph.com/news/bitcoin-cash-could-face-51-attack-for-10-000-in-rented-hashpower https://cointelegraph.com/news/bitcoin-cash-could-face-51-at...
  - hn_acc_2 6y ago
    You're correct, and the market is always right after all. However I am confident based on previous events that BCH prices will trend strongly downward when news of the next successful 51% attack is released.
  - gridlockd 6y ago
    BCH does have a significant amount of hashrate. A 51% attack being possible is one thing, it being profitable is another. If it's not profitable, it is unlikely to happen. If Hashrate was to drop significantly without commensurate price drops, attacks will happen and then the price will drop to compensate.
gridlockd 6y ago
If you're just holding coins you're "safe" in the sense that you will get to keep your account balance. However, if the network is unstable, then that balance will lose value in real terms or even become worthless.
- erik_seaberg 6y ago
  If you sell something in return for coins, and the network could agree on a fork of history in which you never received those coins, not all your balance is safe. Either you trust the network operator(s) not to accept a fork, or you trust the implementation to make a winning fork infeasibly expensive (51% of all miners).
  - gridlockd 6y ago
    The question was "Can a bad actor steal the coins I am just holding, not spending or receiving?" and the answer is generally: No.
AgentME 6y ago
A double spend is generally what people mean by "rewriting history". A 51% attack only lets people steal money by taking back recent transactions and sending them elsewhere. Transactions that have been received a long time ago are generally safe. (An attacker can reach further back in time if they run the 51% attack longer or with even more hashpower.)
- brian_cloutier 6y ago
  This is not quite right. 51% lets you rewrite arbitrarily-deep history, limited only by how much time you're willing to spend on the attack. You have more hash strength than the rest of the network combined! You pick some ancient block and start building blocks off of it, and the very definition of a 51% attack is that your new fork will eventually outgrow the "real" fork. Double spends are possible well before 51%. Even a 10% attacker has the ability to (sometimes) double-spend. A 10% attacker can try to double-spend transactions and has a certain chance of being able to. The chance of success decreases exponentially, the deeper in the chain the target transaction is. However, for a high-enough value transaction, attempting to double spend it can have a higher EV than the mining revenue you lose out on by mining honestly.

Drakar1903 6y ago

I keep wondering what pushes people to stay in the ecosystem of the forked chain that quote unquote lost the fight.