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Comment by brian_cloutier | Hacker News Reader

I haven't read the article, but I believe it will say something like, the continual investment is necessary in order for the transactions to be irreversible (settlement assurance). I'm sure there are some forgotten altcoins out there still chugging along, but it probably does not take much money to hire sufficient hash power to double-spend those alts, or even re-write their history.

For as long as bitcoin miners fight, tooth and nail, to hash incrementally more efficiently than their competitors, it will be prohibitively expensive for most actors to reverse transactions.

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Not saying this isn't a possibility, but you'd think in the last 5 years of thousands of altcoins this sort of thing must of happened at least once? I guess the interest has to be there to attack it? Though if the interest was there more people would be mining it. Something like that.

It... has happened. Multiple times.

Bitcoin Gold: https://cointelegraph.com/news/bitcoin-gold-blockchain-hit-b...

Vertcoin: https://www.coindesk.com/the-vertcoin-cryptocurrency-just-go...

Ethereum Classic: https://cointelegraph.com/news/ethereum-classic-51-attack-th...

Google is happy to show you more.

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?

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.

Good. My long held worthless altcoins will remain worthless and mine regardless.

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-at...

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.

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.

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.

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).

The question was "Can a bad actor steal the coins I am just holding, not spending or receiving?" and the answer is generally: No.

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.)

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.

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

Bitcoin Gold was hit by double spend multiple times, and that's just what I remember off the top of my head.

However there are obviously diminishing returns for such an attack. The alt coin value drops drastically when users find out about the attack and flee to more secure currencies.

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