Quote from: dga on August 20, 2014, 08:02:52 PM

Quote from: devphp on August 20, 2014, 07:20:28 PM

Quote from: rpietila on August 20, 2014, 07:10:24 PM

No. You need the majority of stake in any point of time past, to forge all the history of the future from that point of time on. So any group who ever had 51% of the currency/votes/etc. can rewrite the history at will. Or any group that can coerce any group at any point of time. There is also no mechanism to keep track which of the forks is the correct one.

Yes, there is a mechanism to keep track which fork is correct. It's called Economic Clustering. But it will likely never be utilized, as getting private keys of 51% coins seems a much more insurmountable task than coercing 2-3 biggest PoW pool operators to follow a certain agenda. But EC will be there just in case, so yes, nodes can know which fork is correct and be in consensus. There is also a penalty in EC for forgers who forge on an incorrect fork.

As I stated a month or so ago in this thread, I think people mis-estimate the ease of borrowing large amounts of money if you already have large amounts of money.

https://bitcointalk.org/index.php?topic=624223.msg7689885#msg7689885

If you're a billionaire, you could borrow a billion dollars of a coin (or buy it outright, use it as rpietila said, and then re-sell), and if you worked through proxies, you could probably do it without people even realizing you'd done it.

Well hedged bets like this make people a lot of money.

Heck - you could even do the attack, make money off of it, and then short the currency hard as it looks like it's going to drop and then reveal that it was owned, causing it to collapse faster...

Adversarial finance is tricky.

And let me make this very concrete: I've actually had a conversation along these lines with someone who works at a private investment firm. It was speculative, and probably won't go anywhere, but don't think that people aren't already starting to plan for things like this in the event that bitcoin or foo-coin becomes successful.

You're not wrong, but I fail to see how this doesn't apply to PoW as well. Someone could short/borrow a large amount of BTC and perform any sort of attack. It wouldn't even have to be a 51% attack to cause a massive crash. Someone providing as little as 25% of Bitcoin's hashrate going rogue and attempting double spends or doing any of the attacks outlined on hackingdistributed would cause a panic.

It would seem that PoS is inherently more resistant to the 'rich guy' attack. And as far as borrowing goes apparently NXT is implenting a change that would require someone to have 90% of the stake in order to fork(is that correct?). Keeping 10% of the total stake unborrowed seems like a reasonable challenge that can be met. That almost sounds too good to be true, so I'm pretty skeptical about a cryptocurrency that requires 90% consensus actually coming true.

If it's a question of what's more secure, I'm betting that within two years we'll have at least one working PoS system that's demonstrably more resistant to attacks than PoW. Maybe I'm just being optimistic, but it seems to be heading that way.

edit: Ah, you said at the end there that people are already starting to plan such things for Bitcoin. Which makes perfect sense and will likely be a major problem in the future. How would any cryptocurrency, no matter the type, survive when the financial benefit of attacking it is positive?