Quote from: BurtW on December 24, 2013, 02:47:02 PM

Do you even know what a 51% attack is? Do you know what can be done using it? Do you know what would happen if a pool tried to do it?

Let's do a mind experiment. Assume BTC guild, right now, has 60% of all the hashing power. Now explain to me exactly what they would do in order to pull off a "51% attack", exactly what they would accomplish by doing it and most importantly exactly why they would do it.

Please don't talk down to people like that, it's not constructive, and given the nature of a lot of the conversations on this board, I don't think this was an invalid question.

Answers:

Yes. Yes. : https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power

No. I'm not sure what would happen. If you have a good idea, and think that the consequences make it not something to worry about, I think many people would like to know that information. That's pretty much why I asked. Should we be concerned? It sounds like no, but you haven't really explained why.

Here are my conjectures:

It seems like the double spend is probably not practically useful, since it's less of a double spend and more of a "spend and then unspend", which only helps if you're getting something from someone else for spending the bitcoin. And in that case, the other party would start squawking up a storm about it, so it wouldn't go unnoticed.

Forcing transactions not to be confirmed seems like it might be something to be concerned about. It could be hard to determine why its happening, and could be used as a weapon against people that piss off the pool. Of course, that requires the pool to know which transactions belong to that person, which is sorta the whole point of anonymity in the blockchain.... but I'm not sure how hard it would be in practice.

Preventing other miners from getting blocks seems like the most likely attack vector, since it could be the hardest to recognize. The top two pools could just give themselves 10% more block awards by rejecting 10% of blocks submitted from outside their pools, and waiting until one of them finds the block. I'm not sure how obvious that would be to people watching transactions on the block chain. The benefits are obvious - they get 10% more block awards than they're entitled to.

I'm relatively new to bitcoin, so some or all of this may be off, feel free to tell me where I've gone wrong, after all, I am trying to learn.