Quote from: acoindr on February 17, 2015, 05:30:06 AM

Quote from: StephenMorse on February 10, 2015, 02:58:14 PM

... The disruptive part is that there would be switching back and forth all the time, and the two chains could contain very conflicting data. ...

That wouldn't be Bitcoin, but some other protocol/alt-coin. The Bitcoin protocol isn't only the longest chain; it's the longest chain that everyone agrees follows all the protocol rules, which includes not double spending coins and hash values which match all transactions (switched with malleability or not). A hash power advantage doesn't enable the circumvention of that.

Edit: I just got what you're saying. That would be a bit of a problem if the attacker could maintain 51% for a long period of time. Multiple conflicting transactions would break the attack, though, as there would be multiple orphan chains, but maintaining two chains of approximately equal length with one fork would mean users in the real world would need to wait to see what was the "real version of reality". I think most 51% risk comes from pooled miners, which in a stressful situation (like that of GHash.io) could migrate away, removing the hash power advantage before long.

Yeah, I think you got what I was saying in the second paragraph. The two chains that are kept alive would be completely consistent within the individual chains, but the chain data between the two would be incompatible.

And yeah, I agree that if this were to happen in a pool then everyone would just leave the pool and it wouldn't be a huge problem. I think I was more thinking about some country who hates bitcoin buying up a bunch of hardware and then attacking the system like this to shut it down.

I don't follow when you say that "Multiple conflicting transactions would break the attack, though, as there would be multiple orphan chains". If there were a fork on one of the chains, it must have been on the chain the attacker wasn't working on (because they are always the only ones working on the shorter (less-work) chain). And since it is the chain opposite to the chain the attacker is working on, the attacker will just keep working until it creates a chain with more work than either of the mini-forks on the other side of the chain.