Quote from: StephenMorse on February 17, 2015, 02:45:36 PM

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 think that's very likely. Attacking the system isn't without risk. It would be hard to pull off such a large undertaking without ever being exposed, which then subjects the attackers to whatever backlash there is, political or otherwise, from a growing global community with increasing stakes in the system, all for a risky maneuver which might not amount to more than a temporary network inconvenience.

Quote from: StephenMorse on February 17, 2015, 02:45:36 PM

I don't follow when you say that "Multiple conflicting transactions would break the attack, though, as there would be multiple orphan chains".

I was responding to this:

Quote

The miner could even purposefully spread double spends between the two chains. Even transaction malleability could be utilized between the two chains, putting essentially the same transaction on the two chains, but with different hashes. This would make the network very unreliable and hard to use.

There is no need to spread double spends or utilize transaction malleability. It only takes one inconsistent transaction, of any form, to create a fork. Once you have two chains your vision for the attack could commence. Adding further inconsistencies within either of the two extending chains would cause further splits, wasting valuable hashing power for an attacker trying to keep two relatively similar length, but different, chains.

Quote from: StephenMorse on February 17, 2015, 02:45:36 PM

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.

That's a way the network could break out of that attack, actually. Say things were proceeding normally and an attacker gaining 51% forked the chain, then worked to keep each about equal length. If there was some agreement compelling say 25% of the "honest" network to create an inconsequentially inconsistent, but valid third fork the attacker would quickly find it difficult to maintain equal lengths. The distribution of hashing power would meander over the three then gravitate toward the longer, least split chain, breaking the tie.

Remember, a valid chain contains all valid transactions and no double spends. As long as there is a longest valid chain, even if the 51% attacker is the one that forked and extended it, everything still works. Now an attacker could use 51% to block or filter transactions etc., things already discussed, but that's not the attack you're describing.