Quote from: andes on June 10, 2011, 06:15:26 AM

Quote from: kjj on June 07, 2011, 11:43:45 PM

There are two versions of the 51% attack, and I have made proposals that would address both of them.

Oh, and by the way, neither of these attacks are anywhere near as easy, nor as useful as popularly imagined. And we really have read 100 threads on the topic.

I have seen other people claim that this attack is not easy nor useful, but when asked to explain why their argument crumbles. By the way, I saw this post from kjj in another thread:

Quote from: kjj on May 21, 2011, 08:46:19 AM

No, nothing in this thread is right.
...
The scenarios involving technical manipulation are entirely founded on misconceptions. The network really doesn't work the way you imagine it does. Someone would need several orders of magnitude more computing power than the rest of the world combined to pull off a block chain manipulation, and it would gain them very, very little.

Several orders of magnitude more computing power than the rest of the world combined to manipulate the chain? Hmmm. One thing is clear, you only need 50% or less of the total mining capacity (supposedly currently worth 50 million dollar or less) to create lots of problems. And this would be the most expensive attack possible. Obvioulsy you could come up with more cost efficient ways to attack. Clearly, you dont need alien technology.

It isn't easy because of the gigantic amount of resources necessary for the attack. For example, you could not purchase enough hashing power today to do the attack, because it does not exist in the world in purchasable form. I would even say that there is only one government in the world that would even have the potential ability to confiscate enough hashing power by sending armed troops into peoples houses and stealing their video cards. Someone would notice that. It would require months or years of gathering resources, all while the network is growing, and even a slow acquisition would likely be noticed.

And it isn't useful, because your payback for spending all of this time and money gathering hashing power is the ability to turn back a few transactions. What possible transaction would you reverse that was worth your 50 million dollar investment? Keep in mind that as the value of future transactions grows, so will the cost of doing the attack. Right now you would need to own roughly a quarter of all existing bitcoins, and spend all of them within the attack window, to beat the cost of your investment. That ratio will probably change somewhat in the future, but the attack will never make sense unless you already control a non-trivial fraction of the bitcoins in the world, and can find enough victims to accept all of them in a short period.

And that other thread you link isn't about this sort of attack at all. It is about difficulty and price manipulation.

And again, exponential difficulty can make these attacks even more costly.