Re: [Bitcoin-development] BIP proposal - patch to raise selfish mining threshold.

On Tue, Nov 05, 2013 at 12:43:15PM -0500, Ittay wrote:

On Tue, Nov 5, 2013 at 12:14 PM, Peter Todd wrote:

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On Tue, Nov 05, 2013 at 12:05:41PM -0500, Peter Todd wrote:

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On Tue, Nov 05, 2013 at 11:56:53AM -0500, Ittay wrote:

Oh, and I don't want to give the wrong impression: there's no need to rush to get this problem fixed. Even if someone wanted to launch an attack right now, with a fair amount of resources, there's a lot of counter-measures based on human intervention that can definitely stop the attack in the short-term

The attack can be easily hidden. And be sure that before today, today, and after today, very smart people are at their computer planning attacks on Bitcoin. Exploits must be published and fixed FAST.

Not this exploit. Here's a perfectly plausible worst-case scenario, that could be happening right now: RAND High Frequency Trading Corp (a subsidiary of General Evil) has a globe-spanning low-latency network of fiber, line-of-sight microwave, and some experimental line-of-site neutrino links criss-crossing the globe. They can get data to and from any point on this planet faster than anyone else. Of course, in addition to their spectacular network they have an immense amount of computing power, as well as exotic overclocked liquid nitrogen bathed CPU's that run at clockspeeds double what commercial hardware can do; in short, they have access to scalar performance no-one else has. Of course, they like to keep a healthy reserve so, 99% of all this fancy gear is constantly idle. Whatever, they can afford it. RAND just hired a bunch of fresh MIT graduates, the best of the best. Problem is the best of the best tends to make not so best mistakes, so RAND figures a Training Exercise is in order. Fortunately for them the NSA (a subsidiary of General Evil) slipped a rootkit into my keyboard a week or so ago - probably that time when I woke up in that farmers field with a *splitting* headache - and are reading what I'm typing right now. I go on to explain how an excellent training exercise for these fresh MIT graduates would be to implement this nifty attack some Cornell researchers came up with. It's really simple, elegant even, but to do it best what you really want is the kind of low-latency network a high-frequency-trading corporation would have. I then point out how a good training exercise ideally is done in a scenario where there is genuine risk and reward, but where the worst-case consequences are manageable - new hires to tend to screw up. (I then go on to explain my analog electronics background, and squeeze in some forced anecdote about how I blew up something worth a lot of money owned by my employers at some point in the distant past) Unfortunately for the operators of BTC Guild, one of these new MIT grads happens to have a: passed General Evil's psych screening with flying colors, and b: have spent too much time around the MIT Kidnappng Club. He decides it'd be easier to just kidnap the guy running BTC Guild than fill out the paperwork to borrow RAND's FPGA cluster, so he does. As expected the attack runs smoothly: with 30% of the hashing power, neutrino burst generator/encoders's rigged around the globe to fire the moment another pool gets a block, and the odd DoS attack for fun, they quickly make a mockery of the Bitcoin network, reducing every other miners profitability to zero in minutes. The other miners don't have a hope: they're blocks have to travel the long way, along the surface of the earth, while RAND's blocks shave off important milliseconds by taking the direct route. Of course, this doesn't go unnoticed, er, eventualy: 12 hours later the operators of GHash.IO, Eligius, slush, Bitminter, Eclipse and ASICMiner open their groggy eyes and mutter something about how that simulcast Tuesday party really shouldn't have had an open bar... or so much coke. They don't even notice that the team from BTC Guild has vanished, but they do notice a YouTube video of Gavin right on bitcoin.org doing his best Spock impression, er, I mean appealing for calm and promising that Top Men are working on the issue of empty blocks as we speak. Meanwhile CNN's top headline reads "IS THIS THE END OF BITCOIN?!?!" It takes another hour for the Aspirin's to finally kick in, but eventually get all get on IRC and start trying to resolve the issue - seems that whenever any of them produce a block, somehow by incredible coincidence someone else finds another block first. After a few rounds of this they're getting suspicious. (if they weren't all so hung-over they might have also found suspicious the fact that whenever they found a block they saw a sudden blue flash - Cherenkov radiation emitted when those neutrino's interacted with the vitreous humour in their eyeballs) It's quickly realized that "somehow" BTC Guild isn't affected... GHash.IO and Eligius, 22% and 13% of the hashing power respectively, decide to try a little experiment: they peer to each other and only each other through an encrypted tunnel and... hey, no more lucky blocks! slush, 7% of the hashing power is invited to the peering group next, followed by Bitminter, 6%, and Eclipse, 2%, and finally ASICMiner, 1%, for a grand total of... 51% of the hashing power! Of course, just creating blocks isn't useful for users, they need to be distributed too, so someone quickly writes up a "one-way firewall" patch that allows the group's blocks to propagate to the rest of the network. Blocks created by anyone else are ignored. It takes a few more hours, but eventually the attacker seems to run out of blocks, and transaction processing returns to normal, albeit a little slow. (20 min block average) Of course, soon there's a 3,000 post thread on bitcointalk complaining about the "centralized pool cartel", but somehow life goes on. The next day Gavin goes on CNN, and gives a lovely interview about how the past two days events show how the strength of the Bitcoin network is in the community. For balance they interview this annoying "Peter Todd" guy from "Keep Bitcoin Free!" who blathers on about how relying on altruism or something will doom the Bitcoin network in the long run. After the interview Gavin respectfully points out that maybe next time they find a so called "developer" with a ratio of bitcointalk posts to actual lines of code in the Bitcoin git repository better than one hundred to one. The producer just wishes that "Mike Hearn" guy was available; at least he's got a sense of fashion, sheesh! Anyway, I'm out of space for my little story, but yeah, the ending involves a group of now-rich pool operators who decide to start a large financial services and data networking company, oh, and time-travel...

Nevertheless, I agree that, as you say, we must not rush it. Look at the BIP, find if we missed anything, and let's discuss it.

Indeed. Quite seriously, your attack is a serious long-term risk, but in the short term the social dynamics of Bitcoin are such that it's just not a show-stopping risk. At worst some miners will lose a bunch of money - that's something that's happened before with the March chain fork, and sure enough Bitcoin survived just fine.

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In addition, keep in mind that this attack is very easy to detect, so if one is actually launched we will know immediately and can start taking direct counter-measures at that time.

Not really. Please see the discussion section in our paper.

You can hide *who* is the attacker - you can't hide the fact that an attack is happening if done on a meaningful scale.

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That Gregory Maxwell so quickly identified a flaw in this proposed solution suggests we should proceed carefully.

There is no flaw. You were just reiterating that the solution does not give us the 51% percent security you thought you had before. We showed that we're not getting this back, I'm afraid.

That's not what we're concerned about - what we're concerned about is that your BIP doesn't discuss the issue, and you didn't seem to be aware of it. That suggests that the analysis is incomplete. There's no pressing need to rush changes, as explained above by example, so we're best off understanding the issue thoroughly first. There's a whole spectrum of potential solutions that haven't been discussed - I myself have two approaches I'm working on that may solve this problem in ways you haven't (publicly) considered. I'm sure there are many others out there. -- 'peter'[:-1]@petertodd.org 00000000000000005144f6a7109b9f8543880a0a5f85a054ec53966bc2daa24c ------------------------------------------------------------------------------ November Webinars for C, C++, Fortran Developers Accelerate application performance with scalable programming models. Explore techniques for threading, error checking, porting, and tuning. Get the most from the latest Intel processors and coprocessors. See abstracts and register http://pubads.g.doubleclick.net/gampad/clk?id=60136231&iu=/4140/ostg.clktrk _______________________________________________ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development