Majority is not Enough: Bitcoin Mining is Vulnerable

[acoindr]

Mined Block Reference Nodes

Latency. A thousand curses to latency! It's the reason this thread exists, a questionable attack vector. It's the source of orphaned blocks and lost revenue. Need I mention the continuing block size problem is due to -- you got it -- latency.

If we could get rid of latency the above problems would vanish! What a relief that would be. It may be possible.

When I conceived Mined Block Reference Nodes (MBRNs) in answer to people questioning Litecoin's reduced block time I thought they might come to exist with any blockchain of continued success. I hadn't worked out the details but the prospect seemed promising. This latest issue caused me to think more carefully.

A MBRN does not require a protocol change. They are not an integral part of the network. Whether they exist or not provides incremental, but not critical improvement overall, like hashrate variance.

A MBRN offers miners two key things: the quickest indication of a new valid block, and the largest list of nodes having it.

How to Build a Mined Block Reference Node

The incentive for building a MBRN, and there can be many, is probably altruistic but can include profit. I see this as a web app over http which I think has benefits, like easy use of DNS which can be useful as I'll describe.

An administrator starts with a tipping wallet address. Just as miners expect users to tip fees for valuable service, a MBRN expects this of miners. Reducing orphaned blocks and increasing network performance means the same level hashrate security with less revenue due power companies, a direct savings.

There can be different configurations but a simple dedicated hosted server is probably fine. The app may serve thousands of requests per second, but isn't function or computation complex.

The admin publishes rules of good behavior and keeps a couple whitelists. There are three (high demand) interactions with a node: query for new block, push new block, and query node list; plus the threat of DDOS so whitelists are needed. Querying for a new block may be limited to once say per 5-10 seconds, and returns the highest block number known.

When accepting a pushed block the node screens and prioritizes by whitelist -- IP addresses that provided a valid block prior -- as this is bandwidth and resource significant. Considering the effects miners will desire to stay within the good graces of a MBRN, and bad actors will be quickly identified.

When new blocks are recognized the height number is updated and the MBRN directly serves maybe the last 2 or 3 blocks on a first come first served basis. For each querying node served the MBRN keeps a constantly growing list of known nodes having the block, starting with itself. Every node calls back to the MBRN when they receive the block; they have incentive to do so because they have started mining on top of it and don't want it orphaned. In this way the knowledge about and dissemination of any new block spreads quickly and exponentially, while not requiring significant bandwidth up or down from the MBRN.

Multiple MBRNs may communicate with each other in the spirit of altruism; and tipping for good service provides a market check and balance for weeding out bad actors.

DDOS

Bitcoin ecosystem websites have already been victims of DDOS and a MBRN would be a target too. As I said in an argument with DeathAndTaxes on this subject that attack's strain on a website usually occurs with the database -- often the same fault under legitimate higher than expected traffic. However, the difficulties in dealing with DDOS as a website will be different than for a MBRN. A website can't predict user behavior and usually has multiple pages of various process complexity to serve reliably. Neither of those are true for a MBRN. Normal websites also can't predict which IP addresses to expect on a regular basis. Here again a MBRN has an advantage. I believe a MBRN can defend well against DDOS for these reasons, but also by being nimble.

Bitcoin websites are finding ways to mitigate DDOS. I'm increasingly seeing use of Cloudflare, for example, which works by sitting at DNS level and handling attacks from there. This is a strategy MBRNs can use too as Cloudflare compiles all the bad actor IP addresses it sees naturally. Their DDOS protection starts at around $200 per month. Take that with the cost of a dedicated server and you have only a few hundred dollars per month expense to run a MBRN, while probably at least that in tips. There are other anti-DDOS configurations I've thought about too, some also involving DNS management, say with EasyDNS.com (situated well globally), to load balance across multiple hosts if desired, because the app is so simple. This is in consideration of what DeathAndTaxes mentioned about severe bandwith impacting DDOS. The answer to that is distribution, and is easily managed with DNS for example, or a CDN like Akamai as I mentioned prior and which MTGox now subscribes to.

Block Size

A final consideration is block size. If this model works in practice imagine how it impacts block size. A key argument against raising the block size limit is it results in the best connected, ie the best funded, eventually dominating won blocks which is supposedly random. This puts far less pressure on network connection for even the smallest miner.

I think that covers it.

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[Enochian]

So these two guys at Cornell University, an associate professor and a post-doc, publish a paper on Arxiv about what they believe to be a flaw in Bitcoin.

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."  News outlets, seeing no reason to believe two guys with PhDs employed by prestigious Cornell University are spraying FUD with a large diameter hose, run with the story that a fatal flaw has been discovered in Bitcoin, and its collapse is iminent.

The central thesis of his paper is that you can make mining slightly more profitable by picking when to make public blocks you have mined, and that this will cause miners to join your pool until it increases to a size which threatens the decentralized nature of the Bitcoin cryptocurrency.

Now we've discussed the threat posed by large mining pools for years, and it makes little difference whether they attract clients by gaming the mining protocol, paying a premium for shares, promising a share of the windfall profits that result when they reach 51%, or giving away free toasters.

It is generally accepted that such shenanigans would be noticed by the community before any harm was done, and appropriate policies adopted.  Therefore, it is unlikely people will make the investment to try such things, only to have them nipped in the bud forthwith.

The professor reacts to skepticism about his attack by saying that all rebuttals must "include math" to be seriously considered, and goes on and on about the great burden he endures by having to put up with the stupid.

He deletes critical blog comments claiming they contain "profanity," which apparently includes the word "fraud" and Will Rogers quotes.  After a few days of being slow-roasted on the spit of Bitcoin community displeasure, he defiantly declares" I used to think that money brought out the worst in people, until I saw Bitcoin."

The reaction of the developers to the idea that this attack poses some serious real threat to Bitcoin has been somewhere between "yawn" and "snooze."

Now the paper is interesting, and it has resulted in some serious discussions about how such an attack might play out, although there are some questions whether the methodology is a perfect fit to the problem being described.  Had it simply been released without the accompanying egregious self-promotion and predictions of Bitcoin demise, it probably would have been warmly received, and constructive feedback cheerfully provided.

This whole scenario is like a guy walking up to show everyone a marvelous anti-gravity car he has invented.  He describes the engine in detail with complicated fluid dynamic equations and state diagrams, and refuses to entertain any rebuttals of his design methodology which don't use these tools.  Meanwhile, a less technical crowd has gathered, and noticed there is no hose connecting the engine to the fuel tank.



 

[justusranvier]

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."  News outlets, seeing no reason to believe two guys with PhDs employed by prestigious Cornell University are spraying FUD with a large diameter hose, run with the story that a fatal flaw has been discovered in Bitcoin, and its collapse is iminent.

Based on his behaviour over the last few days, I give him an 8/10 for douchebag.

He's eligible for a perfect score if it turns out he posts on this forum as "revans" and if he's in any way related to the DDoS attempts on the exchanges and charting sites yesterday.

[bee7]

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."  News outlets, seeing no reason to believe two guys with PhDs employed by prestigious Cornell University are spraying FUD with a large diameter hose, run with the story that a fatal flaw has been discovered in Bitcoin, and its collapse is iminent.

Based on his behaviour over the last few days, I give him an 8/10 for douchebag.

He's eligible for a perfect score if it turns out he posts on this forum as "revans" and if he's in any way related to the DDoS attempts on the exchanges and charting sites yesterday.

So, how long before these idiots make a public apology/resign?

They will not. What for?

[RodeoX]

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."  News outlets, seeing no reason to believe two guys with PhDs employed by prestigious Cornell University are spraying FUD with a large diameter hose, run with the story that a fatal flaw has been discovered in Bitcoin, and its collapse is iminent.

Based on his behaviour over the last few days, I give him an 8/10 for douchebag.

He's eligible for a perfect score if it turns out he posts on this forum as "revans" and if he's in any way related to the DDoS attempts on the exchanges and charting sites yesterday.

I hope he sold all his coins before publishing in an attempt to buy back during some kind of crash. I don't doubt the credentials of the guys who discovered this exploit, but the way they released the information in sensational terms is suspect.

[donut]

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."  News outlets, seeing no reason to believe two guys with PhDs employed by prestigious Cornell University are spraying FUD with a large diameter hose, run with the story that a fatal flaw has been discovered in Bitcoin, and its collapse is iminent.

Based on his behaviour over the last few days, I give him an 8/10 for douchebag.

He's eligible for a perfect score if it turns out he posts on this forum as "revans" and if he's in any way related to the DDoS attempts on the exchanges and charting sites yesterday.

I hope he sold all his coins before publishing in an attempt to buy back during some kind of crash. I don't doubt the credentials of the guys who discovered this exploit, but the way they released the information in sensational terms is suspect.

The "exploit" he "discovered" has been discussed in 2010 on these very forums.

[revans]

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."  News outlets, seeing no reason to believe two guys with PhDs employed by prestigious Cornell University are spraying FUD with a large diameter hose, run with the story that a fatal flaw has been discovered in Bitcoin, and its collapse is iminent.

Based on his behaviour over the last few days, I give him an 8/10 for douchebag.

He's eligible for a perfect score if it turns out he posts on this forum as "revans" and if he's in any way related to the DDoS attempts on the exchanges and charting sites yesterday.

You are an idiot.

[RodeoX]

The "exploit" he "discovered" has been discussed in 2010 on these very forums.

Indeed, apparently Satoshi also considered it. And, if I'm not mistaken, it is a modified "Sybil" attack. Which is also known.

[gyverlb]

So these two guys at Cornell University, an associate professor and a post-doc, publish a paper on Arxiv about what they believe to be a flaw in Bitcoin.

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."

Wow, I didn't know about the "sell" tweet. I had to check on Twitter to believe it (and found lots of damage control and whining in the process).

After publishing a supposed security flaw without contacting the devs first which is already amateurish at best, the "sell" tweet now makes the professor suspect of having an agenda.
Even if his intentions weren't dishonest his public life is now checked under the microscope by hordes of people with various motivations. I wouldn't want to be in his position right now especially if he has anything to hide. Go easy on him people: with the way he is currently handling his paper he begins his career with quite an handicap already...

[cunicula]

So these two guys at Cornell University, an associate professor and a post-doc, publish a paper on Arxiv about what they believe to be a flaw in Bitcoin.

Rather than quietly and tastefully wait for some feedback on their ideas, the professor immediately Tweets "You heard it here first: now is a good time to sell your Bitcoins."  He then proceeds to blog "Bitcoin is Broken."  News outlets, seeing no reason to believe two guys with PhDs employed by prestigious Cornell University are spraying FUD with a large diameter hose, run with the story that a fatal flaw has been discovered in Bitcoin, and its collapse is iminent.

The central thesis of his paper is that you can make mining slightly more profitable by picking when to make public blocks you have mined, and that this will cause miners to join your pool until it increases to a size which threatens the decentralized nature of the Bitcoin cryptocurrency.

Now we've discussed the threat posed by large mining pools for years, and it makes little difference whether they attract clients by gaming the mining protocol, paying a premium for shares, promising a share of the windfall profits that result when they reach 51%, or giving away free toasters.

It is generally accepted that such shenanigans would be noticed by the community before any harm was done, and appropriate policies adopted.  Therefore, it is unlikely people will make the investment to try such things, only to have them nipped in the bud forthwith.

The professor reacts to skepticism about his attack by saying that all rebuttals must "include math" to be seriously considered, and goes on and on about the great burden he endures by having to put up with the stupid.

He deletes critical blog comments claiming they contain "profanity," which apparently includes the word "fraud" and Will Rogers quotes.  After a few days of being slow-roasted on the spit of Bitcoin community displeasure, he defiantly declares" I used to think that money brought out the worst in people, until I saw Bitcoin."

The reaction of the developers to the idea that this attack poses some serious real threat to Bitcoin has been somewhere between "yawn" and "snooze."

Now the paper is interesting, and it has resulted in some serious discussions about how such an attack might play out, although there are some questions whether the methodology is a perfect fit to the problem being described.  Had it simply been released without the accompanying egregious self-promotion and predictions of Bitcoin demise, it probably would have been warmly received, and constructive feedback cheerfully provided.

This whole scenario is like a guy walking up to show everyone a marvelous anti-gravity car he has invented.  He describes the engine in detail with complicated fluid dynamic equations and state diagrams, and refuses to entertain any rebuttals of his design methodology which don't use these tools.  Meanwhile, a less technical crowd has gathered, and noticed there is no hose connecting the engine to the fuel tank.



 

This sums it up nicely, particularly the paragraph in bold.

[Meni Rosenfeld]

Here is the tweet about selling bitcoins: https://twitter.com/el33th4xor/status/397219415025934336

They are not acting in good faith. Apparently they don't see a problem with their behavior.

They can't even get their stories straight - https://twitter.com/el33th4xor/status/397827176759697408

[chriswilmer]

Here is the tweet about selling bitcoins: https://twitter.com/el33th4xor/status/397219415025934336

It is clear now they are not acting in good faith. Apparently they have no idea at all there's something wrong with their extreme arrogance and spreading factual lies and FUD.

They can't even get their lies straight between the two of them - https://twitter.com/el33th4xor/status/397827176759697408

Wow. I really liked the paper when I first skimmed through it and I even sent a nice note to the authors... but that was all before the Business Insider press releases and this. I am an academic myself and I think telling people to sell their bitcoins definitely crosses a line when it comes to intellectual discourse.

[justusranvier]

I am an academic myself and I think telling people to sell their bitcoins definitely crosses a line when it comes to intellectual discourse.

I wonder if Cornell professors are accountable for lapses in ethical academic behaviour?

[solex]

I am an academic myself and I think telling people to sell their bitcoins definitely crosses a line when it comes to intellectual discourse.

I wonder if Cornell professors are accountable for lapses in ethical academic behaviour?

Call me old fashioned but I thought a research paper normally involves research (here: discourse with core dev), and also involves peer review before publishing. Or has Cornell abandoned such archaic practices?

[gmaxwell]

Call me old fashioned but I thought a research paper normally involves research (here: discourse with core dev), and also involves peer review before publishing. Or has Cornell abandoned such archaic practices?

On the subject of Bitcoin fairly few academics have bothered to communicate with the experts in "industry" (e.g. core devs, folks here posting with books-badges, etc). It's a shame: the failure has resulted in a lot of really confused papers which could have been very easily improve a lot with some expert pointers.

But there are a lot of broken things out there— e.g. no ethical review on experimentation, etc. (I loved one paper I saw that said something like there was no ethical concerns with their live fire privacy attacks on the unwitting public "because anyone else could have performed the same attacks").

[TooDumbForBitcoin]

I wonder if Cornell professors are accountable for lapses in ethical academic behaviour?

Take away five CS scholarships and ban them from College Bowl for two years. 

[MicroGuy]

Even though it's a little taboo to mention, the truth is: Bitcoin's developer abandoned the project years ago and disappeared into thin air. If he was still around he would be actively evolving the coin and addressing issues like this one in a clever manner. Bitcoin without Satoshi is like Apple without Jobs.

To my understanding, the only coin actively developing a 51% defense is Goldcoin whose developer is actively building upon Satoshi's unfinished work.

[gyverlb]

Even though it's a little taboo to mention, the truth is: Bitcoin's developer abandoned the project years ago and disappeared into thin air. If he was still around he would be actively evolving the coin and addressing issues like this one in a clever manner. Bitcoin without Satoshi is like Apple without Jobs.

To my understanding, the only coin actively developing a 51% defense is Goldcoin whose developer is actively building upon Satoshi's unfinished work.

You mean one of the coins that I blacklisted in my scrypt miners because my up-to-date node was on a forked branch? You have some balls to come advertise Goldcoin in this thread!

[Luke-Jr]

Even though it's a little taboo to mention, the truth is: Bitcoin's developer abandoned the project years ago and disappeared into thin air. If he was still around he would be actively evolving the coin and addressing issues like this one in a clever manner. Bitcoin without Satoshi is like Apple without Jobs.

This is nonsense. Bitcoin still has numerous developers working on it.

To my understanding, the only coin actively developing a 51% defense is Goldcoin whose developer is actively building upon Satoshi's unfinished work.

"51% defence" is nonsensical. As long as you're entrusting decisions to majority hashpower, someone with a majority is making that decision. If you're not doing that, then you're throwing out the entire innovation that Satoshi solved with Bitcoin, and your solution has nothing at all to do with Satoshi.

I'm not saying it's a bad idea to try to solve double spending some other way, just that it's not where Satoshi was going with Bitcoin.

[acoindr]

Another thing about those MBRNs...

Earlier in this tread in response to this quote:

...
Low latency connection itself is expensive, and we can nullify its advantage by relaying unverified block headers. People will always assume a block header is valid unless it is proven otherwise, and always mine on top of the first seen header. (I think creating invalid block header is very expensive and no one is trying to do this. Any stats for this?)
...

I said that broadcasting headers before the final set of transactions doesn't help because miners are never sure what block to mine on top of until they know the winning block. Trolling the network is possible by anyone with any size hash capacity and no worry about block reward.

That's still true, but a MBRN can add trust into the equation. When hearing of a block header from any random node one isn't sure of the rest of the block. Hearing of a block header from a MBRN is different since it's acting as a known source of good, verified information. It's worth the risk mining on top of only a block header then since a header which doesn't match up with the transactions sinks any credibility the MBRN has built up. As justusranvier points out propagation of block headers can be very fast, near instantaneous.

The real value of a MBRN is in the most effective use of hash-rate. It's almost always directed at the most profitable block, near zero waste. This appears to me to completely nullify the Sybil attack dynamic the paper counts as valuable to an attacker.