Topic: How would 51% double spending work in the long term ? Thought experiment (Read 2852 times)

::sigh:: I'm pretty sure I've gone through this before, but perhaps I now know how to explain this in a way that will get through.

Self-consistency and global convergence are mutually incompatible objectives.  Large scale of convergence is the worst possible failure mode for Bitcoin, worse than any amount of transaction reversal.

If participants prefer their past state it means that two participants with different pasts can fail to agree on the current state even when presented with identical information. Small amounts of self-consistency can be tolerated but it is at the direct expense of convergence (which is why bitcoin is often not converged at all after just a single block, though it becomes converged with infinitesimal chance of convergence failure after enough blocks relative to the communication delays).

The problem of non-convergence is made worse for a distributed systems in a universes with a speed of light, as an attacker can pick which nodes have which past by selecting their 'locations' that they use to announce messages and so they can choose the shape of the non-convergence to be most damaging.

To specifically address your particular suggestion:  A byzantine attacker obtains a large amount of hashpower for long enough to mine a couple blocks— say twelve total for your figures— faster than the network, and he maps out the locations of large mining power concentrations with some accuracy.   Then he mines two forks off the current head, starting with blocks A and A'.  He feeds these forks all in parallel to all nodes, giving A to half the estimated hash power and A' to the other half. Congrats: Bitcoin is over, from one currency you have two. (well, at least until people manually intervene, turn off your 'tweak' or force nodes onto the state they're rejecting, and the system causes a reversal of all transactions on one side or another, belonging and trusted by roughly half the participants).

The idea of 'solving' high hashpower attackers by imposing some kind of self-consistency constraint appears to be one of the perennial (bad-)proposals around here. They all damage consistency— which is a more important objective—, they all fail to prevent reversals with certainty (e.g. huge amounts would be reversed in my above attack example when consistency is finally recovered, ones with many many more confirms than the 6 the attacker produced), and to the extent that they might do anything at all they all present a new practical attack surface where none existed before. It's possible to propose forms which are which are so weak— e.g. only bar >=210000 block reorgs— that they do nothing at all and thus also don't create a practical problem, but if you propose one that would stop an actual attack then that same attack effort could instead be put into creating an optimized convergence failure by careful propagation to produce a maximum entropy partitioning.

It's soft cementing (wet cement?). It probably has some advantages over hard cementing, but essentially it has the same tradeoffs.

I may be wrong, but i think that the best solution would be to implement @kjj's idea with difficulty automatically increasing exponentially if block chain fork is longer than Y blocks.

This is still pure Proof Of Work, not really cementing. It would simply require much more than 51% to fork blockchain after Y number of blocks.

In such a case the "cementing feature" would be more than useful, to lock in clients on the "island blockchain".

Just imagine how would you react after trading for a whole year with your neighbors, and then a new neighbor comes in with a satellite link to the Internet and the whole island blockchain is dumped the moment first wallet updates (assuming the main chain is longer). I know I would be pissed, and at least would like to have the option to keep doing business on the "island blockchain". The moment you have a group of bitcoin users that work and trade over a blockchain fork for a period larger than a week, you can confidently say they are trading in their own digital currency, and there isn't an easy way to deal with it at the blockchain level.

This is similar to bitcoin tech ported onto other planets, which in absence of some low latency telecommunication channel, that would naturally create their own local currency.

Another case is when a country or an island gets isolated from the rest of the world.
I guess that's why Satoshi did not put an automatic cementing feature. But such a case can be detected due to a sudden really slower network speed ? At least, the user may know it :p

this

You don't lose the ability to recover.  You only lose the silent, automatic recovery that obfuscates the damage.  You, the human, is going to have a mess that you are personally going to have to deal with.  The difference is that in one scenario, you have no idea what happened or why, and in the other scenario, you have to clear out some invalid blocks and restart your node, but you know exactly why.


Also, I should point out that section 6 of the paper doesn't really apply here, because section 6 is written with the assumption that q < p, while the assumption in this thread is that q > p.  In this case, it means that we are assuming that the attacker will eventually overtake the honest network.  Your only protection in that case is to have done all transactions and converted all of your bitcoins to wealth prior to the block when the attacker publishes their chain that destroys the entire system.

Nice to know that Gavin, thank you. Will be interesting to see how solutions are developed with enough time.

Maybe. I think the ability to recover is important, both practically and for the theoretical "correctness" of the system.

I used "Pure PoW" in the narrower sense of sticking to the longest branch (which depends only on the current observed data and not the history of receiving it).

Thank you for this. I would however like to emphasize that table 2 is probably the least reliable item in the paper, it assumes a specific model for an attack which is likely to diverge significantly from reality. (And doesn't even follow the math of that model to the end because of the complexity of the calculations.)

I see what you're saying. When you say it's only slightly higher than breaking ECDSA, you mean that THIS PROBLEM IS SO EXTREME AND URGENT THAT WE NEED TO FREAK OUT ABOUT IT RIGHT NOW!  No? Ok.

This is not how things work. Let me tell you how you should do it, it you ever want something done (as with my fork I have already learned the hard way how things work in the world).

Nobody will do anything for you (for free). If you ever want to get stuff done, you must work hard, do it and code it yourself, test it thoroughly and only then - when you are 100% sure that it works correctly, you can try to convince people to merge it to the client.

I sent ShadowOfHarbringer's some of my thoughts on this in this private message:

I don't worry much right now about economically irrational, "I'm going to spend millions of dollars to disrupt the bitcoin network" attacks because I don't think anybody is going to spend millions of dollars to disrupt our tiny payment network.

I have no idea what bitcoin payments will look like in 5-10 years; I expect all sorts of trust mechanisms and relationships to develop that are independent of the bitcoin network, and I suspect some of those will make 51% attacks irrelevant.

And I have no idea what the mining landscape will look like in 5-10 years; if thousands of companies around the world are installing bitcoin mining hardware for free in every house built in cold climates (generate bitcoins and get a discount on your heating bill) then it may be completely inconceivable for even a government to amass enough hashing power to mount a 51% attack.

So while I encourage y'all to keep thinking about it as an interesting theoretical problem, it is only slightly higher on my personal priority list than worrying about quantum computers breaking ECDSA.

Erm, don't take this the wrong way, but this seems like a strawman to me.  If an attacker has you isolated, they can already feed you bogus blocks, and there isn't a damn thing you can do about it.  In a parity proof-of-work system, when the isolation is broken, your node will resync with the rest of the world.  But the damage (to you) has already been done, it was done when you accepted blocks that you thought were current, but really weren't.  Throwing them out and loading the correct blocks doesn't fix anything, and might actually make it harder for you to realize what happened.

This is the trade in reality.
You get:  your node can fix itself when reconnected to the global network.
You pay:  The entire global network is vulnerable to hidden chains, whatever damage you suffered while disconnected is unchanged, you don't necessarily know that you were attacked.

Seems like a terrible trade to me.


The best part of my proposal is that it doesn't change the network rules at all.  It is still pure proof of work.  The only change is that the burden of proof for a reorganization is higher than it is now, and it scales with the amount of danger than such a reorganization represents.

Wow, I didn't think about that. Thanks for clearing this up, @Meni.

This is a very good discussion we are having, i wish I did that more often.

Without cementing, new nodes use the longest branch, as do all other nodes. In case there's a majority attacker, he can make attacks but at least the nodes are consistent with each other.

With cementing, if a certain attack is executed, new nodes will be detached from the rest of the network (e.g., the veteran network is cemented on a shorter branch, but the new node will join the attacker's longer branch). At the very least it shows that cementing doesn't make the network (where "the network" includes the ability of nodes to join it) immune to such attacks.

I more-or-less agree with with the rest of your post, this is the part i have doubts about:


How is this different when using cementing ?
I mean clients already take longest chain when offered multiple chains or different length.

No change here.

(Unless you were talking about the majority rule, not cementing itself)

Yes, but checkpoints are much more infrequent, and require upgrading the client.

If the IP vote overrides the longer chain, it is not clear you still need majority hashrate. You can use your IPs to vote on your minority branch. How this works exactly will depend on the specific implementation and attack.

Also, the cost is additive. You'll need the cost of multiple IPs + cost of hashpower + cost of bitcoins. Tripling the resource cost isn't so impressive if the attacker might have a thousand-to-one resource advantage.

Suppose you're a new node joining the network. Of course you are not cemented on anything. You see some nodes broadcasting a short chain and some an incompatible, longer chain. Who do you believe? If you default to the longer chain, you could fall right into the majority attacker's net. If you use something else you make it easier to carry out an attack without majority hashrate.

In this way the attacker can prevent new nodes from joining reliably. Not "some poor victim somewhere will be attacked". Nobody can join the network. Unless, of course, we spend some time thinking about a framework for higher-level overriding.

And I will repeat myself and say there are two primary attacks with majority hashrate - double-spending and rejection. Cementing helps with one. What about the other?

I agree, when we do start seriously thinking about this, we will need more knowledgeable people to take part.

Or when there is an exploit, community detects it, patches are produced, executables distributed and all the legitimate transactions are redone.

https://en.bitcoin.it/wiki/Incidents#CVE-2010-5139

Exactly. This already happens, but only once every few months.
So why not make it more often if there are no known drawbacks ?

Simple solutions are not always bad.

Doesn't this already occur on an ad-hoc basis when new versions of the client are released through the process of creating a "checkpoint"?

Yes, but in no way will this be easy to do.

It would require simultaneously VERY significant resources in multiple countries around the world AND >51% Hashing power, AND few million $ to buy Bitcoins to perform an attack.
Which makes it infeasible for almost every possible adversary on the planet.

Attack like this would be almost impossible to do, even for government of a very rich country (we all know what country I am talking about).


So the only time a block reorg longer than 10 blocks will ever happen is because an advanced double spending attack.
Logically, implementing cementing after Y blocks (where Y = relatively big number, like 15, 20 or 50) should be an absolutely viable solution to our problems.

Unless there are other drawbacks, not mentioned in this discussion.

I surely could use @Gavin's opinion here.