Sabotaging Bitcoin
blog.dshr.org
blog.dshr.org
It looks like a result with complex implications - eg, maybe making it impossible for new miners to set up unless they have a meaningful advantage in operating costs instead of just parity with the entrenched players. It is hard to tell because market reality is a mess but if there is a meaningful strategic choice to be made beyond simply announcing a block when it is mined then there is a lot of room for weird equilibriums even if the paper's specific analysis turns out to have flaws.
There’s nothing inherently valuable about crypto beyond what value people assign to it in their minds.
What are you referring to with “research more”?
But I do wonder if the abstract nature of it will forever hinder its ability to do so universally.
I’m also interested why Bitcoin Cash wasn’t more successful after the fork.
you mean besides it being run by a bunch of crooks and scam artists like CSW?
For most people the value is what they can receive for it in trade. Which holds for all money.
If it wasn't radioactive, poisonous and pyrophoric people would probably all just leap into the Neptunium market.
If it were only worth pennies an ounce, numerous industries wouldn't be paying what they do for it. The fact that many industries value it at several thousand dollars an ounce is self-evident from their continued use of it.
Some bitcoin advocates will talk about how useful it is as a currency, and I wonder how much bitcoin is actually used for purposes other then to hope you can sell it to someone else for more than you paid.
Ultimately the populace could repudiate the whole social contract, which is also just consensus, but that's a far bigger deal than mere money.
The answer is "no, it's not the same". The attack does not require everybody to agree that the bitcoin is worthless.
Obviously if everybody agrees that the bitcoin is worthless, then it is worthless. But that's a separate topic.
I don't know if I have a good comparison here, but maybe something like "if the bank keeps your money for a little longer before validating your transaction, they can use your money for a little longer and make more money from it". Of course if your bank says that a transaction takes 1 year, you will go to another bank. But if they say it takes a day...
How is it wasted if they work on the current chain? If they find a block during those X seconds, they'll propagate it before the waiting pool does. The waiting pool will then just lose the revenue from the block they put on hold. They're the ones wasting mining time when that happens, while the others never do.
A finds a block after 1 minute, then powers off and waits for another minute. They reveal the block after 2 minutes.
B searches for the block for 2 minutes.
After 2 minutes, A has used 1 minute of their compute, and B has used 2.
If B finds a block between minute 1 and 2, they start working on their competing chain, but A is already working on theirs. And A had a headstart because it started working on it somewhere between minute 1. So it's more likely that A's fork wins the race in the end.
I'd even say that B is slightly more likely to keep their reward because they started propagating their block earlier, so it's more likely other miners are mining on this block.
If A finds a second block between minute 1 and 2, then they win, but it would be the same if the didn't withhold their block.
When A is mining on their hidden block, they mine for a potential height of 2 that would win against a miner only able to push a height of 1. But by doing that they put the block they found at risk of being abandoned because another miner found a block in the meantime.
So if you find a block, you get almost 100% chance it'll stay if you publish it immediately. If you withhold it and find another one you get 100% chance of keeping your 2 blocks. If you don't find that 2nd one, you get <50% chance of your block to be the main chain (depending on time of reaction of another block being published, and connectivity). On the other hand, if you don't withhold it and find 2 blocks in a row, you also get almost 100% chance of keeping your 2 blocks. I fail to see how withholding is profitable.
Because you keep ignoring the part where it is profitable :-).
> If A finds a second block between minute 1 and 2, then they win, but it would be the same if the didn't withhold their block.
Except that by withholding their block, they got a headstart so they are more likely to find the second block. So it's not the same.
And you keep ignoring the fact that they don't necessarily have to wait until someone else finds a competing block. Maybe a winning strategy is to always withhold the block for 5 seconds. If you slightly increase your likelihood to find the winning block, you increase your profit, and that's the whole point.
With the interesting consequence (and that's the game theory part) where if everybody starts withholding their block for 5 seconds, then it changes the winning strategy.
Withholding their block (5s or whatever) doesn't make them more likely to find the second block. The probability of finding a block is always the same, given a hashrate.
They are the only ones mining on this particular chain, but that's not an advantage either. How mining on a hidden chain is an advantage?
On the other hand, withholding certainly makes them more likely to lose the reward of the block.
And you would sill have 10% chance of mining another block if you don't withhold.
What advantage does withholding give you?
One last time. By withholding, you have a headstart on the next block. If you can mine for longer, you increase your chances.
The hidden chain can easily be discarded if the miner of the hidden chain doesn't find a 2nd block and if the miners on the public chain find a block and propagate it before the hidden chain is published. In that case, the public chain and hidden chains will be 2 competing heads, and other miners will decide which one wins. They will generally take the first block they saw, so most likely not the (previously) hidden chain. In that situation, mining on the hidden chain was a waste, not a head start. We could even say that the miners on the public chain had a head start. That's why I say there's no such thing as a head start.
But I'll stop discussing that with you. It's pointless and you're way too condescending.
That's just an intuition. You keep saying "having a headstart doesn't help, because those are independent probabilities". Which is wrong: having a headstart does help. How much does it help, and is it worth it? That's the whole question. And it would require more work to answer it.
It's easier to see the argument if you have a head start. Imagine you've somehow created a private chain that's 10 blocks ahead of the public chain. You could publish that now and earn 10 blocks of reward, or you could continue mining until the lead diminishes to 0 blocks, earning the same 10 blocks of reward plus however many blocks you've mined in the meantime.
If you have 50%+ε of the hash rate on the network, this argument would have you bully other miners out by almost always stranding their blocks, since in expectation you'll mine blocks faster than your competitors.
The insight is that this same situation can happen probabilistically with a finite but non-majority fraction of the hash rate on the network. With 49% of the hash rate you'll still be able to build a private chain some fraction of the time, so waiting a little bit to see if this occurs might have positive expected value.
So, you have to risk a lot of rewards, and for what potential gain? If you win you get to discard some blocks of others. You don't get more rewards, you just make others earn less (and you push the difficulty down a bit).
I can see how you get a chance to double spend, though. If you want to double spend a transaction with N confirmations, you've to be N+1 blocks ahead in your hidden chain, publish your first transaction, wait for N confirmations on the public chain, and you publish your chain that's still 1 block ahead (and includes your double spend transaction).
Indeed, it's not "51% expensive", but it's still very expensive because of the rewards lost during the failed attempts before you get ahead enough. Actually, it might even be more expensive, because with 51% you're guaranteed to get ahead enough at some point, so you don't really risk your rewards (if you can maintain 51%).
You KNOW you are ahead, because you found a block and nobody else has published a block.
I think you are missing something very basic here: the longer you compute, the higher the likelihood that you will find the hash before the others.
The extreme case being that if you can try ALL the possibilities before the others can start, then you are guarantee to find the solution before them.
Your advantage is having exhausted a fraction of the search space. But that fraction is tiny.
You're trying to find a hash with a value below a certain threshold (simplified said, a hash starting with a certain amount of zeroes). You do this by trying random inputs to the hash function. Every input has the same probability of getting an output that is low enough in value. You are not advancing by having tried other inputs. It's practically equivalent to rolling multiple dices until enough of them show a one. Every roll has the same probability of succeeding regardless of the rolls before.
That's the whole question: is it relevant or not? Even if it makes mining slightly more profitable, that's a win. No need to remind you that those who mine do it exclusively for profit.
Not necessarily. The whole idea is that it maybe more profitable to withhold a block for some time. "More profitable" means that you make more money at the end. Not that you make billions in a second.
In hopefully simpler words; You want to find a hash with all zeroes. So you start trying inputs from your search space and hash them to see if they match that criterion. Every single input you try has the same probability of matching. After trying a lot of inputs you have exhausted a part of the search space. You have already tried many incorrect inputs. At some point if you keep only trying incorrect inputs you should have exhausted the whole search space and the last remaining possible input has to be the correct one resulting in an all zeroes hash. So the probability of the next hash being the correct one should go up during your search as you learn information about the remaining candidates in the search space. If this information is in any way significant in practice with any feasible amount of computing power, the cryptographic hash function is insecure. Of course with Bitcoin you aren't searching for a full hash inversion with all zeroes but only for a partial one starting with some zeroes, but that does not change the fundamentals. It should be infeasible to learn any significant information about the output of untried inputs by trying other inputs.
If SHA256 was to be broken in that way, we'd be in big trouble and Bitcoin would be the least of our worries.
The benefit there is that if another miner released a block before that 3 minutes this miner still can release their first block and has already spent 2 minutes working on a block that could better validate their first block now that there are competing chains.
1. They don't have to wait until another miner finds a block, they can just wait "for some time" and then release their block. All that time gives them the edge for the next block.
2. My understanding is that if two different blocks are found concurrently for the same head, then the network waits for the next block to select which "new head" is accepted. I.e. when there are competing chains, the longer chain wins. So I could imagine that a strategy could be to wait until some other miner announces their block and release yours precisely at that time, hence creating two competing chains. But you presumably have an edge because you have already been mining for a while on top of your block.
It's a race. Starting earlier obviously gives an advantage?!
It would be like saying you've an edge if you start earlier at the roulette.
In a lottery, the more tickets you buy, the higher your chances to have the winning number.
If we played with a roulette and said "the goal is to be the first to have a winning number at the roulette" and I could try 50 times before you started, obviously I would be more likely to win our game, wouldn't I?
Yes, and it's exactly the same in bitcoin with the hashing power. Each hash is a ticket.
> If we played with a roulette and said "the goal is to be the first to have a winning number at the roulette" and I can try 50 times before you start, obviously I am more likely to win our game, am I not?
In bitcoin the goal is not to be the first. The goal is to find a winning hash that's on a chain that will not be abandoned. As soon as a new block is propagated you start mining on the new head. It doesn't change anything that you previously worked on another chain. The time spent on the previous chain is not wasted, unless finding a block wouldn't have got you the reward.
There is a kind of a race if 2 blocks are found simultaneously. But that's not really what this discussion is about, and in this case the outcome depends mostly on network connectivity.
> The key idea behind this strategy, called Selfish Mining, is for a pool to keep its discovered blocks private, thereby intentionally forking the chain. The honest nodes continue to mine on the public chain, while the pool mines on its own private branch. If the pool discovers more blocks, it develops a longer lead on the public chain, and continues to keep these new blocks private. When the public branch approaches the pool's private branch in length, the selfish miners reveal blocks from their private chain to the public.
> In bitcoin the goal is not to be the first. The goal is to find a winning hash that's on a chain that will not be abandoned.
The goal is to be the first (or very close to the first), because it makes it much more likely that your chain will not be abandoned. If you wait 2 days before you reveal your block, obviously it will be abandoned...
I don't understand how this scenario is beneficial. If the selfish miner doesn't have 51% of the hashing power, they can discover more blocks than the public chain only if they are very lucky. They don't know in advance that they will be that lucky. Withholding blocks in hope of this luck means putting these blocks at a very high risk of being discarded and losing the rewards. Why would they do that, exactly? If they get lucky, they get the rewards of their chain, and discard the rewards of the other miners. If they don't, they lose a lot of rewards. On the other hand, if they just publish the blocks they find, they're almost guaranteed to get the rewards. Why take the risk? It sounds like putting your own rewards at risk just to put others' rewards at risk. It looks like the risks even out.
> The goal is to be the first (or very close to the first), because it makes it much more likely that your chain will not be abandoned.
Yes, if there are blocks that are found at almost the same time. But that's not the situation discussed here.
In other situations, being first doesn't matter. If a miner finds a block before you do, then you just start mining on top of their block. You haven't lost anything.
It VERY MUCH is.
Of course if you take another scenario that doesn't make sense, then it doesn't make sense :-).
> They don't know in advance that they will be that lucky.
Whenever you find a block, you know you are one of the first to find it. It's obvious because nobody else has published a block. So you know you are lucky right now. You can decide to wait 1, 2, 5, X seconds before you reveal your block and start mining the new block in the meantime.
Maybe you just mine for 5 seconds before revealing the block, and that's the winning strategy. Maybe you wait until someone else publishes their block and you immediately reveal yours, ending up with two competing chains but knowing that you had a headstart with yours.
The detail of whether or not this is profitable, and how exactly you should do it (Wait X seconds? Wait until someone publishes a block?) is statistics and game theory ("What if the others are also withholding their blocks now? What is their strategy?"). The whole question is whether or not there is a practical, profitable strategy doing that.
Yes, but everyone else is still buying tickets for yesterday's jackpot, while you're busy accumulating them for tomorrow's.
On the other hand, if someone finds a block while you're keeping yours secret, it's very likely you'll lose the reward of your block.
So, you get a chance to discard the block of another miner, but you have to put your own block at risk of being discarded. Maybe there's a gain here, but it's not clear.
So you can get a head start on the next block from the likely new head block you've found.
It only works on average of course, you might be the one wasting resources if someone else published a block while you're withholding yours, but the trick is for you to gain an edge on average.
Now what happens if everyone is doing that calculation? That's where you need to do the game theory analysis (which I haven't and don't claim to understand).
Finding a block relatively early doesn't affect the odds of others finding a block soon. The odds are always the same, each hash is an independent event.
I don't see why withholding would get you an edge on average. If the others find a block while you're withholding, you lose your reward. If you find another block before them, you get the rewards of 2 blocks, exactly like if the same happened but you didn't withhold.
The only way for you to have an advantage is if you find a 2nd block at the same time as another one finds one on the other chain. You can then publish a height of 2 vs a height of 1, so you win. But to do that you have to first put your first block reward at high risk by withholding it. I don't think the odds are in your favor here.
Edit: I think the strategy does work, but a little differently: if you withhold a block and someone else finds one while you do so, you can still publish yours and win a race with a certain probability, i.e. the expected loss is not as high as one might think.
Then, if you do that and if you have enough hash power, you can end up mining a private chain ahead of the public one often enough, so that the loss you take is less than the loss others take through the hash power they are wasting because of you doing this.
It doesn't have to be arbitrary. You know when a block was "lucky" and you found it ahead of average by a given percentile. You leverage those blocks.
Thus, the time spent mining block is directly dependent on the logarithm of number of transactions in the block.
If one can mine a block with 3000 transactions (11-12 hashes to the header) in 10 minutes, one can mine a block with one transaction (1 hash to header) about ten times as fast.
The construction of the block is negligible if we talk about complete block mining time.
Huh? Surely the attempts for both take exactly the same amount of time after you've initially constructed the block, you're calculating only a single hash for each attempt.
However, as Bitcoin's security inevitably weakens over the coming years due to diminishing miner rewards (denominated in BTC), I believe this "6-confimation" acceptance policy will change to include not only the number of confirmations, but the timing of those confirmations as well. Consider a scenario where an exchange deciding whether a tx with 6-confirmations that took 4 hours to arrive (this happens occasionally) is safe to consider finalized/settled. Even though 6-confimations may be considered safe by today's acceptance policies, this tx would still have a high probability of double spend due to the assumed 4-hour long wait for the 6 confirmations (as the attacker would have 4 hours to produce 7 blocks instead of the normal/expected 1 hour). Instead of ignoring block interarrival timing, it may make sense to include block timing as part of an acceptance policy.
So, going forward Bitcoin acceptance policies may change from today's 6-confirmation standard to something more complicated that involves the amount of time those blocks took to arrive. This would significantly enhance Bitcoin's double spending resistance without adding/altering any code and may give the network a much needed security boost in the coming years to prevent the attack discussed in the post.
That's incorrect. Security scales with USD-denominated rewards, not BTC-denominated. And there are 16 years of real-world data showing they have been generally increasing, so a healthy sign that the Bitcoin experiment is working:
https://newhedge.io/bitcoin/block-reward-per-block
And not only that, but rewards are still expected to stabilize even when measured in BTC (thereby not relying on an increase of BTC's price) as they are progressively composed more and more of tx fees instead of newly mined BTC.
It's puzzling to me why some still don't understand the systemic incentives that make all this work as it has for 16 years and counting...
For L1 fees to actually increase over time, we need increased L1 throughput. Without that, increased demand for transactions causes more batching of transactions (mostly between exchanges).
Given the failure of BCH for pseudo-religious reasons I don't have much hope.
Then I guess you're the type who will be really surprised to learn that with diminishing rewards comes increasing consolidation.
> ... that make all this work as it has for 16 years and counting...
That's convenient way to memory hole the market flash crashes, network forks, the blocks mined without consensus, and everything bad that happened over that timeframe.
I don't see how it could be profitable. If it can't be profitable, then the risk of someone doing it is pretty low. If they already have the necessary hardware, they'd be much better off mining.
If you're not trying to profit from the double spend itself but rather from a collapse following a proven double-spend, you can double-spend the bitcoins to yourself.
Says you, without a hint of a rationale backing your argument.
It seems to me that the historical hashing rate curve tells a different story.
And block rewards have been diminishing regularly (and very predictably) pretty much since day one.
> block rewards have been diminishing regularly
That's exactly what the poster you're replying to argued; the BTC denominated block subsidy halves every 4 years, and so without a corresponding doubling in price, the bitcoin security budget keeps diminishing, at least until tx fees start to dominate the subsidy.
I would have expected such security rules are part of the miner code, no? Don't they need to consider rules related to the comparative security level of a chain when decided which chain to follow when multiple exist?
The Monero PoW community has had to deal with such nonsense, as have other smaller PoW coins.
With ε=1e-3, the expected number of 6 confirmations works only so long as the largest pool size does not exceed 12%. For a pool size of 30%, at least 24 confirmations should be required in Bitcoin, but 49 in Monero with its stricter ε=1e-6. You can see the table and the math at https://gist.github.com/impredicative/0907e1699f5ff97a9fed5d... and again it's all cleanly reproducible from the whitepaper. Anyone who is still requiring only 6 confirmations then will be setting themselves up for a risk of reversal.
Perhaps this is more suitable as a response over months or years to a long-term shift in the composition of Bitcoin miners than as a short-term measure when it appears that someone has suddenly acquired 30% of mining capacity today.
So 240 minutes for Bitcoin, and 98 minutes for Monero.
So even though Monero is more strict, it is still "faster".
--- Starting in late 2020, as shown in The Economist's graphic, the spot market in Bitcoin became dwarfed by the derivatives markets. In the last month $1.7T of Bitcoin futures traded on unregulated exchanges, and $6.4B on regulated exchanges. Compare this with the $1.8B of the spot market in the same month. ---
If I'm buying futures I can enter into a contract that says "I'll buy a contract for 1BTC that says BTC is going to go from $88.5k to $98.5k in 1 year." I don't actually hand over any money. In a year's time, if BTC is now $100k the person who agreed on the contract gives me $10k. If it doesn't go up then I owe the seller $10k. The futures contract is settled in cash - no BTC is involved.
Right now though, I don't have a $88.5k to spend on BTC, so the spot market isn't an option. I probably could find $10k in a year's time so a bet on a BTC future might be viable. The actual derivative 'value' isn't real though. The only money changing hands is the delta of the change in value when the contract is settled.
(Caveat: I am a total noob at finance stuff so this could be quite wrong. One of the many reasons I will not be buying that futures contract. :) )
Futures contracts aren't just pieces of paper traded between people, they are actual promises to pay for physical delivery of the underlying.
It's not surprising to me that crypto people consider them nothing more than leveraged gambling slips but that's really not how one should think about them. Personally I think crypto needs far heavier regulation than it gets.
If you can make a gigantic bet on the price going up and then buy a large amount of Bitcoin that moves the price up you can win from that. See the Jane street India derivatives market issue.
In ~6 more years, Bitcoin will undergo two more halvings, so if the price of BTC is not ~400k by then, then attack will have become more feasible.
Yeah yeah, I've read the arguments about liquidity issues, shutting down the rails, making it illegal to trade, etc. but that's beside the point and depends on a thousand future variables to play out. So I don't know if btc will make it or not, but I do know property rights mean everything to humans. They literally determine whether not one is a slave (I am my own property). So just the ability to have a technology enables pure property rights to a world where nobody really has enforceable property rights over anything seems pretty interesting to me.
Bitcoin doesn't enforce property rights. The only thing you own is your bitcoin. The fact that I "own" my house and the land it is built on is enforced by the state with guns.
It's more like saying a hypothetical car which moves itself by using gasoline as a propellant rather than fuel for its combustion engine would have negative value.
Sure, using fuel (of all things) for propulsion would be one way to move a vehicle, but it would be inefficient by design.
Bitcoin, at least, was created during a time where there was no alternative to security-by-inefficency, but PoS and other consensus mechanisms are pretty battle-tested now
Proof of Stake is an absurd security proposition. Stakeholders are immediately centralized. In every single PoS coin, the governance immediately becomes the exchanges because they always hold the most coins. They can and have used this to guide PoS coins towards governance unfavorable to the users, like as happened with Steem.
I have nothing against bitcoin being the money of the future, but if it is to become that, this is the sort of trial by fire that it should endure.
That said, authorization implies an entity with ownership rights granting some kind of limited license to others to interact with the owner's property.
For a permissionless decentralized network with no owner, where the attack is against the consensus of which chain is valid, I'd have a hard time arguing that "authorization" as a concept is even applicable or relevant.
As wmf suggested, market manipulation laws may still apply, but I'm not sure traditional CFAA "without authorization" / "exceeding authorized access" hacking charges could apply, though I'd be willing to bet a prosecutor could make a case for wire fraud - a scheme to defraud using interstate communications.
Shakeeb Ahmed was convicted of wire fraud for exploiting a smart contract bug.
Avi Eisenberg was also convicted for exploiting a smart contract bug, but he had his conviction overturned on appeal.
The Peraire-Bueno brothers were in court for exploiting a bug in the MEV mechanism but it ended in a mis-trial so we're going to have to wait to find out.
Not legal advice ;-)
But who ultimately controls Bitmain? The Chinese state.
So, by extension, bitcoin is controlled by the CCP.
What a shitshow. Crypto needs to move on from bitcoin already, pick something better... anything better. There are so many options, and bitcoin is the worst of all of them.
Heck, they can embed CSAM into the Bitcoin blockchain and that won't stop anyone from using it, because above all else, line must go up.
"Democratizing finance" my a**.
EDIT: For comparison: https://gridwatch.co.uk/
"Today, Hedera is performing the equivalent of over 10,000,000 transactions and 788,000 transactions for the same amount of energy it takes Bitcoin and Ethereum to process 1, respectively."
[0]: https://hedera.com/blog/going-carbon-negative-at-hedera-hash... [1]: https://discovery.ucl.ac.uk/id/eprint/10160701/
Unlike AI, there's a strong incentive to find the cheapest electricity possible. Because that's what everyone else is doing. With Bitcoin, you now exactly what your costs are and what your yields are. There's a clear threshold, when power in an area becomes too expensive there's no reason left to mine.
AI, on the other hand, is a bet on the future - infinite gains. No matter how much power costs, it's worth it to keep using as much as possible. We can't know how much power AI uses. Unlike Bitcoin, there aren't any metrics from which to extrapolate. But we do know that AI uses more power than Bitcoin already. We just have no idea how much more.
Funny thing about that. Civilized governments put a stop to that, by fining flare-offs to make it economical to not do that.
WTF? Hydro is rarely wasted because it's so dispatchable. Typically, it can only happen during high water seasons. Same for the gas power plants.
> Unlike AI, there's a strong incentive to find the cheapest electricity possible.
Like coal.
7 transactions per second is NOT a sure bet.
I call shenanigans on this statement. We can and most certainly can tell how much power AI is using. The upper bound is the total datacenter usage.
https://www.theguardian.com/technology/2025/apr/24/elon-musk...
We have no idea what's happening in private data centers around the world.
You've drank some kind of kool-aid. You've been gaslit. Don't do it to the rest of us.
Speaking of, I just did a Google search asking if we know how much power AI is using and this was the AI response: "It is difficult to determine the exact amount of power AI is using because most leading AI companies do not disclose specific data"
It doesn't matter how much you hate Bitcoin, it's here and it's not going away. You should probably get over it.
Cmon you remember supply and demand right
There are a lot of ifs and buts here ... but the amount of power used to support the BT mechanism worldwide is roughly the same as the power consumption of the entirety of the UK.
The vast majority of transactions are speculation on what other people might pay for a bitcoin (i.e., a line on a spreadsheet). And even then, that speculation and trading often occurs on secondary markets which rely on trusted third parties - thus rendering the entire ordeal even more pointless.
(*) Approaching no-one on a global scale.
In fact, wiping out the derivative markets would be seen as a net-postive by most individual hodlers.