Table of Contents
Speech TiTle: Spacechains: One-way pegged sidechains for Bitcoin
Speaker: Ruben Somsen
Conference: Pizza Day Prague 2022
I do a lot of work on layer two for bitcoin and I would say, in bitcoin it’s very difficult to make changes. If you need a soft fork for something it’s a lot of effort and that’s what comes with the package of wanting to do something with a system that’s inherently decentralized. Spacechain is one of these layer two solutions that I’ve come up with that tries to fit in between that difficulty of not being able to change Bitcoin but still wanting to do more. It’s a system essentially to achieve sidechains but these are specifically one-way pegged sidechains. The key thing is to get permissionless blockchains that benefit Bitcoin. It allows you to create more blockchains but these blockchains are specific to Bitcoin. They are not altcoins. That is the main important distinction here because that is a big problem in this ecosystem. Whenever you want to innovate and do something new, you can’t immediately do it in Bitcoin, you end up with an altcoin.
The problem with altcoins
There are lots of problems with altcoins. Ultimately, it comes down to this destructive cycle of greed, where you have the people that start it and you have the insiders, who have inside information, know when the token is going to hit the exchange, can insider trade, speculate on it ahead of time and always win out. Unfortunately, that goes at the cost of everybody else who’s participating. Then you’ve got early adopters that if they’re lucky they gain the most. Obviously, some altcoins are going to fail completely. It’s all about marketing. It’s not about creating a project that is useful but it’s about creating a project (for altcoins at least) that looks like it’s going to be useful in the future and therefore, people are going to buy it now. That makes the value go up and that makes everybody who are holding the token happy. This is a key problem with no easy solution and there is no incentive to stick around.
Let’s say, you create a successful altcoin and it goes up in value. Great, you made a lot of money. You were the first person who created it, you gave yourself a lot of tokens. Now you have this new idea and you have to use two options you can say “okay, well let’s try and change the current thing” or “let’s do another altcoin”. That’s what we see in the space and you have a lot of serial altcoin entrepreneurs. I would like to see this cycle broken where we can actually stick with one token in this case bitcoin and do everything there as opposed to trying to repeat this cycle over and over again. Ultimately a lot of tokens go nowhere and the last ones who are holding the bag lose money. Normally, doing a trade is a win-win situation. Both people are better off, you wanted that new phone, Apple wanted to get richer and you’re both happy. But in this case, you’re buying a lemon, you’re buying something that you think is going to be great in the future because people told you so and that might not be the case. I won’t say that there isn’t any positive here. I definitely think a lot of innovation is happening in the altcoin space. There’s so much money involved and it attracts so many people but the positive doesn’t really outweigh the bad. It’s absolutely great that we get a lot of Zero-Knowledge proof work but it’s like stealing money and then donating it to charity. Sure, you’re doing good with the money but the method also matters.
The limitations of two-way pegs
Basically, it’s not an altcoin but it’s also not a two-way peg. We should start with acknowledging what kind of two-way peg methods there are and why a one-way peg might be useful. Generally, the most two-way pegs have a certain limitation, problem or issue that fundamentally makes it a trade-off. Anything with trade-offs (especially on layer 2) has a right to exist. I would like to see all of these come to fruition and have everybody experiment and see what wins out. As long as the trade-offs are different, it deserves to be explored and built.
What are some of these two-way pegs? The simplest and the one most people know are federations. Liquid is a famous example. I have another layer two design called Statechains which also makes use of federations which is a little more secure than Liquid but has other trade-offs. With federations, you give money to a group of people, you put it into a multi-sig, they credit you a token on their chain (the chain that they control) and you hope when you get out, they give you your money back. It’s just a form of custody but it’s better than what we have today because you’re not just depending on one party, you’re depending on multiple parties. It’s very interesting but it moves away from this decentralization what we want in bitcoin.
Then there’s Drivechains which basically is the same as a federation but you’re giving your coins to miners and they control your coins. When you do a peg out and try to get your coins out, miners have to very slowly and visibly say “okay, look, I’m going to give my coins to this person” and it happens in half a year or so. The process is slow, deliberate and visible in hope that miners won’t cheat. It could work out depending on how much value is on that chain. If there’s too much value it certainly seems more fragile than how Bitcoin works today. I’m personally worried that that’s not good enough but I would like to see the experiment.
Then Rollups this is a complex topic, I won’t go into it in detail but I do have an article about it. Traditional ZK-rollups, as we see them in Ethereum, aren’t actually off-chain. So what you do is you put them onto the Bitcoin blockchain or the mainchain (in case of Ethereum) and compress data to some extent but the bulk of it is still there. It lowers the computation which is a really big problem in Ethereum but not in Bitcoin. If you were to put this system in Bitcoin, you end up with a 2x block size increase roughly in terms of transaction throughputs. That’s not really that much and if it wasn’t for the complexity (it’s a complex system with novel cryptography which might not be as secure as what we have in Bitcoin), I think it’d be a tough sell for Bitcoin but maybe we’ll get there eventually.
Then there are collateralized pegs which you see a lot in the altcoin world, where they try to take bitcoin and put it onto their altcoin chain. They do it by using their altcoin as collateral. The big problem is that it requires the altcoins to have value in the first place. You can’t use something as collateral if it doesn’t have value. Considering my personal opinion, most of these altcoins will go to zero. Ultimately, that doesn’t work and even if it does, it only works up to the market cap of that altcoin and you need something like 2x collateral and that is also another limitation where you can’t really put a lot of bitcoins on that kind of chain. Very generally, we’ve seen this with Luna for instance which spectacularly blew up recently. There’s this implication that says “okay, we’re going to create this other token (in this case it was a UST) and we have our altcoin and the altcoin is going to be used as collateral and because people want the UST, the altcoin will have value” and that’s sort of like the circular reference that doesn’t really add up. Where is the actual value in the token? You need to have the altcoin have value first and only then can you use this collateral you can’t say “because we’re going to use this all altcoin as collateral therefore it has value”.
Another one of my layer two ideas is called Softchains and I can’t really go into it now and haven’t really given a presentation on it yet (maybe next year). Basically, it requires some invasive changes to Bitcoin. This might be an interesting model for an altcoin or at least some other chain but it’s not really something that we can get into Bitcoin anytime soon. The general issue here is that a bunch of these require elaborate soft forks that we’re not going to see come to Bitcoin anytime soon.
That brings me to Spacechains. All the fees of Spacechains go to bitcoin miners (despite not being a two-way peg) and all the value goes to bitcoin holders. That’s what you want out of a sidechain (a secondary chain) that is not trying to compete with Bitcoin that’s trying to add value to the Bitcoin ecosystem.
Fees to miners
When I say “fees go to miners”, what do I mean? With Spacechains you can simulate mining inside of Bitcoin. Somebody can create a secondary chain and instead of having its own Proof-of-Work, we actually do it inside of the Bitcoin blockchain and we just use bitcoin. Somehow, we create one freely spendable output per block which can be spent by anyone and that’s the key. Then everybody tries to spend it. I can try to spend it, you can try to spend it. But bitcoin miners will take the transaction with the highest fee through RBF (replace by fee, which is in Bitcoin today). We can try to outbid each other and the miner will take the transaction that has the highest fee and add that one to the block. Whoever wins this bidding war, puts their hash off the Spacechain inside of the transaction that they used to bid. The winner is the person who wanted to create the Spacechain block the most and was willing to pay the most to miners. In return, they get the fees that are on Spacechain. The highest bidder gets the Spacechain fees and the bitcoin miners get the Bitcoin fees. What’s nice about this system is that it completely leaves bitcoin miners out of it. The whole process is just regular Bitcoin transactions and we get consensus on the secondary chain by people, just creating these transactions and miners, doing what they’re already doing (picking the highest fee transactions).
To give you a brief visual overview of what this might look like. Let’s say you have a Bitcoin block and inside of this block you have a transaction with a Spacechain hash and this was this bidding and so one of the transactions got chosen, the transaction entered into the Bitcoin blockchain and there’s a hash and as soon as it hits the Bitcoin blockchain the person who chose that hash will reveal their Spacechain block to the Spacechain network. And this moves forward, the next Bitcoin block comes in with the same process (it’s possible to do a fork just like in Bitcoin). So the Spacechain blocks also choose which Spacechain block they build on and in this example we went from block 1 to 2 and then suddenly there’s another block 2. So, there’s 2a and there’s 2b and now there’s a fork and the reason that needs to be possible is because 2a could just be a nonsense block with no actual valid data and so we need a way to work around it. Or even worse the block is valid but it’s being withheld and nobody has it. You need this forking mechanism. The way it works is basically because we’re tied to bitcoins one Spacechain block per Bitcoin block. We don’t really need to check like it works in bitcoin if block’s coming faster we adjust the difficulty. The difficulty actually follows bitcoin and you can just count and say “okay which chain is longer” and that chain is going to be the valid chain. Basically as soon as another block appears the fork resolves and now we all know we’re following block 3b. Especially if consensus moves forward it becomes even more clear.
How to pay fees?
So far we’ve explained how you can create another blockchain but what we haven’t really discussed is how to get rid of the altcoin? You have a chain now that exists inside of Bitcoin and all the fees go to bitcoin miners. Okay. Great. But now I want to use this chain and make a transaction, what we do is we burn bitcoin so we allow people to destroy their bitcoin in order to create a Spacecoin and this is basically the mechanism where you tie the value of the tokens on the secondary chain (Spacechain) directly to bitcoin via a one-way peg.
What do these Spacecoins look like? 1 BTC gives you 1 Spacecoin the value of these burned bitcoins which is important to realize actually goes to bitcoin holders because what you’re doing is you’re destroying bitcoins you’re lowering the amount of bitcoins that are in existence and because of that basically you make bitcoin more scarce and all the remaining people their bitcoins go up in value. What’s interesting to note here is that bitcoin by definition is more valuable than the Spacecoin and that is really important, that is what makes it not an altcoin. If you want to speculate, the best thing you can do is just hold bitcoin because if the Spacechain is popular and more people burn their Spacecoins it’s bitcoin that goes up in value, not the Spacecoins. Spacecoins only have non-speculative value. You can’t hold it and hope for them to go up because they’re never going to go past 1 BTC. One Spacecoin will never be worth more than 1 BTC. With that we incentivize people to only want Spacecoin if they actually want to do something and access functionality on the Spacechain. There’s no longer this incentive to say “okay well I think Spacechain is going to be popular in the future, therefore, I want Spacecoins now”. No, that doesn’t matter. If you think it’s going to be more popular in the future, you want bitcoins because that’s where all the value goes.
What are spacechains good for?
The second question is “what kind of functionality can you access? Why would somebody want to use the Spacechain?” Ultimately, anything altcoin can do can now be done as a Spacechain in a way that helps Bitcoin. Asset issuance is a perfect example, where you can do stablecoins, NFTs, a federated two-way peg (not thrustless but it’s possible to issue a two-way peg via a federation, via multi-sig). The chain itself will still be fully trustless and decentralized but that token might not be. You could do decentralized DNS, e.g. Namecoin (but Namecoin is a speculative token). Ultimately, if you want to do decentralized DNS. You don’t really care about this new token, you don’t want to buy it but you have to and Spacechain takes the token out of the equation (or at least it replaces it with Spacecoins which are non-speculative tokens).
You can do a BIP47 payment code registration. I’m not going to go into this now (I’m giving a workshop at 3 pm about the subject).
There’s another interesting use case which is low-value-on-chain-payments and so this depends on how much value there’s actually going to be in these Spacecoins but if these tokens hold some amount of value (they won’t hold a lot of value because nobody’s gonna hold thousands of dollars worth of Spacecoins but maybe for a couple of hundred dollars you’d be okay) you could create a Spacechain (a big block Spacechain) for simple payments and that could be an interesting experiment to run. Basically, you can do anything that requires a global consensus (as long as there’s a network of users which obviously is a prerequisite).
It’s also interesting to note that a single hash on the Bitcoin blockchain (a single transaction) can support any number of chains because you can have a Spacechain inside of a Spacechain. As soon as you have your first Spacechain, all other Spacechains exist inside of that Spacechain. You basically have just a single transaction per Bitcoin block supporting an unlimited number of chains. It’s really scalable in that sense where it doesn’t really put a lot of friction on the Bitcoin blockchain while simultaneously the fees go into bitcoin miners. It’s entirely permissionless anyone can create their own chain. My hope is to get this ecosystem that’s similar to lightning where people start creating their own chains and some of them take off, some of them don’t but we get permissionless innovation that is in altcoins today but not in bitcoin.
What’s going to happen to the Spacecoin price? Spacecoin value depends on the current demand of block space. As long as people are constantly wanting to use the block space at a certain rate, the token is going to be stable in value. If demand for block space goes up that’s fine too, you just hit the ceiling. People are going to burn more bitcoin to create more Spacecoin but it goes wrong when demand goes down. At least it’s never going to be overvalued and speculated. Because there’s no pump, nobody’s buying into that expectation. Consequently, there’s not gonna be a dump. That hopefully makes it more stable already. And it’s unattractive for storing value. If you want to store your value, you’re just going to hold bitcoin. Even if its price goes down, it doesn’t really matter because you just want to have these tokens to pay for fees. Maybe you’re going to have $20 worth of Spacecoins and use them to send an NFT to your friend and if those $20 go down in value and become $10 , sure, that’s not ideal but it’s not the end of the world either (you’re not storing your value there).
My theory is that either it’s going to be close to the value of bitcoin (where people think there’s going to be more demand for the block space in the future, therefore, everybody accepts that it’s gonna be worth 1 BTC today) or maybe it will taper down slowly (where the value goes below 1 BTC but it goes down slowly). Either way is perfectly acceptable. The worst case scenario is where you have a popular Spacechain but a new Spacechain comes into existence which people like more and everybody abandons the first one. Then whoever holds those tokens will eventually lose their value (but the assumption is that nobody’s ever going to hold a lot of them).
“Burning feels bad”
People love it or hate it, basically. A lot of people come to me and say “Ruben, you’re asking me to burn my bitcoin, that really hurts”. I understand but it’s not bad at all. It’s just part of the system. If you destroy your bitcoin it’s good for anyone else who doesn’t destroy it. You can also buy Spacecoins off the markets (somebody else will have burnt their bitcoin and you buy it from them).
Instead of arguing, I’m gonna present an alternative. So what you can do is you can pay bitcoin to future miners. Instead of burning 1 BTC to create 1 Spacecoin, you can lock up 1 BTC for two years (it needs to be a long time period because miners need to expect that they’re going to get it in the future) and in two years, the output gets unlocked and can be taken by anyone. This means miners are gonna take that fee because they decide what goes into a block. By paying out bitcoin to future miners, you remove it from the system temporarily and eventually it becomes available for miners who will compete to get it. Instead of destroying the bitcoin you’re turning your bitcoin into Proof-of-Work (it’s still destruction of value but in this case it’s destruction of electricity by generating more Proof-of-Work). Instead of bitcoin going up in value the more Spacecoins enter the system, Bitcoin security goes up (that’s an interesting trade-off). One of the things you can do there is smooth out the future subsidy. Today, you have halving which is very extreme and eventually we run out of subsidy entirely. So instead of deciding what’s going to be two years from now, the miner’s going to get it, you try to smooth it out or you try to extend the period that miners get fees to increase bitcoin security.
Ultimately, we have the question of whether you want to do this or you want to burn bitcoin. It depends on what is going to happen with bitcoin in future. If we don’t have enough fees, if bitcoin security is in danger or people think it’s in danger then it’s preferable to do this, if people don’t think that’s the case then it’s preferable to increase bitcoin’s value by burning bitcoin.
A potential future could be, we have the Bitcoin blockchain, then we have one (main, conservative) Spacechain that’s very similar to Bitcoin and everybody else connects their crazy Spacechains to that.  Maybe we have this big block lightning-ish Spacechain that we use to pay for small fees.  Maybe we have another Spacechain that focuses on confidential assets with a lot of privacy there.  Maybe you have a Spacechain that focuses completely on a dollar stablecoin like Tether and basically you don’t even have Spacecoins on there, it’s just an alternate chain with issued dollars that people use for holding and trading from bitcoin to dollars.  Maybe we have some experimental technology like my Softchain proposal which allows you to have a sidechain system.  Maybe we have a DNS chain or a group of DNS chains that are tied together and what’s interesting there is you have a hierarchy because you always have to validate the parents. So if you connect to DNS chain 3, you also have to connect to chain 2, chain 1, the main Spacechain and the Bitcoin blockchain. So you can get a tier of more preferable to least preferable DNS space.
Stabilizing the peg value
That brings us to a couple of interesting things you can do with Spacechains. For instance, you can stabilize the value of the peg (1 BTC=1 Spacecoin) by having a consensus rule where everyone who creates a block has to burn a certain percentage of the supply and it needs to be very low amounts of 0.01% for something you target, something like 10% per year (This variable is very important and needs to be carefully chosen – once set it is hard to change). When you do that you create a way for Spacecoins to exit the system. So it puts upward pressure on the price and with that, you have a better chance of maintaining the 1BTC=1Spacecoin value. It’s really cool because then you have a “two-way peg” (not quite a two-way peg but gets close). Importantly, the peg is only stable if the burn (the amount of Spacecoins that exit the system) outpaces the drop in demand. If demand drops beyond whatever is being burned you’re out of luck. This may improve the low value payment use case (maybe good for the big block Spacechain) but burnt fees (that’s what you’re doing when you force miners to burn a percentage of the supply) are taken out of their profits and their profit is there to create Proof-of-Work essentially. So when you burn a percentage of these coins you’re lowering the security of the chain. That is a trade-off and whether or not that’s good enough depends on the security of the chain.
Then there’s another interesting idea – and even if some of these ideas are not perfect, don’t work out, or have certain trade-offs, it is permissionless innovation. We can try all these things. Some of them might work out and some might not. That’s ultimately the beauty here that we don’t really need to ask anyone for permission we can just see where it goes.
With the Dollar Spacechain you can have a consensus rule where you need to know the relative price of USD-BTC (you can also use CPI but let’s keep it simple). Essentially, you have to check the price of bitcoin and if you see a new block come in on the Spacechain and you disagree on the price, you reject that block. The idea is that the price of bitcoin is not something we disagree on. If you make it slow enough where we’re not talking about the current price (BTC to USD) but about the price a week ago, we can get consensus (with a little flexibility) even if your full node is running and you’re connected to an oracle that tells you a wrong price. The chain just halts for you and you have to manually look for the price. Since you can verify that yourself and be certain about last week’s bitcoin price, you can get consensus. Once you have this oracle that is part of consensus, we can do the same burning mechanism but instead of creating one Spacecoin (when we burn 1 BTC), we create 30,000 Spacedollars (or whatever the price is). Now, you basically have a system that is a one-way pegged Luna where you can turn your bitcoin into dollars but you can’t go back. Interesting to note: dollars are made to lose value over time by default. Let’s say, there’s a million dollars’ worth of space dollars inside of the Spacechain. In one year, that one million dollars is going to have less purchasing power (because that’s what dollars do).
The interesting thing is that (given that the oracle is good enough which is a weak spot but seems doable) you actually get this decentralized dollar. Then the question is if demand for that dollar going to be stable. If demand tapers off, the peg doesn’t hold (because it’s still a one-way peg similar to Spacecoins).
A couple of other ideas that are also interesting but less worked out. You could have a tokenless chain with out-of-band fees where you don’t have Spacecoins. You just have a chain that doesn’t require any fees. This is going to be somewhat centralizing because you end up having to talk to a miner and pay a miner in order to add your transaction. It’s going to be inefficient with multiple parties because it’s interactive as opposed to the regular, non-interactive fee band process (where you just create a transaction and send it out into the P2P network). There are downsides and upsides. The upside is that fees are really frustrating in Bitcoin. It creates a lot of problems with the lightning network (e.g. how to deal with fees) and taking it out of the blockchain has a bunch of benefits. It’s good and bad, but again, it’s permissionless, why not try and see what happens?
You can actually have awareness of Bitcoin’s consensus inside of the Spacechain and with that you can do some interesting things like a half native Atomic Swap (I say half native because the Spacechain knows what happens on the Bitcoin blockchain but the Bitcoin blockchain doesn’t really know what happens on the Spacechain). You can have a one-way function where you create an output on the Spacechain and it says “you can claim this output if you send 1 BTC to this bitcoin address” and then as soon as somebody sends 1 BTC to this bitcoin address they can claim the output on the Spacechain. This is more than regular Atomic Swaps which require these HDLCs. It’s sort of half native.
Another interesting thing is that you can look at bitcoin’s Proof-of-Work and figure out what the hash rate is that miners are creating in order to receive bitcoins. You can get this number just by looking at the Bitcoin blockchain where you see miners get let’s say 10 bitcoins and they create this number of hashes in order to receive those bitcoins. With that number you can actually create contracts that speculate on the hash rates and so you can allow miners or whoever wants it to speculate on that value inside of a Spacechain.
This brings me to one of the things I created, it’s a Spacechain’s mining demo which is available on signet (you can actually play around with this, create hashes and compete with other people via RBF). There’s no actual blockchain yet (that is the next step) but it’s something you can play around with today. I’ll show you a couple of slides with some information.
First, I promised you some details about what Spacechain mining looks like and I said there’s one output that anyone can spend. But how do we get one output per Bitcoin block? We get to that point by creating a series of transactions. I call this a covenant but it’s a pre-signed covenant. It means one person signs an input and output with a relative lock time of one block (meaning only one of these transactions can enter the Bitcoin blockchain and we’re going to create a sequence of them). So they have this covenant input and output (which repeats every block) and then there’s the secondary output (which is the anyone can spend output, the freely spendable outputs that people can compete over. In this case, 500 Satoshi output).
Then people compete, they create their own transaction with a user input, user output they also put they also try to spend the freely spendable output and they add an op_return with a hash and this is CPFP (child pays for parents). You’re basically paying for the covenant transaction and RBF is taking place where anyone can create the secondary transaction (user input, user outputs) and outcompete each other via RBF. Eventually someone wins, it enters into a Bitcoin block and we get the winner broadcast their Spacechain block on the Spacechain network.
Like I said this is a covenant and it repeats every block. It basically keeps going, you get the exact same thing next block and the covenant transaction just repeats. The way this works is that the covenant input and covenant outputs are pre-signed and basically there is a trust assumption that whoever signed it, isn’t going to keep the key. They have to throw away the key. But it isn’t that damaging if they keep the key. We can do things like 1000/1000 multi-sig and make sure that as long as even one person of this 1000 people throws away the key, the whole thing is safe. But even if it wasn’t, the worst thing that can happen is that somebody breaks the covenant and spends this in a way that is not supposed to be spent and what happens then is that the Spacechain just halts and needs to be restarted with a hard fork. It’s not ideal but it’s acceptable today. We can do better but we need a soft fork. We need specifically either op_ctv or anyprevout. Both of these are high up in the discussions today as to what people want in Bitcoin in future. So there’s a reasonable chance that we’re going to get it but even if we don’t, we can move forward with this.
In this case signet blockchain what it looks like is here you have this transaction so this is basically the top transaction you saw here first right covenant in, covenant out, 500 Satoshi out that is this transaction here you get the covenant input covenant outputs and then you get the CPFP anchor transaction which is the freely spendable output. In this case, there’s 800 Satoshi’s in there (a little bit more because it’s a test I just made it a little higher).
This is the second transaction where the users write user in user out that I showed earlier where someone puts in an input that basically pays for the fees with the change outputs they also spend this freely spendable output and they add their op_return.
This is the freely spendable output so the freely spendable output has a redeem script where (this is for people who know bitcoin is correct and not everybody does) it puts a zero on the stack and then it says CSV (check sequence verify) which is a relative time lock. So these two codes mean this transaction has a relative time lock of zero blocks and it is not a time lock at all. It just means that after zero blocks this transaction can be spent. The reason we do this here is because it actually incidentally enforces RBF. So RBF can actually be disabled on transactions but by putting this into the script, you force the output that’s spending it to have RBF enabled (otherwise they cannot spend the output and this solves certain shenanigans which can also be solved if we change how bitcoin works in the P2P layer today but without that we can do it like this). So it makes sure that RBF cannot be disabled and people can keep outbidding each other.
Then on the op_return side, basically it’s a way to add data to the blockchain and you create an output that’s not an output. The output is not spendable. What that means is that however many coins you put in this op_return outputs they are going to be burned. That’s the mechanism to create these Spacecoins. This is a test transaction, some op_return data and it says one Spacechain’s test transaction. That’s the hash (instead of a hash I just put this text there just to show that it worked).
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