Nuts and Bolts of Cross-Chain Communication (Devcon5)

[Applause] Dominic is also fishery student in our lab and most of will be here in a second is permissive so the work trip is scheduled in two parts we have one part we will talk about trusting communication the principal will also explain why costume communication is where it's not possible without a trusted third party and then we'll also have a technical deep dive into Chamberlin specifically to see relate and then with prepared game so essentially if you have ganache installed in a laptop if not you should download it now you will get early broken VDC relay implementation and you get to fix small parts of it and we'll have a game sir which tests attacks against relay and then we'll have a leaderboard so this workshop is based on a paper we recently released so we wrote up a systematization of knowledge on across your communication so the tiny print if you want to have a look at that I won't be presenting everything because it's just way too much content but yeah if you're interested that's really so I mean I guess we can all agree on riot or probability is necessary but let me just repeat it today there exist over 2000 cryptocurrencies and I think we can agree that most of them different design purpose not only functionality but also the different views of the community of how this thing should be used so we kind of need a way to find have to communicate between this different block channels because it's very unlikely that would be one change in the other one now historically seen crossing communication the principles of crushing communication date back to communications across disparate databases however the main difference here is that databases specifically within us within a single data center you don't expect bit Centene rather bizarre behavior right you soon that everyone's honest you're a single admin and the only thing that can happen to your processes which push and pull data from the databases they can crash so basically not like non-blocking atomic commit is a way because it basically is a problem and they're solutions for that and the problem is that you want to make sure that your process that is running correctly does not fail and does not get stuck even if other processes crashed now in despair Ledger's and watching specifically the difference is that we actually expect participants to misbehave correct we can of selfish - we can have people who try to double spend and just attack the protocol because anyone can join the system and this is basically the major difference between these two things so if you briefly talk about the main scenarios of cross change into nationality if we're talking about the main scenarios of prostration communication I guess the one that everyone knows about is exchange of assets right swapping coins against each other and also trying to transfer coins from one chain to the other right making a payment to merchants who may be only accepts aetherium and you're only am different now there we have atomic swaps who also have the console integrated assets we'll talk about this - imitative anime now another very important thing work so now you're across communication occurs is synchronization estrada systems specifically that you're charting is that you separate the state of a system and each charter responsible to make to maintain and secure the states or a subset of the state now the problem is this only makes sense if you can actually communicate between these charge because otherwise your system of isolated shards and then it's a recharge so we'll talk about the differences in the different hues consensus between shrugging and crushing transfers belayer now what I think not many people know is that cross gene communication was actually the first time implemented to use something else specifically it was used to introduce merge might and that's basically food shopping technique to largely object and then give theories that you have one parent chain and you allow minors to reuse the true for group solutions from the parent chain for example Bitcoin to create lots and other options like name and essentially there that means that name when can use the security assumptions of Bitcoin but it also becomes dependent because the fitment bills so does name because it uses the same and finally and this is the train recovery this bit more in the paper there is his term of site chance now this factor to see many many different definitions of it over the years and it's have become an umbrella term for block change of ability general and I guess the most famous to that we have seen his side chains are an approach to extend functionality of a one block chain which becomes a parent chain and you have small options on the side which are dependent on the security model of the package but the other definitions and also quite famous one specifically in academia which suggested side chains basically referred to one block to verify mistake of the other now in the paper would try to clarify this and we try to not to use this throw at all because it's very confusing to check so again we'll talk about these two things are working in more detail today now maybe a question to the audience how many of you think that cross chain communication is possible and trustless matter okay if you've seen our to be 34 okay so the answer is none and I'll try to explain why this is the concern but before being a being epidemics you know before we explain something we always have to make a system you know to be sure that we cover all cases and that we understand what we're talking about so in cross-chain communication we for simplicity assume that we have two different lectures right in this case bit kinetic theory now we also have two parties or processes as in dispute assistance with Alice on Bitcoin and Bob with you on now now let's assume we have two transactions right transaction what the blue one is all day out so she can push it to the box and since actually two is owned by God now transaction one is a Bitcoin transaction it can only go into the physical monitor as action to is valid and etherium it can only go into theory so far simply right facing now Thank You of cross communication is that Alice ones that Bob writes transaction do to aetherium and Bob would like that Alice read transaction one to Bitcoin now this and basically this also implies that Alice knows exactly how transaction looks like and the other way around both of those what she actually wants to have now if what reduces to practice you can imagine this being an exchange right transaction two gives a theory of Travis transaction one is bit go to Bob right and let's keep this in mind for the rest of the discussion now the goal of crushing communications to synchronize these two processes right we were to make sure that Alice writes transaction one to Bitcoin but bubbles the rest interaction to to theory and we actually wanted to ensure that they both write at the same time or at least atomic now we have three properties for customer communication the paper I will not bore you with a theoretical definitions here I'll just try to give the intuition what she took with each of the means so effectiveness basically states that if everything is alright if Alice and Bob are honest and the transactions are as they expect then cross your communication will receive correct right so this basically process to be executed now atomicity reduces the set of potential behaviors to ensure that either Alice and Bob write transactions to the respective blockchain so either teks one goes into bit hundred and six two thousand two theorem or none of them do right because otherwise Palace work Bob could be the disadvantage and this took great-uncle and finally timeliness basically ensures that at some point the protocol proceeded at some point Bob Morales will initiate the process and then the protocol starts with now let's recall again we're looking at an exchange and essentially cross gene communication between Alice and Bob it can be seen as a fair exchange of assets specifically using our example to emphasize this right because they want to make sure that Alice receives coins of it on aetherium and poverty points of Bitcoin atomic otherwise one of them will be cheap now the thing is that fair exchange is impossible without a trusted her prior this is a result from that dating back to 1999 um if y'all gonna be three month outlook it's written in this paper it's actually fairly easy to follow and there's a really good explanation also the discussion about trusted through our parties and what this actually means well questions what do you consider first place okay let's get back to that out this is double talk about the same because expressed using a few more conferences he's possible no and I'll try to explain this I don't think the question is what you consider the definition before so let I'll explain in a second you won't get to that point but yeah HTC's allow you to reduce trust in the third party but you don't ever get anything here took that the exchange will actually get time we'll get to that in a second so again we conjecture right right now that it's impossible right and there's a whole basically proof sketch in the paper and I'll try to give an intuition here right now because obviously I don't think you should just trust me I'm saying so let's assume again we have a smart contract anything right then the smart contract allows us to do way more things than just teach those so it kind of supersedes patent law contracts in this case and you see this fart project what we can actually do is we can enforce that Bob behaves correctly if Alice writes transaction tips want to bid right because we can require lock to lock in some coins in the contract and the contract will then release these coins if Alice control actually spent send it to Bob in a bit better so this is actually a very weak model using angel season becomes more complicated in fact the we'll cover this in the first lesson there now if Alice memorized transaction went a bit grunt she wants to be sure that she receives the Quincy theorem so what she has to do he has to suggest me to prove or somehow inform this smart contract on the theory side that she actually wrote the transaction to the bit cross-eyed and the problem is within a single chain yes this smart contractors could actually very fine right - good if Alice were to write - theory you meant to call a function of the smart contract the contract will become aware that yes something is happened but since these are separate Ledger's this country by itself also being a program which react system it but it has no other way to verify that's something I've been a bit of an accepted someone who tells them that it's happened and this is actually where the comes in because the only way we can solve this problem place we can make an assumption that either all parties that we have seen from the small that means Alice is also online and that we know that there's a bound on the message delayed so basically in this case Alice isn't mine she is honest to herself obviously and she's an incentive to claim the coins on the theorem set after same fifth grade so she's the one who submits the proof but the problem is if we do not want to make this assumption in a synchronous model and we went without alistair crash which is the typical assumption we have to make the only other will be to achieve actual cross the correct Russian communication is by having a trusted third party which then takes the transaction one proof and submits it to the smart contract or once never trust a third party you might as well trust the third party to be the one who locks the points of the game which allows to be just stratified like so there's a sudden to write his name in this day so the trucks or the trust certify in the sense of disability mean obviously wanted to behave honestly you make this assumption but if you build a game around this trusted third party it doesn't guarantee you that this party will be a vomits her it's just incentive exactly so if you say this party must block collateral it will reimburse you you may not face financial damage but the protocol still fit right you still the actual view me like I'm not security mathematical let's say let's put it that would trustless in game theory is grave definition right I mean essentially if you and we have their protocols which ensure that you don't face man like hence the letter but in all these protocols you have some time outs and some time whoops and some synchrony assumptions because if you don't have a synchronous function and bricks but we'll come to that in a bit I mean we'll we'll discuss atomic swaps cryptocurrency back assets and you'll see where the synchronous assumption comes in actually so your argument in this example was that Ellie's would not be able to do that third party so the thing is in this model you must allow Alistair krei right cannot assume I if we assume that the amount of time international approaches in the second and I'll actually have an example way to still at the problem but essentially the problem your problem but I mean not exactly that's also a problem but in half my contracts they only ensure that you cannot steal it was behave within a certain period of time but if you crash in the time of expires it still fit there's a specific example of this yeah okay sorry question yeah can you talk a little bit about how important it is for us to not rely on the synchrony assumption and like why I mean the thing is so anyway we design protocols often we make the assumption that we have a synchronous the network model because it's way easier to design these things right we assume that if I send a message at some point you will receive however if we cannot make this assumption then I do not know and that we assume that nodes to be malicious then I do not know if your malicious or you just did not get the message there's a really famous FOP miscibility result by Mansu inch which should actually states consensus is impossible in the system where you have more than one but these ones also note if it's a synchronous because again I send you the message but if I cannot make an assumption I don't know whether you received it or not I don't know maybe your malicious and you were just you know pretending not to know it or the network actually has a huge delay usually the synchronous and network assumption and not participant assumption sounds like here it's an assumption regarding Alice so I was just one day affair which one is in this case I mean it's both because you need now is beyond light which is because your she needs to send a messenger but you can usually ensure that somehow but if the now we don't have the super new assumptions and it's hard yes so the problems within a single system you you can try to ensure that Alice sends the message or something that's the time and such right at some point Alice read the message but here we kind of have two separate systems so if two separate networks but yeah I see the question the Phoenicians are more clear than paper hopefully it's just it's not an easy thing to just explain by intuition but I hope this kind of makes sense to everyone are they any more questions okay so if you have questions you'll just have to trust so yeah we conjecture that it's a coolant and then there's a discussion in the 1999 impossibility result on why this is the case and they try to give a similar explanation of why actually having a synchronous model is very similar to having a trust a third party friend cuz again Alice if she has to be online and we must expect the message will be delivered on time it's the same thing as if we have a trust with their price for the trusted third parties online and it ensures that messages are delivered on time and ensures that a vertical progresses if we have assumes if we assume synchrony Alice's her own third party where she trusts herself because she knows she'll be online and necessarily deliver so if you it's basically it's the same thing whether you some interested third party or you some sequences bond Theory respect now obviously as soon give us a third party the processor pipe is still your coins and everything but for the correct execution of the protocol and again this is where the games running perspective comes an even 50 it still is [Music] fire fire yes is basically when you basically first a message unchanged and there is the recovery of both face and so what would be in some cases if you try to swat an asset and you want that swap to be atomic or whether conception to be a moment in the sense that and if you if you we've x2 chain wide but it's X consistently loose chain like there's two different ways of just dealing that this way I forget where you basically customize 19 X say K X will not be beautiful chain wife and there's no will become professors behind that so like there's no way you can fail she say you can say I want to be checked in the next to Y then there's no there's never a more stable recovery person like that [Music] but here the two-phase commit further goals there is a port or recovery phase in the case for our transaction fails okay so so you might change object X we change Y but you might be light but what do you check the world object s chain while it might be the case that you might have to undo that because determine the overall transaction that this process is part of fails so and this is when our board comes in okay and now it will become more clear when we actually go through the actual verticals but it is a classification second classification is a local mover cane so when you actually do push fast communication or first chain communication who is responsible for verifying that medication for some principles local her kitchen which is basically when needed the participants who are involved in that question communication are responsible for verifying that question for example hdl-c basic level swaps so for example if Alice wants to send money to Bob 30 Bob is responsible for verifying that that money has been sent on the other hand with global verification you don't just need the team participants all the participants of the question communication protocol deserve to be involved in their verification you also need the entire consensus to be without verification or you to verify the consensus by some consensus that will header or object and one example of this is public swaps with transaction including roots so transaction inclusion with some Bricklin for example the ethereal chain has to receive a block header from the Bitcoin chain and the etherium chain has to verify that proof of work with that block header answer that's global perfect Asian because the chain is verifying in the consensus of the big question so the entirety of James Ray's be involved in some way of verifying the communication I finally have security model the different security rules for authentication as homogeneous and heterogeneous [Music] heterogenous let's talk with heterogeneous that's most important and tension venus is basically when you have different chains the aren't really kind of connected the or directly connecting to each other for example let's say you want to communicate with from Bitcoin to Syria or any or any of these chains this is considered a true genius because these chains are built differently and have different security assumptions okay so you might not be able to certainly make assumptions about the security of in today's chains like I could just create my own boxing and I'm gonna get with Bitcoin for example using atomic swap but bigger people have Bitcoin can't make assumptions about the security of my chain now with our genius with emojis it's used as a uniform level of security for example father ensure in Charlotte in a Charlotte system there's multiple chains but there's chains above the same system and they use the same consensus protocol and they they designed to be connected to each other so you can make assumptions about the security of those chains and usually each other's systems as I exaggerate that if one chain fails then send us the other because the best chains rely on the state if each other so you might be familiar with cosmos for example or talk about one that I closed most for example have a heterogeneous security model so for example the cosmos of water that you can create your own zone or chain with your exercises well first for example they see empty pointers shopping or any viable because it to be the Virginia's I'd say when I go through different variables and pronounce no we're not going to do much detail but they do give you the next NTFS table with atomic swaps so yeah also just one thing on this on this finger saw I mean we won't have time to go into detail upon all of them but one thing that you may notice is that local verification so when only to the participants are involved in the exchange have to know to do the checks it's actually an agnostic of the blockchain is where this agnostic elicits reading one obviously need to make sure that the locking mechanism is available but otherwise they don't care about any consensus level deeds and so on because again only Ostergard effect so let's talk about cross station transfer than I hope I'll be able to answer your personal agencies may not solve your problem with respect so I get an atomic spouse abuse method I like okay sure so in atomic spots using symmetric locks the idea is that both parties Alice and Bob use the same locking condition on both chains an example our hash locks right so what Alice traits you generates a secret and greets the current expanding condition on Bitcoin for example which says if you really if you can provide me with a secret you can take these quotes and Bob does the same thing so usually they'll have a setup phase in exchange for application transactions and so on but then they both know the hash of the secret so Bob names the same lock now in the next step we'll if Alice wants to spend Bob's corn stress to reveal the secret and this again ensures that bob has a secret and can also stay calluses Clause now typically you also have time locks to prevent one of the parties crashes where the exchange never starts that your friends are not locked up indefinitely but this bears a problem and this is I hope the big is basically this is my digital seized unsolved opponent or the constant communication problem because what can happen is Ibaka crash right Alice spends the coins from ethereal believable reveals a secret but Bob never sees it because he's crashed and what happens then is Alice can wait until the time it expires and take her coins back from the Bigfoot side that's why it looks like this it doesn't matter it doesn't rush is know there are crashes before heat so up never have even sees the secret teachers of life they boasted a lot and he crashed when the secret is still there just have to find it it means everything is public you don't know but love Bob the guy who could spend these coins because this is not just the hash book it's hash lock ended up singing from the business right this can only be spent ballast is can only be spent my bomb right so that's how I shoulda season struggle in scenario and if both crashes and he never sees the secret and he does not come online before the time look of this thing expires Alice can withdraw because that's it that's why I'm not in a way like this so he has enough time to do it okay Jochen it's not a contradiction yes you do that it was a time lapse but if he's offline no man indefinitely the travel is P type of mining definitely of course the youth participant walked resolutions okay yeah well I think these are fine but that's impossibility so you're very good yeah I mean of course it's essentially what a bomb does yeah it's bad but it's it's still impossible with and without anyone enforcing that he stays online that's a result I don't say that you should participate in the exchange if you think you may crash that's where you can always watch stars but still now you can also do is you can try to say well what if Bob is we can make sure that Bob actually is a livable time and he can actually go flying because using this in our contract and that's the same example as we show using a disability zone so Bob walks is fun looks launched this coincidence my contract and says okay well if you can prove to this contract or any way that you send Bitcoin into my account on the Bitcoin side you contain these quotes and now the responsibilities on Alice because she is the one who has to push the coins to Bitcoin and also send the crunch of MIT pen and push the proof the problem is so Bob can go offline for what if Alice crashes so Alice sends the Bitcoin and then she goes offline and of course Bob could say well okay I'm a nice guy I'll give you all out send the proof myself or anyone else could do it that's already an improvement because now anyone can push the proof and the contract will give you the coins but it's again if you think about it's the same thing as when hto seats and furthermore if this time of expires again Bob can take the quiz at so in if Alice crashes and she's a fine or promise the guy who actually has a close to the knowledge service attack against Alice here you can actually steal quote so another approach and this is basically some a different approach across communications use cryptocurrencies back assets so instead of trying to circumvent this impossibility result each time we exchange but we can do is we can create a representation of Bitcoin on a theater and with the whole paper describing this concept term to explain and I'll try to give a very very brief or word so what you do is you send coins to a pulp which is collateralized and insured through internet serum smart contract so you have to give through it to guarantee that if if nothing goes wrong so is something was wrong you'll get reimbursed in this case any theory you still have to meet agrees changes now Allison submits approved to the contract that she locked up the coins and receives bid compact optics and she can use these tokens to nominative atomic swaps we were better guarantees and it can optimize more we sell did possibility result of fair exchange also the free option problem but at least we're not have to synchronize clocks and everything else across two chains and we also don't have to trust that at this point in time there is no attack of it which prevents can actually be included so do you do the slow set that they cost this set that phase once then you can trade these tokens and use them smart contracts options I'd say you don't have that much time but it's it's a fairness problem that I can cancel the exchange and basically get a free option because I know what's your offer is but if you don't know about my Center so now when I want to go back I burn these coins and then the vault has to release these funds to me otherwise it can get slashed and to get the collateral back in some it's proof I'm skipping over this because running out of time but there's a whole paper describing this concept then we also have some Coraline yeah no so the vault so this thing was constructed in a way that you do not need smart contracts only in one chain evolved to be a smart contract or a participant it could be a human on the bitter end side we just need smart contracts on the decide where you issue the topic sorry it's a trusted party from digital assistance perspective for lightness but from financial perspective it's it's insured with collateral so if it fails I can get back collateral from the theory side so I so the construction is if the exchange rates don't crash completely I is a participant did not face financial damage that's a different thing the ball can go flying but then the contract I can if I'm online and I can go to the concert and say wait a second this government of but then I have to be online to accuse him right I have to then say well he didn't do it or in our case actually involved is not submit to prove from time I'm automatically reimbursed but if you think about it that the communication actually failed because nothing happened on the midwestside right I didn't actually trigger a process in the business side I just got reimburse on the theory of site so cross your communication fail but I still because the protocol is constructed that way I got a reimbursement on the one chain that I'm off so there's two different aspects is you have to be careful to separate now that's a most effect I'm gonna give you a brief review of quite sharp communications so this is a very high should be atomic transaction procurement so what is the atomic transaction brother is also the can also be called the training result program this suppose Alice wants to book a track a treatment and she needs a hotel room and the train seat to get to the hotel unless suppose that there's a travel agent contract or the chain and what that contract basically does he booked it books on hotel room and a train seat at the same time just like when you go to Expedia you can book a package with your flights that they tell other same plan now let's suppose that the hotel room object or contract is insured one and the train seat is in short tube so that means the travel agent contract will have to communicate with birth chart to execute this transaction where others wants to book a hotel room and a fancy at the same time okay but the key thing here is that Alice wants to put first in her stroller and transit we don't want to end up in a situation where one of those transactions payable because if the Train see transaction fails then at least has a hotel room but nowhere that gets written or if the hotel fails in the Trinity but nervous state now so this opposed each transaction happen this is trick she tries to make this transaction but this person lost second boat comes along and tries to book the same and celebrate one second earlier right but Alice has already submitted this transaction also missing one second can end up the situation where Bob ends up at the hotel room and analyst has a train seat but she has failed to put the head celebrate so that transaction say the transaction was an atomic okay now the goal of a short communication is to have atomic transactions so that it should be that either Alice gets the birthday metal roof of the train seat or nothing it's all the mouthing we don't want to out we don't want a half complete transaction so there's there's two kind of broad categories of doing flash-flood communication the first one is basically we tax based returning family plot first focus on services as she called it for short acting but it's actually basically Texas which was invented the seventies and the idea is very simple so basically what we do is you issued a Yank command for the interpreter household room and what that does is it removes that hotel room from that shot and that shot generates a receipt that this has happened that the hotel room has been removed from the shot and that was it simply a message in the block and the receipt itself is basically can be proven using a relative proof and then what else does is sends that receipts and that child to reach this receipt and we creates this hotel room in that shot so until now the hotel room and the train seats are now in the same shot and so now there's no no classified communications be even needed which was actually going to happen in the same car so no so yeah so I just described was a fire and forget protocol so this was a here it's a fire on forget protocol there's no recovery phase there's no water face right there's no way that there's not there's no failure bird here the potential failure bird here is if when during the time that Alice moved to this hotel room to this chart but might move this train seats to talk to trot one so then chassis beef and so the hotel room back to your I thought one but in terms of the actual first pole is through the protocol itself does not have a built-in support orthographies well the other mechanism for doing computation is called the C phase grid and then the derby of to involve everyone diagram like spin it and T phase commit is a very old principle that I told is because using distributed systems design but it can also be applied to shotting it's called defense commit because there's two phases okay so this ability have short one and for two short one manages a hotel trust to manage the Train the user sends the transaction to birth charts and then what they first do is they check that the transaction is valid using by Santana and they run up my son time for at home whatever just like a bit risky 114 is to agree the transaction on function an ordering of the transaction second taste and then they lock they lock these objects okay that means that once these objects are locked no other transaction can come along and try to use these objects because this one's a this objects these objects will be locked in this transaction so it's like this transaction is reserving reserving these objects it sounds like when you're an event right for example and you try to book a ticket it says this ticket has been reserved for seven minutes or so grab that if you don't cook it within seven minutes then you will need dessert again exactly it's the same principle you're basically reserving these objects for this transaction and once both of these what all of the shots have locks all the objects necessary as needed for this transaction they basically communicate with each other to tell them that yes these objects have been locked to this transaction and then they can execute the transaction and unlock the objects or specifically actually just destroy objects depending on how depending on your puzzle they see you chase a model you will destroy the object because it is already spent what once there is a contract well open it's basically on top and so that's basically the principle what could happen is another transaction another transaction it welcomes long as fancy reserved the same hotel is locking will this locking will fail and then versatile objects will be unlocked but the transaction will be aborted the transaction will be committed it will be reported [Music] okay so now we're going to look pretty about question for every camera a pretty poor showing the verification validation and this I think we're going demo don't okay workshop actually how many how many people actually have a laptop here and can't participate in the coding part okay can you raise like just we see so okay cool yeah then let's just go to a survey up just okay so question verification and validation when you do question credit transactions means you validate that's the messages that you're receiving question are actually valid verified now there's different levels of verification for watching okay as we with question for same communication first level state verification like that means verifying that states correct I mean just verifying what the state is or what or what or what the consensus thinks the state is for example checking that some sort of locator state is just for work right yeah that's government is not fitting but so checking this is valid checking what does the consensus was what states that the consensus we're gonna agree on like what next it's just taking a piece of work so if you have a half mmm oh good or sorry a block header then this simply involves checking that that Bach header has a piece of work but it all has has a crest sensors this is my business flashing maybe has from the tube sorry I've ever stopped yes sir I'm not over to use this consensus so step number one is just saying some particular that the high flow is constant is to separate checking if the consciousness is the consensus agrees on what the state is so this is simply my plants so that OS pv s pv verification so one you download a follow it for particular example Android Android where Bob Willis is downloading that small piece of the block through the blog header and his stuff is not actually checking what the state is it's just checking that the state has consensus instead level is ticking what transactions are fruits in the state that are relevant to you so not necessarily every transaction so for example you might want some what you might want to check but someone sent to use transaction so you will ask for notice to give you a proof that a translation has been included in the dock simple memory and number four is stake validity so just because you know that the state has consensus does not mean that the state is valid okay so it's very possible that this one is majority of miners or speakers could produce a blog that is invalid or even like a single miner to do that actually got a diamond size majority so in the big quake principle or if they report for a block to be valid it's not it's a common misconception actually that's a lot of people think for a block to be valid it needs to have people needs to have its consensus but that's not sure nobody exactly has consensus the actual the actual transactions in the block has to be correct as well so a dishonest majority of consensus this fence not be able to concerns by the transactions it's a block that's still people's money so that's why it's also important you've actually validate the state and what just as trusted consensus and that's what basically the purpose of full notes if you run a full minute to verify a entire part in your checking every single transaction you're not just simply trust you know trusting with consensus and the fifth level which we'll talk about is data availability which is basically checking I guess that's kind of coupled with checking well is he checking that is all the block data is actually available for a verification and this can be actually done as alternative sustainability instead of checking it's not checking that the state is correct yourself this is assume that I see is correct and rely on something called flood proof do you tell people generated if the stick is incorrect so they can prove to you that the city's incorrect so I'm normally not use a like a guilty until proven is a model where you achieve that every block is incorrect as where you prove otherwise whereas for previous user in a sense about proven guilty and yeah the green stuff is basically what my plans are this degree clarity and which you'll be relevant to their top and the other is Oliver African descent direct observation you run a full know yourself sexy chick predators give the change Watchtower so you can then ask ask someone to run a full note on your behalf as a full battle if you fail I'd like this they say ways to do this does not that does not require trusting at work shower or just simply record narrow just if we trust someone oh this verification games so what so presently like tribute to these the computation happens off chain but if if the execution is correct you can challenge the execution result or chain and there's like an interactive game that is stated to challenges our verification result and basically the best news to me that I've had told to reduce the money okay so into a really good technical deep them into Chamberlain's which is relevant for the practical part and then since then everyone has a laugh that I suggest we split the room so wondering can do the corium sample will also provide this align the afterwards so we have to prep the serve and everything will have a UI and you can play the game later on so we'll make it a game and then in the other side if you have any more questions ones are dispersed we have okay so but let's do a five minutes because that's kind of relevant Fred even if you're not spending today so I mentioned genuine lays are basically to be quite right so the allowance verify the state the state agreement estate volusia of another budget now state can also be considered as a part six in transactional data right you just know well you can parse the data of a big transaction but you've no clue where it's actually included or even yeah just now the thing is when SP declines and that's a kind of a problem because it's not really been defined in practice I mean you have a lot of formal frameworks for this now the back for model but I have not really seen a lot of clear definitions of what has to be planned should we mean basically everyone knows but then we don't really write a plan but if we build a study plan that's when we actually not start to think about it so what do we need to verify and the first thing is we need to be aware of the difficulty just because otherwise since we know that bit where the difficulty is switched every 2016 blocks and experiments different but we need to make sure that our Chamberlain program knows what the difficulty is expected at this point in time otherwise I could submit flops which are too easy next we have to verify that before that's clear we have to check that each block that we submit is an ancestor but because they want to make sure it's a chain then we also have to make sure we can detect its forms right