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0 sats \ 7 replies \ @BitcoinHistory OP 14 Nov 2023 \ parent \ on: Bitcoin P2P e-cash paper | Satoshi Nakamoto satoshi at vistomail.com Fri Oct 31 bitcoin
Satoshi Nakamoto https://www.metzdowd.com/pipermail/cryptography/2008-November/014849.html satoshi at vistomail.com Thu Nov 13 17:56:55 EST 2008 Previous message: Bitcoin P2P e-cash paper Next message: WSJ Story on NSA history Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] James A. Donald wrote:
It is not sufficient that everyone knows X. We also need everyone to know that everyone knows X, and that everyone knows that everyone knows that everyone knows X
  • which, as in the Byzantine Generals problem, is the classic hard problem of distributed data processing.
The proof-of-work chain is a solution to the Byzantine Generals' Problem. I'll try to rephrase it in that context.
A number of Byzantine Generals each have a computer and want to attack the King's wi-fi by brute forcing the password, which they've learned is a certain number of characters in length. Once they stimulate the network to generate a packet, they must crack the password within a limited time to break in and erase the logs, otherwise they will be discovered and get in trouble. They only have enough CPU power to crack it fast enough if a majority of them attack at the same time.
They don't particularly care when the attack will be, just that they all agree. It has been decided that anyone who feels like it will announce a time, and whatever time is heard first will be the official attack time. The problem is that the network is not instantaneous, and if two generals announce different attack times at close to the same time, some may hear one first and others hear the other first.
They use a proof-of-work chain to solve the problem. Once each general receives whatever attack time he hears first, he sets his computer to solve an extremely difficult proof-of-work problem that includes the attack time in its hash. The proof-of-work is so difficult, it's expected to take 10 minutes of them all working at once before one of them finds a solution. Once one of the generals finds a proof-of-work, he broadcasts it to the network, and everyone changes their current proof-of-work computation to include that proof-of-work in the hash they're working on. If anyone was working on a different attack time, they switch to this one, because its proof-of-work chain is now longer.
After two hours, one attack time should be hashed by a chain of 12 proofs-of-work. Every general, just by verifying the difficulty of the proof-of-work chain, can estimate how much parallel CPU power per hour was expended on it and see that it must have required the majority of the computers to produce that much proof-of-work in the allotted time. They had to all have seen it because the proof-of-work is proof that they worked on it. If the CPU power exhibited by the proof-of-work chain is sufficient to crack the password, they can safely attack at the agreed time.
The proof-of-work chain is how all the synchronisation, distributed database and global view problems you've asked about are solved.
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0 sats \ 6 replies \ @BitcoinHistory OP 14 Nov 2023
Ray Dillinger https://www.metzdowd.com/pipermail/cryptography/2008-November/014857.html bear at sonic.net Fri Nov 14 21:20:23 EST 2008 Previous message: NSA history Next message: Bitcoin P2P e-cash paper Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] Okay.... I'm going to summarize this protocol as I understand it.
I'm filling in some operational details that aren't in the paper by supplementing what you wrote with what my own "design sense" tells me are critical missing bits or "obvious" methodologies for use.
First, people spend computer power creating a pool of coins to use as money. Each coin is a proof-of-work meeting whatever criteria were in effect for money at the time it was created. The time of creation (and therefore the criteria) is checkable later because people can see the emergence of this particular coin in the transaction chain and track it through all its "consensus view" spends. (more later on coin creation tied to adding a link).
When a coin is spent, the buyer and seller digitally sign a (blinded) transaction record, and broadcast it to a bunch of nodes whose purpose is keeping track of consensus regarding coin ownership. If someone double spends, then the transaction record can be unblinded revealing the identity of the cheater. This is done via a fairly standard cut- and-choose algorithm where the buyer responds to several challenges with secret shares, and the seller then asks him to "unblind" and checks all but one, verifying that they do contain secret shares any two of which are sufficient to identify the buyer. In this case the seller accepts the unblinded spend record as "probably" containing a valid secret share.
The nodes keeping track of consensus regarding coin ownership are in a loop where they are all trying to "add a link" to the longest chain they've so far recieved. They have a pool of reported transactions which they've not yet seen in a "consensus" signed chain. I'm going to call this pool "A". They attempt to add a link to the chain by moving everything from pool A into a pool "L" and using a CPU- intensive digital signature algorithm to sign the chain including the new block L. This results in a chain extended by a block containing all the transaction records they had in pool L, plus the node's digital signature. While they do this, new transaction records continue to arrive and go into pool A again for the next cycle of work.
They may also recieve chains as long as the one they're trying to extend while they work, in which the last few "links" are links that are not in common with the chain on which they're working. These they ignore. (? Do they ignore them? Under what circumstances would these become necessary to ever look at again, bearing in mind that any longer chain based on them will include them?)
But if they recieve a longer chain while working, they immediately check all the transactions in the new links to make sure it contains no double spends and that the "work factors" of all new links are appropriate. If it contains a double spend, then they create a "transaction" which is a proof of double spending, add it to their pool A, broadcast it, and continue work.
If one of the "new" links has an inappropriate work factor (ie, someone didn't put enough CPU into it for it to be "licit" according to the rules) a new "transaction" which is a proof of the protocol violation by the link-creating node is created, broadcast, and added to pool A, and the chain is rejected. In the case of no double spends and appropriate work factors for all links not yet seen, they accept the new chain as consensus.
If the new chain is accepted, then they give up on adding their current link, dump all the transactions from pool L back into pool A (along with transactions they've recieved or created since starting work), eliminate from pool A those transaction records which are already part of a link in the new chain, and start work again trying to extend the new chain.
If they complete work on a chain extended with their new link, they broadcast it and immediately start work on another new link with all the transactions that have accumulated in pool A since they began work.
Do I understand it correctly?
Biggest Technical Problem:
Is there a mechanism to make sure that the "chain" does not consist solely of links added by just the 3 or 4 fastest nodes? 'Cause a broadcast transaction record could easily miss those 3 or 4 nodes and if it does, and those nodes continue to dominate the chain, the transaction might never get added.
To remedy this, you need to either ensure provable propagation of transactions, or vary the work factor for a node depending on how many links have been added since that node's most recent link.
Unfortunately, both measures can be defeated by sock puppets.
This is probably the worst problem with your protocol as it stands right now; you need some central point to control the identities (keys) of the nodes and prevent people from making new sock puppets.
Provable propagation would mean that When Bob accepts a new chain from Alice, he needs to make sure that Alice has (or gets) all transactions in his "A" and "L" pools. He sends them, and Alice sends back a signed hash to prove she got them. Once Alice has recieved this block of transactions, if any subsequent chains including a link added by Alice do not include those transactions at or before that link, then Bob should be able to publish the block he sent Alice, along with her signature, in a transaction as proof that Alice violated protocol. Sock puppets defeat this because Alice just signs subsequent chains using a new key, pretending to be a different node.
If we go with varying the work factor depending on how many new links there are, then we're right back to domination by the 3 or 4 fastest nodes, except now they're joined by 600 or so sock puppets which they use to avoid the work factor penalty.
If we solve the sock-puppet issue, or accept that there's a central point controlling the generation of new keys, then generation of coins should be tied to the act of successfully adding a block to the "consensus" chain. This is simple to do; creation of a coin is a transaction, it gets added along with all the other transactions in the block. But you can only create one coin per link, and of course if your version of the chain isn't the one that gets accepted, then in the "accepted" view you don't have the coin and can't spend it. This gives the people maintaining the consensus database a reason to spend CPU cycles, especially since the variance in work factor by number of links added since their own last link (outlined above) guarantees that everyone, not just the 3 or 4 fastest nodes, occasionally gets the opportunity to create a coin.
Also, the work requirement for adding a link to the chain should vary (again exponentially) with the number of links added to that chain in the previous week, causing the rate of coin generation (and therefore inflation) to be strictly controlled.
You need coin aggregation for this to scale. There needs to be a "provable" transaction where someone retires ten single coins and creates a new coin with denomination ten, etc. This is not too hard, using the same infrastructure you've already got; it simply becomes part of the chain, and when the chain is accepted consensus, then everybody can see that it happened.
			Bear
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0 sats \ 4 replies \ @BitcoinHistory OP 14 Nov 2023
Satoshi Nakamoto https://www.metzdowd.com/pipermail/cryptography/2008-November/014858.html satoshi at vistomail.com Fri Nov 14 23:43:00 EST 2008 Previous message: Bitcoin P2P e-cash paper Next message: Bitcoin P2P e-cash paper Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] I'll try and hurry up and release the sourcecode as soon as possible to serve as a reference to help clear up all these implementation questions.
Ray Dillinger (Bear) wrote:
When a coin is spent, the buyer and seller digitally sign a (blinded) transaction record.
Only the buyer signs, and there's no blinding.
If someone double spends, then the transaction record can be unblinded revealing the identity of the cheater.
Identities are not used, and there's no reliance on recourse. It's all prevention.
This is done via a fairly standard cut-and-choose algorithm where the buyer responds to several challenges with secret shares
No challenges or secret shares. A basic transaction is just what you see in the figure in section 2. A signature (of the buyer) satisfying the public key of the previous transaction, and a new public key (of the seller) that must be satisfied to spend it the next time.
They may also receive chains as long as the one they're trying to extend while they work, in which the last few "links" are links that are not in common with the chain on which they're working. These they ignore.
Right, if it's equal in length, ties are broken by keeping the earliest one received.
If it contains a double spend, then they create a "transaction" which is a proof of double spending, add it to their pool A, broadcast it, and continue work.
There's no need for reporting of "proof of double spending" like that. If the same chain contains both spends, then the block is invalid and rejected.
Same if a block didn't have enough proof-of-work. That block is invalid and rejected. There's no need to circulate a report about it. Every node could see that and reject it before relaying it.
If there are two competing chains, each containing a different version of the same transaction, with one trying to give money to one person and the other trying to give the same money to someone else, resolving which of the spends is valid is what the whole proof-of-work chain is about.
We're not "on the lookout" for double spends to sound the alarm and catch the cheater. We merely adjudicate which one of the spends is valid. Receivers of transactions must wait a few blocks to make sure that resolution has had time to complete. Would be cheaters can try and simultaneously double-spend all they want, and all they accomplish is that within a few blocks, one of the spends becomes valid and the others become invalid. Any later double-spends are immediately rejected once there's already a spend in the main chain.
Even if an earlier spend wasn't in the chain yet, if it was already in all the nodes' pools, then the second spend would be turned away by all those nodes that already have the first spend.
If the new chain is accepted, then they give up on adding their current link, dump all the transactions from pool L back into pool A (along with transactions they've received or created since starting work), eliminate from pool A those transaction records which are already part of a link in the new chain, and start work again trying to extend the new chain.
Right. They also refresh whenever a new transaction comes in, so L pretty much contains everything in A all the time.
CPU-intensive digital signature algorithm to sign the chain including the new block L.
It's a Hashcash style SHA-256 proof-of-work (partial pre-image of zero), not a signature.
Is there a mechanism to make sure that the "chain" does not consist solely of links added by just the 3 or 4 fastest nodes? 'Cause a broadcast transaction record could easily miss those 3 or 4 nodes and if it does, and those nodes continue to dominate the chain, the transaction might never get added.
If you're thinking of it as a CPU-intensive digital signing, then you may be thinking of a race to finish a long operation first and the fastest always winning.
The proof-of-work is a Hashcash style SHA-256 collision finding. It's a memoryless process where you do millions of hashes a second, with a small chance of finding one each time. The 3 or 4 fastest nodes' dominance would only be proportional to their share of the total CPU power. Anyone's chance of finding a solution at any time is proportional to their CPU power.
There will be transaction fees, so nodes will have an incentive to receive and include all the transactions they can. Nodes will eventually be compensated by transaction fees alone when the total coins created hits the pre-determined ceiling.
Also, the work requirement for adding a link to the chain should vary (again exponentially) with the number of links added to that chain in the previous week, causing the rate of coin generation (and therefore inflation) to be strictly controlled.
Right.
You need coin aggregation for this to scale. There needs to be a "provable" transaction where someone retires ten single coins and creates a new coin with denomination ten, etc.
Every transaction is one of these. Section 9, Combining and Splitting Value.
Satoshi Nakamoto
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0 sats \ 3 replies \ @BitcoinHistory OP 14 Nov 2023
Ray Dillinger https://www.metzdowd.com/pipermail/cryptography/2008-November/014859.html bear at sonic.net Sat Nov 15 02:04:21 EST 2008 Previous message: Bitcoin P2P e-cash paper Next message: Bitcoin P2P e-cash paper Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] On Sat, 2008-11-15 at 12:43 +0800, Satoshi Nakamoto wrote:
I'll try and hurry up and release the sourcecode as soon as possible to serve as a reference to help clear up all these implementation questions.
Ray Dillinger (Bear) wrote:
When a coin is spent, the buyer and seller digitally sign a (blinded) transaction record.
Only the buyer signs, and there's no blinding.
If someone double spends, then the transaction record can be unblinded revealing the identity of the cheater.
Identities are not used, and there's no reliance on recourse. It's all prevention.
Okay, that's surprising. If you're not using buyer/seller identities, then you are not checking that a spend is being made by someone who actually is the owner of (on record as having recieved) the coin being spent.
There are three categories of identity that are useful to think about. Category one: public. Real-world identities are a matter of record and attached to every transaction.
Category two: Pseudonymous. There are persistent "identities" within the system and people can see if something was done by the same nym that did something else, but there's not necessarily any way of linking the nyms with real-world identities. Category three: unlinkably anonymous. There is no concept of identity, persistent or otherwise. No one can say or prove whether the agents involved in any transaction are the same agents as involved in any other transaction.
Are you claiming category 3 as you seem to be, or category 2? Lots of people don't distinguish between anonymous and pseudonymous protocols, so it's worth asking exactly what you mean here.
Anyway: I'll proceed on the assumption that you meant very nearly (as nearly as I can imagine, anyway) what you said, unlinkably anonymous. That means that instead of an "identity", a spender has to demonstrate knowledge of a secret known only to the real owner of the coin. One way to do this would be to have the person recieving the coin generate an asymmetric key pair, and then have half of it published with the transaction. In order to spend the coin later, s/he must demonstrate posession of the other half of the asymmetric key pair, probably by using it to sign the key provided by the new seller. So we cannot prove anything about "identity", but we can prove that the spender of the coin is someone who knows a secret that the person who recieved the coin knows.
And what you say next seems to confirm this:
No challenges or secret shares. A basic transaction is just what you see in the figure in section 2. A signature (of the buyer) satisfying the public key of the previous transaction, and a new public key (of the seller) that must be satisfied to spend it the next time.
Note, even though this doesn't involve identity per se, it still makes the agent doing the spend linkable to the agent who earlier recieved the coin, so these transactions are linkable.
In order to counteract this, the owner of the coin needs to make a transaction, indistinguishable to others from any normal transaction, in which he creates a new key pair and transfers the coin to its posessor (ie, has one sock puppet "spend" it to another). No change in real-world identity of the owner, but the transaction "linkable" to the agent who spent the coin is unlinked. For category-three unlinkability, this has to be done a random number of times - maybe one to six times?
BTW, could you please learn to use carriage returns?? Your lines are scrolling stupidly off to the right and I have to scroll to see what the heck you're saying, then edit to add carriage returns before I respond.
If it contains a double spend, then they create a "transaction" which is a proof of double spending, add it to their pool A, broadcast it, and continue work.
There's no need for reporting of "proof of double spending" like that. If the same chain contains both spends, then the block is invalid and rejected.
Same if a block didn't have enough proof-of-work. That block is invalid and rejected. There's no need to circulate a report about it. Every node could see that and reject it before relaying it.
Mmmm. I don't know if I'm comfortable with that. You're saying there's no effort to identify and exclude nodes that don't cooperate? I suspect this will lead to trouble and possible DOS attacks.
If there are two competing chains, each containing a different version of the same transaction, with one trying to give money to one person and the other trying to give the same money to someone else, resolving which of the spends is valid is what the whole proof-of-work chain is about.
Okay, when you say "same" transaction, and you're talking about transactions that are obviously different, you mean a double spend, right? Two transactions signed with the same key?
We're not "on the lookout" for double spends to sound the alarm and catch the cheater. We merely adjudicate which one of the spends is valid. Receivers of transactions must wait a few blocks to make sure that resolution has had time to complete.
Until.... until what? How does anybody know when a transaction has become irrevocable? Is "a few" blocks three? Thirty? A hundred? Does it depend on the number of nodes? Is it logarithmic or linear in number of nodes?
Would be cheaters can try and simultaneously double-spend all they want, and all they accomplish is that within a few blocks, one of the spends becomes valid and the others become invalid.
But in the absence of identity, there's no downside to them if spends become invalid, if they've already recieved the goods they double-spent for (access to website, download, whatever). The merchants are left holding the bag with "invalid" coins, unless they wait that magical "few blocks" (and how can they know how many?) before treating the spender as having paid.
The consumers won't do this if they spend their coin and it takes an hour to clear before they can do what they spent their coin on. The merchants won't do it if there's no way to charge back a customer when they find the that their coin is invalid because the customer has doublespent.
Even if an earlier spend wasn't in the chain yet, if it was already in all the nodes' pools, then the second spend would be turned away by all those nodes that already have the first spend.
So there's a possibility of an early catch when the broadcasts of the initial simultaneous spends interfere with each other. I assume here that the broadcasts are done by the sellers, since the buyer has a possible disincentive to broadly disseminate spends.
If the new chain is accepted, then they give up on adding their current link ... and start work again trying to extend the new chain.
Right. They also refresh whenever a new transaction comes in, so L pretty much contains everything in A all the time.
Okay, that's a big difference between a proof of work that takes a huge set number of CPU cycles and a proof of work that takes a tiny number of CPU cycles but has a tiny chance of success. You can change the data set while working, and it doesn't mean you need to start over. This is good in this case, as it means nobody has to hold recently recieved transactions out of the link they're working on.
Is there a mechanism to make sure that the "chain" does not consist solely of links added by just the 3 or 4 fastest nodes?
If you're thinking of it as a CPU-intensive digital signing, then you may be thinking of a race to finish a long operation first and the fastest always winning.
Right. That was the misconception I was working with. Again, the difference between a proof taking a huge set number of CPU cycles and a proof that takes a tiny number of CPU cycles but has a tiny chance of success.
Anyone's chance of finding a solution at any time is proportional to their CPU power.
It's like a random variation in the work factor; in this way it works in your favor.
There will be transaction fees, so nodes will have an incentive to receive and include all the transactions they can. Nodes will eventually be compensated by transaction fees alone when the total coins created hits the pre-determined ceiling.
I don't understand how "transaction fees" would work, and how the money would find its way from the agents doing transactions to those running the network. But the economic effect is the same (albeit somewhat randomized) if adding a link to the chain allows the node to create a coin, so I would stick with that.
Also, be aware that the compute power of different nodes can be expected to vary by two orders of magnitude at any given moment in history.
			Bear
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0 sats \ 0 replies \ @BitcoinHistory OP 14 Nov 2023
Satoshi Nakamoto https://www.metzdowd.com/pipermail/cryptography/2008-November/014860.html satoshi at vistomail.com Sat Nov 15 13:02:00 EST 2008 Previous message: Bitcoin P2P e-cash paper Next message: Bitcoin P2P e-cash paper Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] Ray Dillinger wrote:
One way to do this would be to have the person recieving the coin generate an asymmetric key pair, and then have half of it published with the transaction. In order to spend the coin later, s/he must demonstrate posession of the other half of the asymmetric key pair, probably by using it to sign the key provided by the new seller.
Right, it's ECC digital signatures. A new key pair is used for every transaction.
It's not pseudonymous in the sense of nyms identifying people, but it is at least a little pseudonymous in that the next action on a coin can be identified as being from the owner of that coin.
Mmmm. I don't know if I'm comfortable with that. You're saying there's no effort to identify and exclude nodes that don't cooperate? I suspect this will lead to trouble and possible DOS attacks.
There is no reliance on identifying anyone. As you've said, it's futile and can be trivially defeated with sock puppets.
The credential that establishes someone as real is the ability to supply CPU power.
Until.... until what? How does anybody know when a transaction has become irrevocable? Is "a few" blocks three? Thirty? A hundred? Does it depend on the number of nodes? Is it logarithmic or linear in number of nodes?
Section 11 calculates the worst case under attack. Typically, 5 or 10 blocks is enough for that. If you're selling something that doesn't merit a network-scale attack to steal it, in practice you could cut it closer.
But in the absence of identity, there's no downside to them if spends become invalid, if they've already received the goods they double-spent for (access to website, download, whatever). The merchants are left holding the bag with "invalid" coins, unless they wait that magical "few blocks" (and how can they know how many?) before treating the spender as having paid.
The consumers won't do this if they spend their coin and it takes an hour to clear before they can do what they spent their coin on. The merchants won't do it if there's no way to charge back a customer when they find the that their coin is invalid because the customer has doublespent.
This is a version 2 problem that I believe can be solved fairly satisfactorily for most applications.
The race is to spread your transaction on the network first. Think 6 degrees of freedom -- it spreads exponentially. It would only take something like 2 minutes for a transaction to spread widely enough that a competitor starting late would have little chance of grabbing very many nodes before the first one is overtaking the whole network. During those 2 minutes, the merchant's nodes can be watching for a double-spent transaction. The double-spender would not be able to blast his alternate transaction out to the world without the merchant getting it, so he has to wait before starting.
If the real transaction reaches 90% and the double-spent tx reaches 10%, the double-spender only gets a 10% chance of not paying, and 90% chance his money gets spent. For almost any type of goods, that's not going to be worth it for the scammer.
Information based goods like access to website or downloads are non-fencible. Nobody is going to be able to make a living off stealing access to websites or downloads. They can go to the file sharing networks to steal that. Most instant-access products aren't going to have a huge incentive to steal.
If a merchant actually has a problem with theft, they can make the customer wait 2 minutes, or wait for something in e-mail, which many already do. If they really want to optimize, and it's a large download, they could cancel the download in the middle if the transaction comes back double-spent. If it's website access, typically it wouldn't be a big deal to let the customer have access for 5 minutes and then cut off access if it's rejected. Many such sites have a free trial anyway.
Satoshi Nakamoto
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0 sats \ 1 reply \ @BitcoinHistory OP 14 Nov 2023
Nicolas Williams https://www.metzdowd.com/pipermail/cryptography/2008-November/014864.html Nicolas.Williams at sun.com Mon Nov 17 16:54:28 EST 2008 Previous message: Bitcoin P2P e-cash paper Next message: Bitcoin P2P e-cash paper Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] On Fri, Nov 14, 2008 at 11:04:21PM -0800, Ray Dillinger wrote:
On Sat, 2008-11-15 at 12:43 +0800, Satoshi Nakamoto wrote:
If someone double spends, then the transaction record can be unblinded revealing the identity of the cheater.
Identities are not used, and there's no reliance on recourse. It's all prevention.
Okay, that's surprising. If you're not using buyer/seller identities, then you are not checking that a spend is being made by someone who actually is the owner of (on record as having recieved) the coin being spent.
How do identities help? It's supposed to be anonymous cash, right? And say you identify a double spender after the fact, then what? Perhaps you're looking at a disposable ID. Or perhaps you can't chase them down.
Double spend detection needs to be real-time or near real-time.
Nico
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0 sats \ 0 replies \ @BitcoinHistory OP 14 Nov 2023
James A. Donald https://www.metzdowd.com/pipermail/cryptography/2008-November/014866.html jamesd at echeque.com Mon Nov 17 20:26:31 EST 2008 Previous message: Bitcoin P2P e-cash paper Next message: ADMIN: end of bitcoin discussion for now Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] Nicolas Williams wrote:
How do identities help? It's supposed to be anonymous cash, right?
Actually no. It is however supposed to be pseudonymous, so dinging someone's reputation still does not help much.
And say you identify a double spender after the fact, then what? Perhaps you're looking at a disposable ID. Or perhaps you can't chase them down.
Double spend detection needs to be real-time or near real-time.
Near real time means we have to use UDP or equivalent, rather than TCP or equivalent, and we have to establish an approximate consensus, not necessarily the final consensus, not necessarily exact agreement, but close to it, in a reasonably small number of round trips.
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0 sats \ 0 replies \ @BitcoinHistory OP 14 Nov 2023
James A. Donald https://www.metzdowd.com/pipermail/cryptography/2008-November/014865.html jamesd at echeque.com Mon Nov 17 18:57:39 EST 2008 Previous message: Bitcoin P2P e-cash paper Next message: unintended? Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] Ray Dillinger wrote:
Okay.... I'm going to summarize this protocol as I understand it.
I'm filling in some operational details that aren't in the paper by supplementing what you wrote with what my own "design sense" tells me are critical missing bits or "obvious" methodologies for use.
There are a number of significantly different ways this could be implemented. I have been working on my own version based on Patricia hash trees, (not yet ready to post, will post in a week or so) with the consensus generation being a generalization of file sharing using Merkle hash trees. Patricia hash trees where the high order part of the Patricia key represents the high order part of the time can be used to share data that evolves in time. The algorithm, if implemented by honest correctly functioning peers, regularly generates consensus hashes of the recent past - thereby addressing the problem I have been complaining about - that we have a mechanism to protect against consensus distortion by dishonest or malfunctioning peers, which is useless absent a definition of consensus generation by honest and correctly functioning peers.
First, people spend computer power creating a pool of coins to use as money. Each coin is a proof-of-work meeting whatever criteria were in effect for money at the time it was created. The time of creation (and therefore the criteria) is checkable later because people can see the emergence of this particular coin in the transaction chain and track it through all its "consensus view" spends. (more later on coin creation tied to adding a link).
When a coin is spent, the buyer and seller digitally sign a (blinded) transaction record, and broadcast it to a bunch of nodes whose purpose is keeping track of consensus regarding coin ownership.
I don't think your blinding works.
If there is a public record of who owns what coin, we have to generate a public diff on changes in that record, so the record will show that a coin belonged to X, and soon thereafter belonged to Y. I don't think blinding can be made to work. We can blind the transaction details easily enough, by only making hashes of the details public, (X paid Y for 49vR7xmwYcKXt9zwPJ943h9bHKC2pG68m) but that X paid Y is going to be fairly obvious.
If when Joe spends a coin to me, then I have to have the ability to ask "Does Joe rightfully own this coin", then it is difficult to see how this can be implemented in a distributed protocol without giving people the ability to trawl through data detecting that Joe paid me.
To maintain a consensus on who owns what coins, who owns what coins has to be public.
We can build a privacy layer on top of this - account money and chaumian money based on bitgold coins, much as the pre 1915 US banking system layered account money and bank notes on top of gold coins, and indeed we have to build a layer on top to bring the transaction cost down to the level that supports agents performing micro transactions, as needed for bandwidth control, file sharing, and charging non white listed people to send us communications.
So the entities on the public record are entities functioning like pre 1915 banks - let us call them binks, for post 1934 banks no longer function like that.
But if they recieve a longer chain while working, they immediately check all the transactions in the new links to make sure it contains no double spends and that the "work factors" of all new links are appropriate.
I am troubled that this involves frequent retransmissions of data that is already mostly known. Consensus and widely distributed beliefs about bitgold ownership already involves significant cost. Further, each transmission of data is subject to data loss, which can result in thrashing, with the risk that the generation of consensus may slow below the rate of new transactions. We already have problems getting the cost down to levels that support micro transactions by software agents, which is the big unserved market - bandwidth control, file sharing, and charging non white listed people to send us communications.
To work as useful project, has to be as efficient as it can be - hence my plan to use a Patricia hash tree because it identifies and locate small discrepancies between peers that are mostly in agreement already, without them needing to transmit their complete data.
We also want to avoid very long hash chains that have to be frequently checked in order to validate things. Any time a hash chain can potentially become enormously long over time, we need to ensure that no one ever has to rewalk the full length. Chains that need to be re-walked can only be permitted to grow as the log of the total number of transactions - if they grow as the log of the transactions in any one time period plus the total number of time periods, we have a problem.
Biggest Technical Problem:
Is there a mechanism to make sure that the "chain" does not consist solely of links added by just the 3 or 4 fastest nodes? 'Cause a broadcast transaction record could easily miss those 3 or 4 nodes and if it does, and those nodes continue to dominate the chain, the transaction might never get added.
To remedy this, you need to either ensure provable propagation of transactions, or vary the work factor for a node depending on how many links have been added since that node's most recent link.
Unfortunately, both measures can be defeated by sock puppets. This is probably the worst problem with your protocol as it stands right now; you need some central point to control the identities (keys) of the nodes and prevent people from making new sock puppets.
We need a protocol wherein to be a money tracking peer (an entity that validates spends) you have to be accepted by at least two existing peers who agree to synchronize data with you - presumably through human intervention by the owners of existing peers, and these two human approved synchronization paths indirectly connect you to the other peers in the network through at least one graph cycle.
If peer X is only connected to the rest of the network by one existing peer, peer Y, perhaps because X's directly connecting peer has dropped out, then X is demoted to a client, not a peer - any transactions X submits are relabeled by Y as submitted to Y, not X, and the time of submission (which forms part of the Patricia key) is the time X submitted them to Y, not the time they were submitted to X.
The algorithm must be able swiftly detect malfunctioning peers, and automatically exclude them from the consensus temporarily - which means that transactions submitted through malfunctioning peers do not get included in the consensus, therefore have to be resubmitted, and peers may find themselves temporarily demoted to clients, because one of the peers through which they were formerly connected to the network has been dropped by the consensus.
If a peer gets a lot of automatic temporary exclusions, there may be human intervention by the owners of those peers to which it exchanges data directly to permanently drop them.
Since peers get accepted by human invite, they have reputation to lose, therefore we can make the null hypothesis (the primary Bayesian prior) honest intent, valid data, but unreliable data transmission - trust with infrequent random verification. Designing the system on this basis considerably reduces processing costs.
Recall that SET died on its ass in large part because every transaction involved innumerable public key operations. Similarly, we have huge security flaws in https because it has so many redundant public key operations that web site designers try to minimize the use of https to cover only those areas that truly need it - and they always get the decision as to what truly needs it subtly wrong.
Efficiency is critical, particularly as the part of the market not yet served is the market for very low cost transactions.
If we solve the sock-puppet issue, or accept that there's a central point controlling the generation of new keys,
A central point will invite attack, will be attacked.
The problem with computer networked money is that the past can so easily be revised, so nodes come under pressure to adjust the past - "I did not pay that" swiftly becomes "I should not have paid that", which requires arbitration, which is costly, and introduces uncertainty, which is costly, and invites government regulation, which is apt to be utterly ruinous and wholly devastating.
For many purposes, reversal and arbitration is highly desirable, but there is no way anyone can compete with the arbitration provided by Visa and Mastercard, for they have network effects on their side, and they do a really good job of arbitration, at which they have vast experience, accumulated skills, wisdom, and good repute. So any new networked transaction system has to target the demand for final and irreversible transactions.
The idea of a distributed network consensus is that one has a lot of peers in a lot of jurisdictions, and once a transaction has entered into the consensus, undoing it is damn near impossible - one would have to pressure most of the peers in most of the jurisdictions to agree, and many of them don't even talk your language, and those that do, will probably pretend that they do not. So people will not even try.
To avoid pressure, the network has to avoid any central point at which pressure can be applied. Recall Nero's wish that Rome had a single throat that he could cut. If we provide them with such a throat, it will be cut.
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