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Author Topic: Post-inflation security  (Read 4482 times)
Mike Hearn
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May 15, 2011, 10:08:40 AM
 #1

Recently there's been a lot of discussion of the "tragedy of the commons" with respect to mining when inflation gets low. It bothered me because it seems this is one part of Bitcoin that wasn't fully thought out ahead of time by Satoshi, which is unusual. I came up with the idea of miners being paid to work on a transaction by the gigahash, with more fees paid meaning quicker burial under blocks, but it wasn't a full solution for a bunch of reasons.

Yesterday Stefan posted this great comment in Jeffs thread proposing a smaller tx fee, I think it's worth pulling out into its own discussion.

Quote from: justmoon
I was recently doing the business plan for a double-spend insurance firm. The firm would charge merchants who need protection from double spends and can't wait for lots of blocks a fee and in exchange would guarantee transactions. It's rates would be tiered based on delay, so there would be a fee for 2-sec guarantee, 5-sec guarantee, 10-sec guarantee, etc.

The costs for such a firm would depend heavily on the number of double spends, so it would seek to minimize them. The more double spends happen, the more money it would be willing to spend on double-spend defense. One of the measures it would do is to pay miners for guaranteed inclusion in their blocks. If double spends happen more, it would pay more miners more money, if double spends happen less, it would pay less miners less money.

Note that such a company would also watch very closely for network takeovers, as it would have to carry potentially significant costs if somebody takes over the network and starts double spending or rejecting transactions.

I think this is the missing feedback loop that connects mining income with network security.

"Paying miners for guaranteed inclusion" is another way of saying paying miners to work on a transaction, but this scheme builds on that idea to make it understandable for regular merchants. Rather than requiring people to understand what a gigahash of work means for them, a middleman handles those details along. They'd gather intelligence about what black market reversal miners are available, how much they cost, how risky any given merchant is etc. All the merchant has to do is find an insurer who charges a reasonable cost.

There are several differences in such a world to traditional Bitcoin:

  • Forced chain splits/reversals could happen, potentially quite often. If the cost of security is higher than the value being protected taking the risk and occasionally losing makes sense.
  • Transactions would often not be broadcast at all. Instead they'd be given directly to your insurer, who then passes them on to a set of miners of their choice along with fees calibrated to that particular clients risk.
  • The fees charged would reflect the value of security to participants, not actually the cost of doing the proof of work. Miners might conceivably charge a fraction of the total BTC value, for instance, despite the fact that including the tx costs them virtually nothing. In a competitive market though, profitability of mining is likely to eventually converge on a bit under 10%, which is the average profitability across all industries.
  • Mining barriers to entry in this world would not be cartels who agreed to only build on each others blocks, but the difficulty of getting contracts with the insurance companies, as most participants won't want to deal with miners directly.
  • Many users have little or no need for security because pre-existing trust relationships are strong enough. These people should not need to pay fees, and can simply pass around transactions themselves outside of the P2P network.

Lately I became more and more convinced the difficulty is too high right now. Too much computational power is being expended to protect too little value, as a consequence of how Satoshi chose to distribute coins. It wouldn't surprise me if over time difficulty actually decreases quite a bit until it converges on the actual power needed to keep double spends to an acceptable minimum (not zero).
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May 15, 2011, 01:05:44 PM
 #2

I'm totally in favor of merchant insurance who take care of watching for double spends and insuring against them.  IMHO it is pretty much the solution to the whole not-enough-miners problem.  It allows merchants the security they have come to expect from more traditional sources and allows the merchants to "pay" the transaction fees on transactions sent to them instead of sent by them. I could also very much see such services rolled into a payment processor as part of the merchant contract/fees.

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May 15, 2011, 02:56:46 PM
 #3

Nice to see this important issue discussed.

Here is another (more simple, but maybe too simple... or unsound?) proposal:
Multiple bitcoin chains with different inflation rates, that enforce different security levels.
People could use a chain with high security (for faster transactions) and/or a chain with low inflation rates (to store gathered funds).

How does your proposed model compare with this one?
Looks like it provides a way to simulate the first option, but relying on a third party instead of another chain (and currency unit).
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May 15, 2011, 07:36:42 PM
 #4

this model points the way toward evaluating bitcoin's capacity for success as the foundation for a payment system. if the double-spend insurance costs are lower for merchants in the steady state than the present transaction fees for visa and mastercard, it is likely a superior payments technology. if not, it isn't. the latter appears to be more likely without changes to the technology, but it is an empirical question.
Mike Hearn
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May 15, 2011, 09:09:32 PM
 #5

Sure, I'm not saying the additional security is a bad thing. Just that if there was no inflation it wouldn't make much sense to have so much proof of work being done.

I agree that what will ultimately decide Bitcoin as a credible competitor to VISA et al will be fees. It's hard to know what they might be at this time.

Payment to miners via fees has a number of problems. One is that you have to pick a fee up front and then broadcast the tx. If you pick too low and no miner accepts it your transaction stays pending forever. If you pick too high you waste money. There's no way to replace a transaction with a higher or lower fee one today and changing Bitcoin to allow it would be tricky.

Another problem is that the most you can pay for with this is inclusion into a single block. It probably makes sense for important transactions to pay for more work than a single block would require, to bury it faster. You can't set up any kind of flexible business plan like this, eg, loyalty bonuses or whatever.

But the biggest problem is the race-to-the-bottom aspect of it.
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May 16, 2011, 04:04:38 AM
 #6

I think it will be pools, who provide double spend protection for bitcoin billing: they will get payment in exchange on building block chain on blocks with "protected" transactions in case of double spending attempt. You make contracts with several pools to be sure you are well above 50%+, and you can accept bitcoins instantly.

And it's in the interest of such billing to publish transactions openly, so other miners could use them (or else they will be generating orphaned blocks in case of double spend attempts).
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May 16, 2011, 09:23:53 AM
 #7

Payment to miners via fees has a number of problems. One is that you have to pick a fee up front and then broadcast the tx. If you pick too low and no miner accepts it your transaction stays pending forever. If you pick too high you waste money. There's no way to replace a transaction with a higher or lower fee one today and changing Bitcoin to allow it would be tricky.

The bitcoin protocol supports a version number in transactions, and there is code in the main client (though disabled) for handling this. I believe it should be possible to enable it again. This will allow you to create new versions of a transaction that override the old version as long as they are not included in a block.

aka sipa, core dev team

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Mike Hearn
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May 16, 2011, 01:00:37 PM
 #8

Yes, but it doesn't let you change the inputs. So you can't add more money into the transaction if it needs a higher fee, unless the recipient is OK with getting a bit less.

At any rate, there is still the problem of knowing how many fees to pay.
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May 16, 2011, 01:07:33 PM
 #9

Yes, but it doesn't let you change the inputs. So you can't add more money into the transaction if it needs a higher fee, unless the recipient is OK with getting a bit less.

You're right, it seems. This seems strange to me - I would expect the outputs to remain identical instead of the input, so that someone who sees a payment coming in will know for sure it will either confirm, be replaced by something with the same effect (or remain in limbo forever...). Any reason why there is a requirement on the inputs remaining identical?

aka sipa, core dev team

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Mike Hearn
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May 16, 2011, 01:17:47 PM
 #10

It's to do with contracts, I think.

I've written up a description of how distributed contracts are supposed to work, but I wasn't 100% sure I got it right. So I mailed Satoshi and asked him to check my explanation. I'll give it a few more days as sometimes he is slow to respond. If I get no answer by the end of the week I'll post what I have anyway and we'll have to figure out the details together.
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May 16, 2011, 01:33:01 PM
 #11

All these concerns about future protocol failure seem to be predicated on the assumption that there is not a trivial solution, I propose this off the top of my head:

1) the default protocol implements the rule that if the transaction pool is 110% of the block size at least 60sec before a solution is found, no non-full (with .5k grace) block will be accepted as valid --  no permanent refusal of non-paid blocks -- accepting free transactions is part of getting paid

2) at reward reassessment points the blocksize is recalculated to be the smallest n^2 * 1M block that can hold the entire average daily PAID transaction pool size -- all paid transactions get done, but the pool is kept too small to guarantee that all free transactions are cleared quickly.

Even without a block size increase, assuming an average fee of .005 BTC in a deflationary economy will be at least as profitable as mining right now.

If these two, simple, back compatible rules were added to the default client they would reach saturation long before these rules are needed, and we can stop all the hand ringing in the meantime.

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May 17, 2011, 01:18:03 PM
 #12

All these concerns about future protocol failure seem to be predicated on the assumption that there is not a trivial solution

We know there is a trivial solution. It's to have developers decide on limits and rules to estimate the correct level of security.

This thread is about pointing out that there is also a null solution. In other words, we as developers don't have to do anything. The market will negotiate the optimal level of security.

We still have to worry about miners creating huge blocks, but this is less of a problem because there is no economic incentive to create unnecessarily large blocks (see [mike]'s proposal for sharing Bitcoin's work if you need to timestamp large amounts of data for your custom application.) So you only need to prevent individual "troll" miners from blowing up the block chain to unreasonable size by themselves. For that you could indeed use some kind of limit that grows with the average of the last 2016 blocks like the difficulty does.

Again, if the block size limit only needs to protect against spam rather than regulate network security, what that means is it can be much, much higher, much less deliberately chosen and everything still works fine.

Perhaps the most important point is to not to try and prevent double spends. Double spends are fine, they are simply a cost that comes from the imperfection of the real world. If we try and aim for a hash rate that prevents all double spends, it will be far, far more costly than paying for the double spends that occur at the margins. More importantly, if we aim for a higher hash rate, we get no feedback about changing conditions. So instead of the hash rate adapting to slightly increasing or decreasing rates of double spend attacks, we would have absolutely no signals to base our decisions on.

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May 17, 2011, 11:50:22 PM
 #13

This thread is about pointing out that there is also a null solution. In other words, we as developers don't have to do anything. The market will negotiate the optimal level of security.

The null solution is leaving the current block limit in place, which is a pointless restriction on the size of the market. While that is "bad", the other choice you propose is not better.

I agree that the "market" - in this case the mining market, not the BTC market - will make decisions about how the rules end up determining the size of the mining pool, but that does not mean that those decisions are of necessity not detrimental to the currency itself.

There is a very real possibility that if the block size restriction is eliminated entirely that the transactional funding of the pool will force out any but the hard-core fanboys, and the most efficient pros. This gives the pros a strong incentive to collude to not accept any blocks which contain free/nearfree transactions. The fanboys even if they want to throw all the outstanding frees into their block, will leave them out so they have a better chance of their block being accepted.

Under this scenario the pros, who have much capital invested, would make a small residual profit, unless they enforce, through cartel, rules which allow them to collude to inflate the price of transactions over time until it reaches parity with the next most efficient system of monetary transaction, where it will stabilize due to outside competition.

If, on the other hand, we can agree on a mechanism to modulate blocksize to the market, then we can have a profitable mining pool, by having a  market for transactional priority, thereby encouraging competition between miners, instead of collusion against users.

The best thing about bitcoin is that it is based on rules which allow users and miners to make plans based on the future state of the market. If we change the rules in such a way that it encourages/enables some players to impose their own rules on the market, then we have broken the system, it goes from defensive democracy of preserving the rules to the oppressive democracy of colluding to gouge the market.

The need for mining means that scarcity will be enforced, whether it is enforced by the protocol, or by the collusion of miners is a matter of protocol definition. If we don't define the protocol in such a way that miners can remain profitable, they will rip the market apart (artificially remove the low transaction cost)  to get whatever they can out of their investment, which does not strike me as the best long term plan.

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May 18, 2011, 01:41:00 AM
 #14

There is a very real possibility that if the block size restriction is eliminated entirely that the transactional funding of the pool will force out any but the hard-core fanboys, and the most efficient pros. This gives the pros a strong incentive to collude to not accept any blocks which contain free/nearfree transactions.

Many people think this, when they first approach bitcoin.  But either way (with/out blk sz limit), free/near-free transactions give miners a userbase, which gives bitcoins their value.  Turning away users is a clear negative incentive.

I predict that the free TX area will increase in size, even as it suffers a Tragedy of the Commons, because we want to pull in more users.

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May 18, 2011, 01:59:57 AM
 #15

Under this scenario the pros, who have much capital invested, would make a small residual profit, unless they enforce, through cartel, rules which allow them to collude to inflate the price of transactions over time until it reaches parity with the next most efficient system of monetary transaction, where it will stabilize due to outside competition.

A mining cartel wouldn't last very long. Miners inside the cartel would have the temptation to secretly mine a little on the side with a lower minimum fee in order to get some extra profits. Miners outside the cartel will also be more profitable than miners inside the cartel, because they get the same amount of money from the expensive transactions plus additional money from lower fee transactions.


If, on the other hand, we can agree on a mechanism to modulate blocksize to the market, then we can have a profitable mining pool, by having a  market for transactional priority, thereby encouraging competition between miners, instead of collusion against users.

A rule would be great if we had any basis for deciding one. You will have to predict:

- Fixed/variable costs of mining
- Economies of scale of mining
- Size of the Bitcoin economy
- Scale of ideological threats to Bitcoin
- Scale of criminal threats to Bitcoin
- Price elasticity on consumer side for fast confirmations

If you get one of these wrong by say a factor of ten, fees will too high or too low by a factor of ten. Even if you get everything right for the year 2016, you might still be wrong by a factor of ten for the year 2021.

Sure you can say that developers could get together every year to update the rule. But then you've created a political body governing the prices for Bitcoin.

Quick sidenote: Note that if your rule creates fees that are too low for a long period of time, the whole merchants buying insurance, which raises prices mechanism will still kick in. So you wouldn't do very much damage, only introduce a bit of uncertainly about how/when the rule is going to be changed. It's only when you accidentally set it too high that you cause a massive amount of wasted resources including extra CO2 emissions, wasted electricity and mining hardware.


The best thing about bitcoin is that it is based on rules which allow users and miners to make plans based on the future state of the market. If we change the rules in such a way that it encourages/enables some players to impose their own rules on the market, then we have broken the system, it goes from defensive democracy of preserving the rules to the oppressive democracy of colluding to gouge the market.

Nobody would be able to impose any rules on the system. The only person who wants to impose a rule is you. I'm arguing against instituting a rule.

Insurance companies would have no influence on mining fees and neither would miners. The only factor influencing mining fees would be how much users (particularly merchants) are willing to pay to make sure their transactions get confirmed. If the network got under attack, they would be willing to pay more. If everything was running smoothly, they would be willing to pay less. That way, the hashing rate would auto-regulate depending on the actual real-life cost that attacks incur upon the Bitcoin economy.

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May 18, 2011, 07:03:59 AM
 #16

I think we should distinguish between the rules that are in place now, vs the rules that are likely to be in place in future.

The rules that exist now can and probably should be simplified. Sipa has been making some proposals for this recently that sound reasonable. Fees aren't really an incentive to mine today because the income from them is so low, it's all being driven by inflation.

The rules that are in place in the future are what this thread is about, and I tend to agree that the block size limit should basically be some kind of multiple of recent average block sizes (over the last two weeks * 10 for example).

A moving block size limit gives some basic troll-resistance without constraining the market. It would, assuming the current set of rules for block inclusion never change, result in fees approaching zero very fast but nobody says miners have to use the current rule set. It's just a convenient bunch of defaults. Even if some miners use the default rules and include every transaction that fits into the block, other miners may choose to only include transactions that pay them specifically (so they are guaranteed to claim those fees eventually) which can lead to the insurance model.
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May 18, 2011, 11:20:19 AM
 #17

There is a very real possibility that if the block size restriction is eliminated entirely that the transactional funding of the pool will force out any but the hard-core fanboys, and the most efficient pros. This gives the pros a strong incentive to collude to not accept any blocks which contain free/nearfree transactions.
Many people think this, when they first approach bitcoin.  But either way (with/out blk sz limit), free/near-free transactions give miners a userbase, which gives bitcoins their value.  Turning away users is a clear negative incentive.

I predict that the free TX area will increase in size, even as it suffers a Tragedy of the Commons, because we want to pull in more users.

having a userbase is only important to miners if processing transactions is profitable, without blk sz limits there is no market for transactional fees. The only way for miners to force mining to be profitable is to set an entry bar by colluding to block all free/nearfree traffic.

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May 18, 2011, 12:30:00 PM
 #18

A mining cartel wouldn't last very long. Miners inside the cartel would have the temptation to secretly mine a little on the side with a lower minimum fee in order to get some extra profits. Miners outside the cartel will also be more profitable than miners inside the cartel, because they get the same amount of money from the expensive transactions plus additional money from lower fee transactions.

Apparently I have not explained well what I mean by cartel, my scenario:

-the block bounties are effectively gone
-the block size is abolished
-almost nobody pays fees, because there is no competition for block inclusion
-mining is slightly better than break even, everyone who can't pay their bill already dropped out
-some group(pool operator,company etc) has >20% of hashes
-that group decides not only not to accept free tx for their block, they refuse to validate any block that contains free tx

result:

-20%+ of blocks will be, by default, paid only, since they are generated by the cartel
-submitting blocks with no free transactions results in a 25%+ advantage in a validation race ( when two blocks are submitted to the network close together)
-other miners, in a low profit fee-as-donation system, don't want to take a 25%+ hit in races so they don't submit free tx, gaining an advantage, as well as creating a defacto barrier to entry for transactions.
-once they are onboard with not accepting frees, it's obvious that some, probably more than half the cash strapped market, will join the revolution and resign all pending frees to the tx pool forever.
-mining is now more profitable
BUT
-the market is broken because the miners are incentivized to continue to raise the entry barrier till they face outside(credit card) transactional cost competition.
-the market is also broken due to an ever growing pool of permanently abandoned tx that are blocked by arbitrarily imposed barriers.

A rule would be great if we had any basis for deciding one...

Which is exactly why I proposed a modulated, enforcible, protocol rules that sets the block size to be larger than the paid market.

If free riders don't like the delay they can include fees, get their tx processed, and indirectly increase the size of the paid market by paying, thereby allowing more free riders to ride, and the miners to make more money.

In my proposal everybodies interests are aligned. Each group competes amongst themselves, but the users and the producers are not at odds, and they can all make well informed decisions about the likely consequences of their movements in the market.

Quick sidenote: Note that if your rule creates fees that are too low for a long period of time, the whole merchants buying insurance, which raises prices mechanism will still kick in. So you wouldn't do very much damage, only introduce a bit of uncertainly about how/when the rule is going to be changed. It's only when you accidentally set it too high that you cause a massive amount of wasted resources including extra CO2 emissions, wasted electricity and mining hardware.

If the equation for adjustment is part of the protocol, and the recalculation is at a set interval (like everything else in the protocol), then everybody will be able to trivially predict the date, and outcome the the recalculation.

Nobody would be able to impose any rules on the system. The only person who wants to impose a rule is you. I'm arguing against instituting a rule.

There are two ways to get a rule applied in bitcoin:
-add it to the protocol
-game the validation system

If you refuse the first, and then create the conditions such that the second is the only way to profit as a miner, then you are setting a rule through negligence, I prefer that we consider these things proactively.

Insurance companies would have no influence on mining fees and neither would miners. The only factor influencing mining fees would be how much users (particularly merchants) are willing to pay to make sure their transactions get confirmed. If the network got under attack, they would be willing to pay more. If everything was running smoothly, they would be willing to pay less. That way, the hashing rate would auto-regulate depending on the actual real-life cost that attacks incur upon the Bitcoin economy.

If mining is unprofitable, and free transactions are unreliable, I suggest that a failure scenario is much more likely than any sort of market correction (since there would, of course, be no market, since there is no limit or marginal cost to the commodity in question).

The block size limit may not have been intended to create market scarcity necessary for the maintenance of the network after block bounties drop off, but it does do that. I don't see why we should just throw it out, instead of setting up a few other trivial rules to deal with its single short coming.

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May 18, 2011, 12:57:24 PM
 #19

In the long run, block size will NOT be the bottleneck, so it will NOT determine the marginal cost of a transaction.

The bottleneck is, and I believe will continue to be, the number of ECDSA signature verifications a miner can perform per second.  Each miner (or mining pool operator) will have a transaction processing capacity of N transactions per second.

If there are more than N transactions per second going across the network, then the smart miners will select the most profitable N for inclusion in their blocks and drop the least profitable.

And the smart miners will keep track of how much it would cost them to invest in more CPUs or specialized ECDSA-verification hardware so they can process N+M transactions per second.  And figure out how much they would make in fees or side-deals (or whatever) when they handle those extra M transactions per second.  If it is profitable, they will increase their transaction processing capacity.


I think what bitcoin is missing right now is code in the clients to figure out the "right" amount of fees.  We're currently relying on hard-coded rules that match in the client and in the miners (because it was All One Application to start).  We need to move to something more dynamic.  Some thoughts I jotted down last night:

Users want to know what fee to pay, given the constraints "I want this transaction confirmed in B or fewer blocks with probability greater than P".

If we think of that as an equation:
Code:
fee = f(txn, B, P)
... then the question is can a client estimate f by looking at the block chain and/or observing transactions as they fly across the network and (eventually) get included in blocks?  Or can we come up with a protocol to communicate f between clients and miners?


How often do you get the chance to work on a potentially world-changing project?
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May 18, 2011, 01:26:09 PM
 #20

Users want to know what fee to pay, given the constraints "I want this transaction confirmed in B or fewer blocks with probability greater than P"...

Code:
fee = f(txn, B, P)

That might be true for current users. Future users won't want anything more complicated than "I want this transaction to be confirmed (a) in about an hour, or (b) in about a day."

Or even better, just a checkbox labelled "Priority transaction (fee applies)".
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