sipa changed the topic of #bitcoin-wizards to: This channel is for discussing theoretical ideas with regard to cryptocurrencies, not about short-term Bitcoin development | http://bitcoin.ninja/ | This channel is logged. | For logs and more information, visit http://bitcoin.ninja
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<gmaxwell> This is relevant to my interests: http://eprint.iacr.org/2016/263.pdf log scaling ZKP in the plain discrete log security setting. Verifier wouldn't be ultrafast, but for most cryptocurrency applications-- at least ones outside of the blockchain-- it doesn't need to be.
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<gmaxwell> to prove a sha1 preimage, their prover takes 6.5 seconds, generates a 4992 byte proof, which is verified in 4.3 seconds... this is via some implementation they created which is a mishmash of python/ntl/and openssl (for EC operations).
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<bramc> Hey everybody
<bsm1175321> o/
<bramc> I sort of broke my pretty proof of space technique and have what I think are simple fixes. They're a little strange though.
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<bramc> And I really need to learn what the actual break of the broken thing is
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<bsm1175321> You'll have to be a bit more specific. ;-)
<bsm1175321> My head is in UTXO set sharding, and proving storage and consistency of the subset you're not holding...
<bramc> Well, I have this slightly goofy but sort of makes sense improvement to the 'trivial' proof of space algorithm
<bramc> The trivial algorithm is that you make take a public key, hash it, and the difference between that hash and the challenge is the quality of that proof, smaller is better.
<bramc> The problem is that this has time/space tradeoffs, something about repeatedly hashing things and finding ones which loop back. So my idea is to make it so that instead of coughing up one thing you have to cough up two.
<bramc> So you have a public key and a size, k. In response to a challenge you need to cough up two strings of k bits such that hashing them together with the key matches the first k+3 bits of the challenge.
<bramc> You can use a number other than 3 but there are tradeoffs. When the number is lower there's more advantage to collective mining. When the number is higher there's more overhead in responding to queries, because more responses fail so you need to parallelize
<bramc> Although that also fights against another mild form of pooling bonus. Running the numbers, it looks like the right value is 3.
<bramc> To find the quality of a successful response to the challenge, you hash together the two values you found plus the challenge and right shift it K bits.
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<bramc> Again, smaller is better.
<bramc> There are some small percentage improvements which can be had to this technique using cpu tradeoffs, but they're readily analyzable and small. I think it kills all the deep algorithmic tricks.
<bramc> Come to think of it 3 is likely a bit aggro, maybe 2 is better, I'll have to crunch some numbers later.
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<bramc> The reason for having that slop is to make it unlikely for someone who gets a hit to have several different possibilities to choose from. The tradeoff is that they often come up with nothing at all.
<bramc> If an attacker happens to have three things in the bucket, they have three different ways of choosing two. If they have four things, they get twelve. That's suboptimal
<bramc> That can also be fought with a slightly crazy usage of proofs of time I came up with the other day.
<bsm1175321> So you're issuing a hashcash challenge? What's the advantage of your more complex algorithm over a simpler give-me-sha256(x+challenge) such that it's < 10^-3?
<bsm1175321> Or maybe, what's your reasoning behind the more complex challenge?
<bramc> I have to run. The short answer is that there's a deep algorithm I don't understand which busts the (somewhat underspecified) thing you just gave. Lots of people independently came up with that approach and it's busted.
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<bsm1175321> Would be happy to understand it if you can ref it ;-) Otherwise have a good night.
<bsm1175321> bramc: It seems to me the only difference is the difference in difficulty in generating a valid pubkey. So, for ECC this is almost nil, but for RSA may be interesting if you're draining the entropy of the challengee...but the challengee can use a PRNG and you'd never know it.
<bsm1175321> From a sharding perspective it's interesting to figure out how to get logical groupings of the sharding-space (UTXO space or address space = hash(pubkey)) to be "near" each other. e.g. my wallet lives on the same shard. I've suggested before "grinding" addresses so they all live on the same shard (are within some \epsilon of each other).
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<bsm1175321> The Ethereum sharding proposal has contracts running on a single shard, with an extra gas cost for crossing shards. This lacks any kind of proposal to get the relevant stuff on the same shard, and hopes on unicorns that other people will figure it out due to the economic incentive.
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<bsm1175321> Nondeterministic gas costs for calling a smart contract API seems like a losing proposition to me.
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* bsm1175321 has nightmares of x86 16-bit memory segmentation.
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<bramc> I don't find sharding very interesting, and my thoughts on actually doing it are that first of all you need a merkle root, and to get a merkle root you need an implementation that's performant, and not entirely coincidentally I'm working on exactly that.
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<lmatteis> kanzure: interesting
<lmatteis> if the supply is controlled, i'd guess this was done before
<lmatteis> such a field of "transparent databases" maybe? :)
<lmatteis> it's funny how everything fin-tech nowadays is tied to bitcoin related technology
<lmatteis> even if it's basically using something else
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<jonasschnelli> Hmm... coinbase transactions use UINT32_MAX for prevout n? Right?
<jonasschnelli> n == (uint32_t) -1)
<aj> jonasschnelli: (uint32_t) -1 is UINT32_MAX
<jonasschnelli> ah. damit,.. right 0-1 = MAX!
<jonasschnelli> thanks ak
<jonasschnelli> aj
<aj> jonasschnelli: no worries :)
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<funkenstein_> * gribble has quit (Read error: Connection reset by peer) <-- removes hat
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<bramc> Hey everybody. I don't remember if I've previously explained my improved idea for using proofs of time to stop withholding attacks in a proofs of space or proofs of steak system, so here it is:
<bramc> (note that this is separate from the use of proofs of time to stop remining since genesis attacks. That's orthogonal and a good idea in its own right)
<bramc> As a point of comparison, a paper on proofs of space suggested having the challenge for each block be based off the block 100 below it. So block 100 has a challenge derived from zero, 101 from 1, 102 from 2, etc.
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<funkenstein_> attachment missing?
<bramc> A method of improving that approach to prevent withholding attacks would be to make it so that the challenge for block N isn't directly from block N-100 but from a proof of time which takes 20 blocks worth of time to calculate. Obviously this does a great job of obliterating withholding attacks, because the attacker has no way of knowing what their challenge will be before they have to publish it so they have no way of selectin
<bramc> Sorry I can only think and type so fast, bear with me.
<bramc> This is a great improvement, except it's a huge amount of proofs of time, especially for the not so great proofs of time we have right now, and the 100 blocks thing makes grinding work across the next 100 blocks at once. That isn't anywhere near lethal, but I'd like that constant factor to be a bit lower.
<bramc> Note that these proofs of time need to be canonical, which is the exact same requirement as proofs of time need to fix mining since genesis attacks. Same caveats apply about the current state of best known ones, and same lament about how there's no deep reason known why there can't be vastly better ones so somebody please find them.
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<bramc> My new idea is basically an optimization on this technique to require vastly less proofs of time. Now blocks come in batches, 0-31, 32-63, 64-95, etc. Each of those batches has all of their challenges derived from a single block. For 0-31 it's genesis, 32-63 it's 0, 64-95 it's 32, etc.
<bramc> Where the proofs of time come in is that instead of the challenges coming off the one block, they're based off a proof of time which is 8 times as long as the average time it takes to generate a block. Again there's no way to apply withholding attacks because you have no idea what your challenge actually is when you publish your block, but now the multiplier on proofs of time vs. real time passed is 1/4 instead of 100, which is
<funkenstein_> i thought the block withholding attack was not a thing anyway, being overall a loss to one who tries to use it
<bramc> funkenstein_: Block withholding attacks are a serious problem in proofs of space and proofs of steak systems because an attacker can immediately tell whether a particular block will advantage them in the future, which isn't the case for a proofs of work based system.
<funkenstein_> ah ok thanks
<bramc> My reasoning behind the constant factors given is that a factor of 8 will nearly always be longer than the time to finish the very next block, and 32 will nearly always be greater than the time to finish the 8. Maybe 8 is a bit aggressive because 2^8 = 256 which isn't all that huge, maybe 10 and 40 is a bit safer. I don't know a simple closed form way of calculating how often the second one overruns, but that's easy enough to w
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<bramc> So that's my latest trick. Hopefully somebody reading this understands it.
<funkenstein_> its still unclear to me that that advantage (knowing how a found block could help in the future) would outweigh the disadvantage of having someone else broadcast a solution first
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<bramc> The problem is that if you have a large enough fraction of all mining power, you sometimes have not only the best but also the second best block, and have a choice of which one to broadcast, and can advantage yourself by picking the one which will be better in the future, which causes a very strong advantage for larger pools.
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<lmatteis> like selfish mining?
<funkenstein_> i can see that, but when you find the first of those two blocks - there is a time period in which you have not yet found the second - and you must decide to broadcast or not. How do you decide?
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<bramc> lmatteis: This is selfish mining
<bramc> funkenstein_: That's the problem. In proofs of space and steak there is no such time lag.
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<lmatteis> there are so many different stake implementations so it's hard to follow your reasoning. anyway i don't quite understand how it solve the double-spending problem. surely you'll run into the 'nothing at stake' problem
<bramc> This doesn't fix the nothing at stake problem. Proofs of steak have a lot of problems :-)
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<funkenstein_> bramc, no time lag between block?
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<bramc> funkenstein_: No time lag between when you find your first, second, third, etc. best responses to the last challenge. They all happen instantly, as a fundamental result of the property that mining doesn't require work.
<bramc> Bitcoin mining requires work, so it doesn't have these problems (although it does have more selfish mining problems than people think.)
<funkenstein_> excuse my ignorance on steak and space systems
<funkenstein_> there must be a mechanism to control block rates?
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<funkenstein_> http://rxiv.org/abs/1504.0072 <-- related
<lmatteis> depends on implementation really
<lmatteis> funkenstein_: to be honest, not much data shown in that paper.
<bramc> funkenstein_: Techniques for controlling the rate vary a lot. I'm mostly glossing over that detail but assuming that the actual rate is stochastic with a target like in bitcoin.
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<bramc> The details of my new trick are heavily dependent on those other details of the system.
<bramc> btw when I say proofs of steak I mean cow systems where everybody votes based on their proportion of the cow, or their 'steak'. People usually call these proofs of stake, I'm being caustic.
<funkenstein_> lol
<lmatteis> ah so you're not typoing
<bramc> Proofs of steak have the fundamental problem that they need to pick a threshold fraction of all outstanding steak which is capable of generating a quorum. If there are two different histories both of which exceed that threshold, there's no way of determining which one is the 'real' one. The lower that threshold is the easier the system is to attack. The higher it is the more likely the system is to spontaneously die.
<lmatteis> then why bother at all :)
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<funkenstein_> you are referring to a maximum depth of reorganization?
<bramc> There's a long list of other technical problems they have but I'm not terribly interested in solving them because the fundamental limitations make cow systems not terribly interesting. Mostly when I describe things which apply to cows it's because they're techniques which I came up with for proofs of space systems, which I do find interesting, which also happen to apply to cows.
<bramc> funkenstein_: Having a maximum depth of a reorg is a hack which helps a little but is implicitly accepting checkpointing.
<funkenstein_> burst coin
<lmatteis> without it anybody can create chains as they see fit with random transactions and there would be now way to know which is the right chain
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<lmatteis> proof of stake systems simply lack formal guarantees of system convergence
<funkenstein_> lmatteis, i thought all byzantine consensus lacks formal guarantee
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<funkenstein_> i think we have just barely caught up to the intro of bramc's idea :)
<lmatteis> not when the guarantees are outside the system itself. such as energy
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