
Optical Proof of Work - ArtWomb
https://arxiv.org/abs/1911.05193
======
jlokier
In my view, the idea that optical PoW will reduce the energy cost of PoW
mining is only temporary, because the Capex aspect of photonic miners is
likely to be only a temporary competitive advantage - just like the Capex
advantage of building ASIC miners is only a temporary, until each newer and
more efficient ASIC is rolled out.

Photonic technology is new and not as readily available - for now.

If it turns out to be a way of minting money, demand will rise, and the
competitive process of PoW will surely result in every photonic-miner owner
trying to build a bigger, faster farm, just like all the others, and then
we're right back to energy.

~~~
mzdubrov
The ratio of HW cost to energy use is different for each computing platform
and depends on algorithm etc. For example, it seems Cuckoo Cycle PoW has
somewhat higher capex/opex in ASICs because of the heavy memory use.

For photonics the ratio changes significantly due to fundamental physical
properties of analog computation, photons vs. electrons and so on.

Anyhow the process of the Si Photonics (and memristors/other analog approaches
to AI) maturing to the same degree as digital ASICs will likely be long and
gradual so this isn't a practical issue for next decade.

~~~
jacobush
I had high hopes for Cuckoo Cycle PoW, but it turns it cannot be scaled
linearly, or even at all predictably. So unless the consensus is to regularly
tweak the algorithms pretty heavily, it runs the risk of reaching a situation
in the future where the currently immense memory requirements being a piece of
cake.

~~~
tromp
See the long discussion in [https://www.grin-forum.org/t/grin-improvement-
proposal-1-put...](https://www.grin-forum.org/t/grin-improvement-
proposal-1-put-later-phase-outs-on-hold-and-rephrase-primary-pow-commitment)

It will take many years for 1GB of on-chip cache to be common, but when that
happens, hopefully commodity computing devices can support somewhat efficient
Cuckoo mining...

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andrewla
Does anyone have any insight into what the actual nature of HeavyHash is?

> This leads to the selection of a hybrid design that composes digital hashing
> with low precision vector-matrix multiplication (intended for photonic
> acceleration) to produce HeavyHash. HeavyHash is an iterated composition of
> an existing hash function, i.e. SHA256, and a weighting function such that
> the cost of evaluation of HeavyHash is dominated by the computing of the
> weighting function.

What is the weighting function? How do we verify that the result is valid?
There have been other attempts to make proof of work more capex sensitive
(especially memory-hard variants, like birthday paradox) but they all end up
suffering from the fact that the very fact that you can verify the result
means that you can brute force the outcome, and often that's a tradeoff that
works.

Without knowing specifics it's very hard to say whether this particular proof
of work algorithm does not permit an energy-inefficient brute force solution
that will end up making the energy problems just as bad -- my intuition is
that of course this won't work, as a matter of "no free lunch" \-- the cost to
secure a coin will be equal to the value of keeping it secure.

I guess we'll have to wait:

> Beyond these intuitions, the specifics of the algorithm and a detailed proof
> of its security will be published in a separate manuscript. [54]

> [54] Michael Dubrovsky and Marshall Ball. Towards optical proof of work;
> oPoW. Unpublished Manuscript, 2019.

~~~
tromp
> the very fact that you can verify the result means that you can brute force
> the outcome

No; you cannot brute-force the outcome in any realistic sense. For example, a
Cuckoo Cycle [1] proof consists of 42 n-bit indices of edges that together
form a cycle in a random bipartite graph on 2^n+2^n nodes, with typically n >=
29. Brute forcing over all possible size-42 subsets of 2^n indices will take
well beyond the heat death of the universe. It's way easier to brute force the
256-bit private keys of all bitcoin balances...

[1] [https://github.com/tromp/cuckoo](https://github.com/tromp/cuckoo)

~~~
jacobush
Can you please expand or refute on my comment above. (User jacobush.)

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bb88
So the big problem is that Market Forces are the largest decision factor in
any distributed system.

Right now the barrier to entry is relatively low. Miners just need cheap
electricity and off the shelf GPUs to get started. It's also well known to use
and get started with building your own mining cluster.

The only way to fix it is to introduce graduated electricity rates that
increase as more electricity is used.

~~~
mzdubrov
Actually, electricity rates drop with economies of scale and willingness to
set up in remote areas.

Have you looked into mining Bitcoin with GPUs lately? You would need free GPUs
and free electricity to justify the effort to set it up...

BTC is only profitable with a. ASICs b. Very cheap energy source

Same is increasingly true for other coins.

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gtrubetskoy
I've spent a lot of time trying to understand PoW and came to the conclusion
that is a distributed clock of sorts, described here
[https://grisha.org/blog/2018/01/23/explaining-proof-of-
work/](https://grisha.org/blog/2018/01/23/explaining-proof-of-work/)

~~~
jmeyer2k
Well, it relies on a synchronized clock, so it can't provide a clock. PoW
adjusts the difficulty based on the time to try to meet some difficulty
target.

In fact, if the time of nodes is not synchronized, it can cause significant
problems and vulnerabilities. If time is too fast, the difficulty adjustment
algorithm will think it mined too few blocks and decrease the difficulty.

~~~
derefr
Those aren’t really “significant problems and vulnerabilities”: any given node
can lie about what time it is, but you’re not trusting a particular node for
more than the outcome of a single contiguous block—and block difficulty
“velocity” is capped—so you’d need a Sybil attack to actually walk the
difficulty down. Otherwise, even at 49% malicious nodes, consensus is just
going to bounce between nodes that say the time was really short, and nodes
that give “regular” timestamps, keeping the difficulty roughly constant within
the network’s margin of error.

Really, the timestamp field in most PoW systems’ “block” structs (Bitcoin’s,
Ethereum’s, etc.) is just defined as “a number that is higher than the one in
the parent block, and not so high that when interpreted as a POSIX timestamp
it would land 30+ seconds in the future relative to the local node’s time.” So
you just need >50% of the nodes to have a ±30s clock sync in order to agree on
which blocks are valid for consideration; and even if you don’t have that
level of synch, those blocks will still become valid _eventually_ , once
they’re old enough that all the nodes do consider them to be in the past. (And
most PoW systems keep around near-“future” blocks until they’re valid for just
such a case.)

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ttul
This is a really fascinating paper. Given that cryptocurrency is here for the
long haul - a reasonable assumption, even if it’s only for money laundering -
finding a way to cut the power cost and make mining resistant to bots makes a
lot of sense.

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iamleppert
If the cost of operation is minimized and transferred to at the time of
purchase, it will just make people buy MORE hardware, not less. Instead of
realizing that money in operations, it will be shifted to purchase of
increasingly more amounts of hardware as the primary mode of competition.

Building the hardware could maybe be optimized, but is unlikely to be energy
efficient or environmentally friendly. At least with energy you can optimize
the point source of pollution at the source, and as such use sources of energy
that are renewable or environmentally friendly.

This scheme seems to be robbing Peter to pay Paul.

~~~
mzdubrov
Yes, more HW would be purchased.

There's much less embodied energy in $1 of chip (especially cutting edge HW
where you are covering R&D) than in $1 of energy.

Also once hardware (access to capital much better distributed than access to
huge quantities of discounted power) is purchased it's portable and condensed.
Much better for decentralization.

~~~
wmf
For existing PoW hardware (even GPUs) it seems like proximity to the Chinese
supply chain is more important than capital. I don't see why that would
change.

~~~
mzdubrov
For existing PoW, proximity to cheap energy is required before anything else.

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ealexhudson
I'm not sure this theory works. If the opex is sufficiently low, even if the
capex is higher then miners would migrate to it as the total ROI is greater.
They seem to admit this partway through the paper - "a low energy PoW can be
achieved by tailoring a PoW algorithm to a hardware paradigm with a CAPEX
dominated cost perhash/trial".

But while they've demonstrated a low-energy way of computing an equivalent
hash, presumably this is in no way currently competitive. Therefore this
proof-of-concept itself is not an example of an algorithm with capex-dominated
costs.

Given that capex versus opex is primarily a matter of accounting (i.e. do I
buy a PC, or do I rent a VM from AWS?), I don't understand how that
algorithmic distinction can even be achieved. If the ongoing cost of running
the device become negligible, then you just incentivise the miners to "spend
the saving" by buying more mining devices up-front.

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Schoolmeister
Naively, it seems to me that as long as the profit you get out of mining
scales linearly with the amount of money you put in, the richest player will
eventually control all of the hash rate. Yet this doesn't seem to be the case
(so far?). Is my thinking incorrect, if so, why?

~~~
vilhelm_s
Why would it? E.g. if mining has a 1% return on investment and there are three
actors with 4, 5, and 7 units of hash power, and they reinvest profits into
more mining rigs, then a year later they will have 4.04, 5.05, and 7.07 units
of hash power---the ratios stays the same.

~~~
Schoolmeister
Turns out, I really didn't think this through. Thanks for the correction.

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sidhantgandhi
They’re falling just shy of mentioning which silicon photonics providers we
should be buying :)

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BluLess
These designed optical chip devices look like alien technology really...

