Date: Tue, 18 May 2021 11:36:49 +0000
To: "mike@powx.org" <mike@powx.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Proposal: Low Energy Bitcoin PoW
Precedence: list

Good morning Michael,

> That=E2=80=99s a fair point about patents. However, note that we were car=
eful about this. oPoW only uses SHA3 (can be replaced with SHA256 in princi=
ple as well) and low precision linear matrix multiplication. A whole indust=
ry is trying to accelerate 8-bit linear matrix mults for AI so there is alr=
eady a massive incentive (and has been for decades).
>
> See companies like Mythic, Groq, Tesla (FSD computer), google TPU and so =
on for electronic versions of this. Several of the optical ones are mention=
ed in the BIP (e.g. Lightmatter)


Please note that ASICBOOST for SHA256d is based on a layer-crossing violati=
on: SHA256 processes in blocks, and the Bitcoin block header is slightly la=
rger than one SHA256 block.

Adding more to a direct SHA3 (which, as a "sponge" construction, avoids blo=
cks, but other layer-crossing violations may still exist) still risks layer=
 violations that might introduce hidden optimizations.

Or more succinctly;

* Just because the components have (with high probability) no more possible=
 optimizations, does not mean that the construction *as a whole* has no hid=
den optimizations.

Thus, even if linear matrix multiplication and SHA3 have no hidden optimiza=
tions, their combination, together with the Bitcoin block header format, *m=
ay* have hidden optimizations.

And there are no *current* incentives to find such optimizations until Bitc=
oin moves to this, at which point we are already committed and it would be =
highly infeasible to revert to SHA256d --- i.e. too late.

This is why changes to PoW are highly discouraged.


Remember, ASICBOOST was *not* an optimization of SHA256 *or* SHA256d, it wa=
s an optimizations of SHA256d-on-a-Bitcoin-block-header.
ASICBOOST cannot speed up general SHA256 or even general SHA256d, it only a=
pplies specifically to SHA256d-on-a-Bitcoin-block-header.

Regards,
ZmnSCPxj