Subject: Re: [bitcoin-dev] Upgrading PoW algorithm

From: nullius <[email-masked]>
Date: Sat, 20 Jan 2018 06:30:37 +0000

On 2018-01-17 at 22:31:52 +0000, Jefferson Carpenter 
<[email-masked]> wrote:
>Bitcoin's difficulty will be maxed out within about 400 years, by 
>Moore's law.

On 2018-01-19 at 20:54:52 +0000, Jefferson Carpenter 
<[email-masked]> wrote:
>In other words, max difficulty for SHA256 might be significantly faster 
>than forcing the first 256 bits of a SHA512 hash...

“Moore’s law” is not a law of nature.  Indeed, chipmakers began bumping 
up against the limitations of *actual* natural laws about 15—20 years 
ago.  That is why instead of increasing core clock, they play the tricks 
which opened the way for Meltdown and Spectre.  Feature size, and thus 
transistor counts, will soon enough run into physical limitations, too.

But the scenario you describe does not even require such a discussion.

2^256 work for brute force is on the order of 10^77 hashes.  For the 
number of atoms in the observable universe, I’ve seen estimates ranging 
from 10^78 to 10^82.  Thus, you are suggesting that within 400 years, 
computers will be able to compute one hash for every myriad of atoms in 
the observable universe—perhaps one hash for every *ten* atoms.  
Moreover, you suggest that twenty-fourth century computers will do this 
fast enough to meet Bitcoin’s ten-minute target rate.

Such a proposition bypasses science, leaps over science fiction, and 
lands in the realm of religion.  Perhaps a deity could do this—using a 
computer made of other than matter, powered by other than energy.  
Humans will *never* be capable of such a feat:  Not now, and not in a 
billion years.  Certainly not a mere four centuries hence!

(I do not here positively exclude the possibility, however slim, that 
mathematical breakthroughs may yield a preimage attack on SHA-256 which 
is significantly better than bruteforce.  I *do* positively declare it 
impossible that Earth-beings will ever be capable of performing 2^256 
work.  Or even 2^128 work, for that matter.)

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