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Comment by esnard | Hacker News Reader

The captcha is considered solved if the browser finds N nonces (with 0 < N ≤ 10) where a SHA-256 hash starts with 20 bits set to 0 in less than 5 minutes.

A nonce is valid on average every 2 * * 20 hashes computations, which is about 1 million hashes.

A modern GPU can compute several billion SHA-256 hashes per second.

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Since this protection is uncommon, it will protect against spam, until someone creates a dedicated bot to bypass this captcha.

Also, I suspect phone users will have a hard time getting through. My phone took 30s to validate the captcha used in the demo, which has the N variable set to 2.

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Yep, to adapt the old trope[1]:

This strategy involves a

(X) technical ( ) legislative ( ) market-based ( ) vigilante

approach to fighting bots. This idea will not work. Here is why it won't work.

...

(X) It is defenseless against brute force attacks

(X) It will stop bots for two weeks and then we'll be stuck with it

[1] https://craphound.com/spamsolutions.txt

So this is like mining Bitcoin on visitor's device, except that nobody gets the Bitcoin.

> My phone took 30s to validate the captcha used in the demo

I tried it on my phone as well and I thought it did not work, because the progress bar did not move. I didn't even notice there was a progress bar.

The idea is great, but I would not use it for my apps, because I would expect a heavy loss of conversions.

Isn't there a mathematical puzzle which cannot be optimizer by GPU usage so that desktop and phones are on par? Maybe something like scrypt which takes less computation and more ram? I guess ram is also a very sparse resource for bots, no?

> Isn't there a mathematical puzzle which cannot be optimizer by GPU usage so that desktop and phones are on par?

If you can answer that question, you can create a blockchain Proof-of-Work algorithm that won’t use half the world’s electricity.

Estimates of Bitcoin's energy consumption are around 73 TWh annually.[1]

The global annual consumption for 2017 was 21,372 TWh. [2]

While Bitcoin's energy consumption is notable, it accounts for nowhere close to half of the world's electricity usage, but rather about 0.3% of it.

1. https://digiconomist.net/bitcoin-energy-consumption

2. https://www.iea.org/statistics/electricity/

Is it just me, or is the fact that Bitcoin uses almost half a percent of the world's electricity completely insane!? It's unfathomable! Half a percent of the sum total of human energy expenditure! Just for some pointless hashing!

Not the sum total of human energy expenditure. Of electricity expenditure.

And how much would it use if we used it to replace fiat currency?

It's not possible to replace fiat currency with bitcoin because the transaction rate (# of tx/s) of the bitcoin ledger is far too slow (5tx/s).

https://hackernoon.com/the-blockchain-scalability-problem-th...

There are solutions to that, I wouldn't go so far as to say it's not possible. But I don't advocate for it or think it's in any way going to happen either.

There is no relation between the hashrate and the amount of transactions being mined. So to answer your question: That depends on a few dynamics, mostly the price of electricity compared to the price of a bitcoin given an amount of inflation (the blockreward).

Are you sure there no unintended side effects either that might cause the energy usage to increase with transactions?

In some cases more onchain transactions does mean more rewards for miners, as transactions will need to outbid with more fees for blockspace. How this relates to hashrate depends on things like the ratio of the block reward vs the block fees.

But in the context of "what if the the whole world uses bitcoin", the vast majority of transactions will not be onchain. But instead will be transacted via other more efficient layers (sidechains - like Liquid, or level 2 - like lightning).

Yeah, that all makes sense and fits with my understanding. Thanks for explaining it.

I wonder if it would be comparable if you took the costs of printing and handling fiat.

What incentive is there for blockchains to not eventually use strictly more than half the world's energy?

The cost of electricity skyrocketing and causing mining to no longer be profitable maybe?

Maybe, but what makes it less profitable than the use of electricity for anything else? Miners probably have far more capacity to pay high amounts for electricity than, say, someone who just wants to light their home or keep their refrigerator running, no?

It took maybe 10 seconds on my iPhone 6S.

Edit: Huh, based on other comments in this thread where it took more than a minute on iPhone X’s, this makes no sense. Uh, I only know what I saw...

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