How does proof of work stop bots?

Because it destroys the economics of scraping. It’s too expensive with proof of work, or at least not as economically viable

Depends on what type of scraping you're trying to stop. For the dumb scrapers that would try to scrape every page on a git forge (for which there are a bazillion pages for a modest project, because of how the site works), yeah it might deter them enough to stop. For anything high value (eg. reddit comments or retail prices), 10s of cpu time isn't going to stop them.

It will not scare away bots but 10 seconds of wait (CPU or only a sleep) will turn away many real users. "This site is so slow, I'll use something else." A kind of reverse captcha.

Maybe, the proof of work can run in the background.

Or it can run as part of a checkout wizard's "verifying your browser and processing your payment, don't close your tab" step.

At which point it exists solely to punish real human users? What scraper bot is going through checkout?

The credit card tester bots go through the checkout process.

PoW wouldn't be a big issue for them though since their volume is much lower.

Sure, the whole premise is exactly that proof of work reduces the value of scraping, while having negligible impact on users. If the data is so valuable that bot operators are willing to pay 10s of cpu, then other measures are necessary.

Nevertheless even for these high value cases, you can still argue that it disincentivizes the business model, it becomes less efficient.

If it's high value, there isn't really much you can do that will be completely effective. Traditional captchas can often be beaten by AI, or by "captcha farms" where impoverished people are paid pennies to complete captchas. Fingerprinting can be beaten by using a full browser to make the requests. Basically anything you do is just a matter of making it more expensive for bots to access it.

Beating fingerprinting and beating traditional captcha is far more expensive than solving pow. Pow doesn't stop anyone, not even the most novice bot operators

You can just download all of Reddit from torrent sites

Except it doesn't

5W load for 2 seconds is 0.002Wh, I think we'll be fine

If it gets too expensive/time-consuming to scrape then it won't happen at scale (as much)?

Speaking from the scraper’s perspective, I like proof of work; a ten year old 96-core server will cost a couple of quid to run for a few hours and will grab an absurd number of pages thanks to the access granted by repeatedly solving proofs of work. Small slick codebases too!

There's also the Anubis idea where your PoW is persistent until your IP address or session cookie changes, so you get to skip PoW in exchange for making yourself identifiable, which means the PoW can then be ramped up to take a couple of minutes.

I don't use Anubis though. I just make my site not take five seconds to render a page so bots can overload it easily? It's not actually that hard?

It would be more profitable to mine bitcoin.

PoW doesn't stop bots.. It's an annoyance at most. A rate limiter and nothing more

PoW difficulty can be scaled, eg: all cookies must work 1s, but 2nd cookie from the same ip, might have to do 2s of work

ideally one would pick something a bit more forgiving than a linear function, to avoid penalizing too much users connecting from CGNAT

I think we're talking about 2 different things. PoW is annoying for basic scrapers but it really doesn't affect enterprise grade bot operations with access to unlimited residential proxies.

look, if you have the kind of bank to pay $1/GB of web pages to get to whatever we're hosting, maybe just send an email to sales so we can give you discount on that.

Can this be repurposed as some kind of distributed cryptocurrency mining mechanism? Pay websites by mining some monero in order to access them?