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It's not enabled yet in production - but the main mechanism is by increasing the difficulty as more requests are made from an IP in a certain timeframe (it's basically rate limiting at that point). Think: every 3rd request in a minute doubles the difficulty with some cooldown period.

With that the cost (and complexity) of an attack can hopefully be in the same ballpark (or higher) than ReCaptcha - without your end user having to label cars or send data to Google.

But in the end a determined spammer will get through any captcha cheaply (for reference: ReCaptcha solves are sold by the thousands for $1) - we just hope we can do better than ReCAPTCHA, especially UX-wise.

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I love this concept of proof-of-work captchas, but there's a growing number of tools and ways to bypass IP blocks via IP rotation[1], specially after the explosion of IaaS providers. How do you intend to tackle this?

[1] Some examples: https://rhinosecuritylabs.com/aws/bypassing-ip-based-blockin... https://oxylabs.io/products/real-time-crawler https://github.com/alex-miller-0/Tor_Crawler https://www.scrapinghub.com/crawlera/

There are free and paid list of all ip addresses from datacenters like https://udger.com/resources/datacenter-list, they probably existing for specifically preventing this, so maybe thats an option here.

The obvious follow-up question is how IPv6 impacts this, because I think it's supposed to be easy for someone to get their hands on a decent chunk of IPv6 addresses.

Maybe the difficulty could scale as a property of how similar the IP address is to previously seen addresses... so the addresses in the same /64 block would be very closely related, for example. (I think that's how IPv6 works... but definitely something I haven't researched lately, so I could just sound very confused)

I don't have all the answers yet, but indeed rate limiting a larger block (at least /64), or even at multiple prefix sizes with different weighting makes sense.

So the way this is supposed to work is that providers hand out /48s and each site should be allocated a /64. In practice if you for example rent a VPS, you'll be handed a /64 for it by your service provider from their /48.

I would personally treat any /64 as the same. Depending on your local network setup the second half of the address could be anything and could change frequently. You might also get multiple addresses. Whereas getting a new /64, or /48, requires slightly more effort.

Of course there's a risk you'll block a /64 and that takes out some whole company or whatever, but I've seen that happen to corporate proxies that got flagged as a source of spam as well so this is not an easy problem even without the 2^128 address space.

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