[bastawhiz]

> I don't think it will stop a bot machine, just make it very expensive to use

My phone solves the captcha puzzle in about three seconds. I assume it's working on one core. If you're running this on a server and it's able to do one every, say, two seconds, and you have sixteen cores, that's still about eight per second. At that point, what is this defending against? You're running into API rate limit territory.

The whole point of a captcha is to make it unsolvable for a machine. Not to make it more expensive. Because the bad actors will eventually make it cheap, and then it's not effective anymore. Consider that today, it's often cheaper to farm out CAPTCHA puzzles to a room full of humans on laptops than it is to solve them. Making it a purely computational challenge is almost certainly saving money for the bad actors.

[mschuster91]

> At that point, what is this defending against?

I have seen spam attacks against webforms running with hundreds of calls per seconds. We in the end ran our own solution - a simple math captcha was all it took.

[bastawhiz]

In college (2010) I built a honeypot to test this. Simply adding a field that blocks anything that doesn't run JavaScript worked in most cases. And that makes sense: a lot of this junk is garbage like malicious WordPress plugins that crank away to just fire off HTTP requests.

But you don't need proof of work to stop that abuse. The simplest JS with a fallback to a "I'm not a bot" checkbox would do the trick. So you're defending against folks that do run JavaScript, but...not fast?

[Kiro]

> If you're running this on a server and it's able to do one every, say, two seconds, and you have sixteen cores, that's still about eight per second.

That's no problem. It's supposed to protect against bots making billions requests a second.

[bastawhiz]

> It's supposed to protect against bots making billions requests a second.

Billions of requests per second is the sort of traffic that Google receives in total. Not the traffic to your blog.

The spam isn't the bottleneck here: at the point where you're caring about the actual load it's putting on your system, you're talking about open connections and the number of occupied workers in your HTTP server. Captcha doesn't help with that. You still need to accept the request in order to reject it.

But even if the goal is to just slow down a botnet that's pounding your server into oblivion, this still ain't it. There's no 16xlarge ec2 instance somewhere beating on your server. It's a bunch of malicious chrome extensions and garbage mobile apps. Why pay for servers when you can have ten thousand people install your software and run it for nearly nothing? The cost of the compute load isn't felt by the bad actor.

[hot_gril]

Captchas are not just ddos protection, and even if it were, the botnets don't send tons of spam from any single device. Otherwise it's too easy to identify and block.

[Kiro]

That's why you use something like this, where each request incurs a cost for the attacker so it doesn't matter if the origins are distributed.

[hot_gril]

The attacker doesn't have to calculate the puzzles in one central place. It can do that on the hacked devices.