Comment by diggan

I think this being called a "recaptcha alternative" to be slightly misleading.

There are two problems some website hosters encounter:

A) How do I ensure no one DDOS (real or inadvertently) me?

B) How can I ensure this client is actually a human, not a robot?

Things like ReCaptcha aimed to solve B, not A. But the submitted solution seems to be more for A, as calculating a PoW can be (probably must be actually) calculated by a machine, not a human. While ReCaptcha is supposed to be the opposite, could only be solved by a human.