I've used SHA-256 HashCash before, for PoW authentication on an API built to support a mobile app. The nice bit about using HashCash is increasing the difficult

I've used SHA-256 HashCash before, for PoW authentication on an API built to support a mobile app. The nice bit about using HashCash is increasing the difficulty for successive failed attempts. I implemented this in response to our API being hammered by exploratory bots/scripts sending malformed requests. By adding this, it dropped failed requests by 99%.