According to Wikipedia, CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. This code shouldn't be called CAPTCHA as

According to Wikipedia, CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. This code shouldn't be called CAPTCHA as it makes no attempt to tell humans from machines. (If anything, SHA256 proof-of-work is a problem that's easy for a computer but hard for a human, the opposite of a Turing test.) I'd call it "Proof-Of-Work Client-side RAte Protection" -- it's a POWCRAP, not a CAPTCHA. (I invented the acronym just now in this post; it could perhaps use some work.)