CAPTCHA comes from "Completely Automated Public Turing test to tell Computers and Humans Apart". These tests don't fulfill that requirement. Random guessing gi

CAPTCHA comes from "Completely Automated Public Turing test to tell Computers and Humans Apart". These tests don't fulfill that requirement. Random guessing gives you 3%, which is worse than random guessing on either the MSFT or reCAPTCHA. This is far worse though. A simple loop over the 30 positions, running the output through an OCR engine would give you nearly 100%. FWIW, I wrote the paper and AFAIK was the first person to break reCAPTCHA. I worked on the original MSFT Passport/Hotmail CAPTCHA system and improved MySpace's CAPTCHA which took spammer registrations from ~1,000,000/day (automated) to a few thousand (manual) in late 2007.