Image Recognition CAPTCHAs

摘要

CAPTCHAs are tests that distinguish humans from software robots in an online environment. We propose and implement three CAPTCHAs based on naming images, distinguishing images, and identifying an anomalous image out of a set. Novel contributions include proposals for two new CAPTCHAs, the first user study on image recognition CAPTCHAs, and a new metric for evaluating CAPTCHAs.