Characteristics of CAPTCHA

This chapter presents the core elements characterizing the analysis and study of a CAPTCHA test. Each element is introduced and described by its main features. Then, an overview of advantages and limitations in the context of the core elements regarding the CAPTCHA test is described. It is accomplished by shortly surveying on different relevant works concerning the CAPTCHA elements and previously introduced in the literature, together with their limitations. In the end, a summary of the main open questions about the presented literature is provided.