CAPTCHA Basics

This chapter describes the basic concepts underlying the CAPTCHA test. Specifically, the aim of the test is described, together with the main tasks of the CAPTCHA and the requirements that the test has to meet in order to guarantee a high security level. The CAPTCHA test is framed inside the human–computer interaction model and strongly connected with the concept of human interactive proofs, which are used to make a differentiation between the human users and computer bot programs.