Distinguishing Human Users from Bots

Abstract

Human Interactive Proof (HIP) systems have been introduced to distinguish between various groups of users. CAPTCHA methods are one of the important branches of HIP systems, which are used to distinguish between human users and computer programs automatically and block automated computer programs form abusing Web services. The goal of these systems is to ask questions, which human users can easily answer but current computer programs cannot. In this chapter, the authors collect different pioneering works, which are done on CAPTCHA systems and create a complete survey of them. They collect more than 100 published works and classify them into 3 categories. This chapter contains different works, which are done for creating CAPTCHA methods and assessing CAPTCHA methods from different aspects, including the attacks done against CAPTCHA methods. This chapter can be used by researchers in CAPTCHA domains to quickly find previous works.