By Alex Y. Chen and York Li CAPTCHA, which stands for Completely Automated Public Turing test to tell Computers and Humans Apart, plays a crucial role in enhancing security measures and preventing unauthorized access. It functions as a challenge-response mechanism designed to differentiate between automated bots and human users. CAPTCHA technology has advanced remarkably in recent years, so much so that many service providers offer free CAPTCHA tools to protect websites from cyberattacks.