Resumen
CAPTCHA stands for Completely Automated Public Turing Test to Tell Computers and Humans Apart. CAPTCHA is an Automated Turing Test. Several companies offer free email services that have suffered from a specific type of attack--"bots" that signed up for thousands of email accounts every minute. This situation has been improved by requiring users to prove they are human before they can get a free email account. CAPTCHA prevents dictionary attacks in password systems by preventing a computer from being able to iterate through the entire space of passwords by requiring a human to type the passwords. CAPTCHA shows that open problems in Artificial Intelligence (AI) can be useful--adversaries cannot vote thousands of times in online polls or obtain millions of free email accounts unless they can solve an open problem in AI. CAPTCHA is used by Yahoo, Hotmail, PayPal and many other popular Web sites to prevent automated registrations, and they work because no computer program can currently read distorted text as well as humans.