Captcha

A CAPTCHA (Completely Automated Public Turing Test to tell Computers and Humans Apart) is a test, used with challenge-response systems, that's designed to differentiate humans from automated senders. A CAPTCHA sets some task that should be fairly easy for a human to perform, but impossible for an e-mail spammer's program. Typically, when a challenge-response system receives an e-mail message, it sends a reply with a URL linking the user to a Web page with a CAPTCHA. There are a number of different types of test used. Carnegie Mellon University's CAPTCHA Project Web site has demo versions of several tests. "Gimpy," for example, is a program that randomly selects seven words from a dictionary and then presents them, somewhat distorted, in an image. The user is asked to type three of the words that appear in the image. Another test, "Pix," presents the user with six images of a single subject, such as babies or horses, and asks them to define the subject of the pictures. Once senders have satisfied the CAPTCHA test, their addresses can be added to the recipient's whitelist of permitted senders, so they won't have to prove themselves each time they send a message.