Think Before You Engage: 100 Questions to Ask Before Starting a Social Media Marketing Campaign

32. What Is a CAPTCHA?

A CAPTCHA is a type of test used to ensure that a response is from a person and not generated by a computer. The process usually involves the user being asked to complete a simple test, which a computer program is able to generate and grade. Because other computers are supposedly unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human.

CAPTCHA is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human, in contrast to the standard Turing test that is typically administered by a human and targeted to a machine. A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen.

The term “CAPTCHA” was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University). It is a contrived acronym based on the word “capture” and standing for “Completely Automated Public Turing test to tell Computers and Humans Apart.” Carnegie Mellon University attempted to trademark the term, but the trademark application was abandoned on 21 April 2008.

A CAPTCHA is a means of automatically generating challenges that intend to:

• Provide a problem easy enough for all humans to solve.
• Prevent standard automated software from filling out a form, unless it is specially designed to circumvent specific ...