What is Image Verification ( Captcha )?

A CAPTCHA or Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. Thus, it 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 that the user type letters or digits from a distorted image that appears on the screen. The term "CAPTCHA" (based upon the word capture) 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 for "Completely Automated Public Turing test to tell Computers and Humans Apart." * Alan Turing — father of computer science, codebreaker, cognitive scientist, theoretician in artificial intelligence—achieved fame in 1936 at the age of 24 with a paper in which he showed that no universal algorithm exists that can determine whether a proposition in a given mathematical system is true or false. In the process of his proof he invented what has been called the Turing machine, an imaginary idealized computer that can compute any calculable mathematical function. The essentials of this machine (an input/output device, a memory, and a central processing unit) formed the basis for the design of all digital computers. After World War II broke out, he worked in England as a cryptanalyst, where he put his extraordinary talents to work on breaking the famous Enigma code used by the German military. By 1940, Turing was instrumental in designing a machine that broke the German code, allowing the Allies to secretly decipher intercepted German messages for the rest of the war. At war's end, Turing was hired to help develop the world's first electronic computer and ultimately designed the programming system of the Ferranti Mark 1, the first commercially available digital computer, in 1948.