Origin

The potential difficulty of differentiating humans from computers pretending to be humans was addressed at least as early as 1950, when Alan Turing described his now-famous Turing test. The first discussion of automated tests which distinguish humans from computers for the purpose of controlling access to web services appears in a 1996 manuscript of Moni Naor from the Weizmann Institute of Science, entitled "Verification of a human in the loop, or Identification via the Turing Test" Naor, Moni (1996). "Verification of a human in the loop or Identification via the Turing Test".

Primitive CAPTCHAs seem to have been developed in 1997 at AltaVista by Andrei Broder and his colleagues to prevent bots from adding URLs to their search engine. In order to make the images resistant to OCR (Optical Character Recognition), the team simulated situations that scanner manuals claimed resulted in bad OCR. In 2000, von Ahn and Blum developed and publicized the notion of a CAPTCHA, which included any program that can distinguish humans from computers. They invented multiple examples of CAPTCHAs, including the first CAPTCHAs to be widely used, which were those adopted by Yahoo!.