About CAPTCHA
From Wikipedia, the free encyclopedia
A CAPTCHA (IPA: is a type of challenge-response
test used in computing to determine whether the
user is
human. "CAPTCHA" is a contrived acronym for
"Completely Automated Public Turing test to tell Computers
and Humans Apart", trademarked by Carnegie Mellon University.
A CAPTCHA involves one computer (a server) which asks a user
to complete a test. While the computer is able to generate and
grade the test, it is not able to solve the test on its own.
Because computers are unable to solve the CAPTCHA, any user
entering a correct solution is presumed to be human. The term
CAPTCHA was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper
(all of Carnegie Mellon University), and John Langford
(then of IBM). A common
type of CAPTCHA requires that the user type the letters of a
distorted image, sometimes with the addition of an obscured
sequence of letters or digits that appears on the screen. Because
the test is administered by a computer, in contrast to the standard
Turing test that is administered by a human, a CAPTCHA is sometimes
described as a reverse Turing test.