Captcha is an automated public Turing test that helps to tell humans and the computers apart. This is discovered at CMU by Luis von Ahn and Manuel Blum. This is a challenging program and it includes a response tests to segregate computers from the human programs. Some of the features of captcha are”

It provides distorted characters to the users

Generic captcha include numbers and distorted letters.

If the letters to be assumed are correct then the user is identified as a human being and thus allow accessing of the system otherwise the user is not permitted to access the system.

The present OCR cannot read the noisy text as well as distorted text and that is possible only by human brains. Therefore whenever the typed letters and numbers do not match the given informations the system will understand the he user is not a human being but a bot which will prevent from any kind of further accessing. The first captcha was used by Altavista in their own sites which gives 95% reduction of spams. Yahoo becomes partner with CMU to reduce the threats they get in the chat messenger service. In the year 2000 captcha was trademarked by Manuel Blum and Luis von Ahn.

The Turing test was proposed by Alan Turning and it helps to test the intelligence level of the ocean. Human judges usually ask two participants and the machine is one among them but he will not know which one id the machine participant. If the judge cannot identify which is the machine participant the the machine will pass the test. A reverse Turing test is employed by the captcha which means a judge= program of captcha and participant = user. If the captcha is passed by the user then he is a human being otherwise it is a machine.

Download Technical Seminar Topic CSE 2011 PPT on Captcha .