1. CAPTCHA Presented By Sayani Chandra (Roll - 09127106017)

2. What is CAPTCHA ?

3. CAPTCHA is …  Completely Automated Public Turing test to tell Computers and Humans Apart.  Created in 2000 for Yahoo to prevent automated e-mail account registration,by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford, Carnegie Mellon University.

4. Introduction …

5.  A program that can tell whether its user is a human or a computer.  It uses a type of challenge-response test to determine that the response is not generated by a computer.  Generic CAPTCHAs distort letters and numbers.  Distorted characters are presented to user.  User has to recognize the distorted letters.  If the guessed letters are correct, the user is inferred to be a human and allowed access.  Else, user is a bot and denied access.  Humans can read the distorted and noisy text.  Current OCRs cannot read them.

7. Background …

8.  In November 1999, slashdot.com released a poll to vote for the best CS College in the US.  Students from the CMU and the MIT created bots that repeatedly voted for their respective colleges.  This incident created the urge to use CAPTCHAs for such online polls to ensure that only human users are able to take part in the polls.  Altavista first used a crude CAPTCHA in their sites.  Resulted in 95% spam reduction.  Yahoo partnered CMU to counter these threats in Messenger chat service.  Luis von Ahn and Manuel Blum of CMU trademarked CAPTCHA in 2000.

9.  Sabotage of online polls.  Spam emails.  Abusing free online accounts.  Tampering with rankings on recommendation systems (like EBay, Amazon). Why CAPTCHA was needed ?

10.  A program that can generate and grade tests that: Most humans can pass. Current computer programs cannot pass. So, CAPTCHA is …

11. What is Turing test?

12.  Proposed by Alan Turing.  To test a machine’s level of intelligence.  Human judge asks questions to two participants, one is a machine, he doesn’t know which is which.  If judge can’t tell which is the machine, the machine passes the test.  CAPTCHA employs a reverse Turing test, judge = CAPTCHA program, participant = user, if user passes CAPTCHA, he is human. if user fails, it is a machine. Turing test is …

13. Classification …

14. CAPTCHA are of … 3 types :-

15. Text Based CAPTCHA …

16.  Simple, normal language questions: What is sum of three and thirty-five? If today is Saturday, what is day after tomorrow? Which of mango, table, water is a fruit?  Very effective, needs a large question bank.  Cognitively challenged users find it hard.

17.  Designed by Yahoo and CMU.  Picks up 10 random words from dictionary and distorts, fills with noise.  User has to recognize at least 3 words.  If user is correct, he is admitted. Gimpy …