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CAPTCHA

CAPTCHA. Presented By Sayani Chandra (Roll - 09127106017). What is CAPTCHA ?. CAPTCHA is …. C ompletely A utomated P ublic T uring test to tell C omputers and H umans A part.

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CAPTCHA

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  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.

  6. Background …

  7. 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.

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

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

  10. What is Turing test?

  11. Turing test is … • 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.

  12. Classification …

  13. CAPTCHA are of … 3 types :- CAPTCHA Text Based Audio Based Graphics Based MSN Passport Service Bongo PIX Gimpy Ez-Gimpy

  14. Text Based CAPTCHA …

  15. 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.

  16. Gimpy … • 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.

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