Captcha & its types

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Information about Captcha & its types

Published on November 24, 2016

Author: indrathakur001


1. CAPTCHA & its types

2. CAPTCHA Completely Automated Public Turing test to tell Computers and Humans Apart. Captcha word comes from capture. First developed by Alta Vista in 1997. The term coined in 2000 at CMU(Carnegie mellon university) by Luis von Ahn, Manuel Blum, Nicholas J. Hopper & John Langford.

3. What is CAPTCHA? CAPTCHA is a program that protect website against bots by generating & grading test that  Human can pass but  Computer program can’t It is a computer program that can tell whether its user is a human or a computer.

4. Continue..... Captcha requires that the user types letters or digits from distorted image appears on the screen. If correct solution is entered than system assumes that response is generated  by user. else  by bots & access denied.

5. Reverse Turing Test CAPTCHA is also known as Reverse Turing Test. It is administrated by a Machine & targeted to human whereas a Turing test is administrated by human & targeted to a Machine. Turing test is used to test a machine’s level of intelligence.

6. Types of CAPTCHA Text based CAPTCHA  Gimpy  Gimpy-r, Google CAPTCHA  Simard’s Graphics based CAPTCHA  Bongo  Pix Audio based CAPTCHA

7. Text based CAPTCHA It is simple CAPTCHA type & carries normal questions. o What is the sum of three & thirty-five ? o If today is Saturday, what is day after tomorrow ? Very effective, needs a large question bank. Cognitively challenged users find it hard.

8. Gimpy Initially used by Yahoo. Yahoo used this version in Messenger. In this CAPTCHA two steps are followed as: a)Pick random characters. b)Distort them and add noise at background Has only 1 random string of characters.

9. GIMPY-R Modified version of Gimpy. Picks up 10 random words from dictionary & distorts, fills with noise. User has to recognize at least 3 words. If the user is correct, then he is admitted.

10. Simard’s Here further advances made and arcs being made into it. Hence steps followed are as: a)Pick random letters and numbers b)Distort them and add arcs

11. Graphic based CAPTCHA: Bongo  After M.M.Bongard, pattern recognition expert.  User has to solve a pattern recognition problem.  Has to tell the distinct characteristic between two sets of figures.

12. Graphic based captcha: Pix Uses a large database of labelled images. It shows a set of images, user has to recognize the common feature among those.

13. Audio based captcha These are based on humans ability to depict sounds that may be distorted, following algorithm is followed in using it: a) Pick a word or a sequence of numbers at random b) Render them into an audio clip using a TTS software c) Distort the audio clip d) Ask the user to identify and type the word or numbers

14. 3-D captcha The 3-D CAPTCHA and modern video games use the same basic mechanism to generate unique images. Designing a computer vision program that can recognize the objects within the 3-D CAPTCHA images is intrinsically difficult.

15. EXAMPLE OF 3-D CAPTCHA We now have the code: CKT

16. Applications of captcha Prevent comment spam in blogs & spam e- mails. Prevent web registration abuse, protect passwords from brute-force attack. Protect Online polls. Prevent emails from scrapers. Preventing dictionary attacks.

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