Published on November 24, 2016
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.