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CAPTCHA

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CAPTCHA

  1. 1. CAPTCHA<br />RavindraShetty<br /> 4NM09MCA60<br />
  2. 2. Topics<br /><ul><li>DEFINITION
  3. 3. BACKGROUND
  4. 4. TYPES
  5. 5. APPLICATIONS
  6. 6. CONSTRUCTING CAPTCHA
  7. 7. BREAKING CAPTCHA
  8. 8. ISSUES WITH CAPTCHA
  9. 9. CONCLUSION</li></li></ul><li>INTRODUCTION<br /><ul><li>CAPTCHA – Completely Automated Public Turing test to tell Computers & Humans Apart.
  10. 10. Invented by Luis von Ahn,Manuel Blum.
  11. 11. It is a program that is a challenge response to test to separate humans from computer programs.</li></li></ul><li>Contd…..<br /><ul><li>Generic CAPTCHAs distort letters and numbers:
  12. 12. Distorted characters are presented to the user.
  13. 13. User has to recognize the distorted letters.
  14. 14. If the guessed letters are correct, the user is inferred to be a human & allowed access.</li></li></ul><li>Contd…..<br /><ul><li>Humans can read the distorted & noisy text.
  15. 15. Current OCRs(Optical Character Recognition) cannot read them.
  16. 16. Simple CAPTCHA image:</li></li></ul><li>BACKGROUND<br /><ul><li>Why CAPTCHA was needed ?
  17. 17. Sabotage of Online Polls.
  18. 18. Spam e-mails.
  19. 19. Abusing free Online accounts.
  20. 20. Tampering with rankings on recommendation systems (like EBay, Amazon).</li></li></ul><li>What is TURING TEST ?<br /><ul><li>Proposed by Alan Turing.
  21. 21. To test a machine’s level of intelligence.
  22. 22. Human judge asks questions to two participants, one is a machine & the other human.
  23. 23. The judge doesn’t know which is which.
  24. 24. After listening to the answer, if the judge fails to recognize which one is the machine, then the machine passes the test. </li></li></ul><li>Contd…<br /><ul><li>CAPTCHA employs a Reverse Turing Test.
  25. 25. Judge = CAPTCHA program, participant = user
  26. 26. If the user passes CAPTCHA, he is human otherwise it is a machine.</li></li></ul><li>Types of CAptcha<br /><ul><li>Text Based
  27. 27. GIMPY
  28. 28. EZ-GIMPY
  29. 29. MSNS Passport Service CAPTCHAs
  30. 30. Graphics Based CAPTCHAs
  31. 31. Audio CATCHAs</li></li></ul><li>Text Based CAPTCHA<br /><ul><li>Simple, normal questions like :
  32. 32. What is the sum of three & thirty-five ?
  33. 33. If today is Saturday, what is day after tomorrow ?
  34. 34. Which of mango, table & water is a fruit ?
  35. 35. Very effective, needs a large question bank.
  36. 36. Congnitively challenged users find it hard.</li></li></ul><li>Gimpy<br /><ul><li>Designed by Yahoo & CMU(Carnegie Mellon University)
  37. 37. Picks up 10 random words from dictionary & distorts, fills with noise.
  38. 38. User has to recognize at least 3 words.
  39. 39. If the user is correct, then he is admitted.</li></li></ul><li>EZ-Gimpy<br /><ul><li>A modified version of Gimpy.
  40. 40. Yahoo used this version in Messenger.
  41. 41. Has only 1 random string of characters.
  42. 42. Not a dictionary word, so not prone to dictionary attack.
  43. 43. Not a good implimentation , already broken by OCRs(Optical Character Recognition).</li></li></ul><li>MSNs passport service <br /><ul><li>Provided for Microsoft’s MSN services.
  44. 44. Use of 8 characters.
  45. 45. Warping is used to distort.
  46. 46. Very strong implementation, hasn’t been broken.
  47. 47. It is segmentation-resistant.</li></li></ul><li>Graphic based CAPTCHAs<br />Two Types:<br /><ul><li>BONGO
  48. 48. PIX</li></li></ul><li>BONGO<br /><ul><li>After M.M.Bongard, pattern recognition expert.
  49. 49. User has to solve a pattern recognition problem.
  50. 50. Has to tell the distinct characteristic between two sets of figures.
  51. 51. Then tell to which set a given figure belongs to.</li></li></ul><li>PIX<br /><ul><li>Uses a large database of labeled images.
  52. 52. It shows a set of images, user has to recognize the common feature among those.
  53. 53. Eg :- pick the common characteristic among the following 4 pictures = “aero plane”.</li></li></ul><li>Audio CAPTCHAs<br /><ul><li>Consists of downloadable audio clip.
  54. 54. User listens & enters the spoken word.
  55. 55. Helps visually disabled users.
  56. 56. Below is the Google’s audio enabled CAPTCHA</li></li></ul><li>Applications<br /><ul><li>Protect Online polls.
  57. 57. Prevent web registration abuse, protect passwords from brute-force attack.
  58. 58. Prevent comment spam & spam e-mails.
  59. 59. E-ticketing, prevent scalping.</li></li></ul><li>Contd…<br /><ul><li>Verify digitized books : “RE-CAPTCHA”
  60. 60. Used in Google books project.
  61. 61. Two words are shown, the program knows the first word.
  62. 62. If the user enter the first word correctly, it assumes that the second unknown word will also be entered correctly.
  63. 63. Second word becomes “known”.</li></li></ul><li>Constructing CAPTCHAs<br /><ul><li>Things to keep in mind :-
  64. 64. Don’t store CAPTCHA solution in web page’s metadata.
  65. 65. A CAPTCHA is no good if it doesn’t distort.
  66. 66. Need a large database of different CAPTCHA questions.
  67. 67. Avoid repetition of question.</li></li></ul><li>CAPTCHA logic<br /><ul><li>Generate the question.
  68. 68. Persist the correct answer.
  69. 69. Present the question to the user.
  70. 70. Evaluate the answer, if incorrect start again- Generate a different CAPTCHA.
  71. 71. If correct allow the access to the user.</li></li></ul><li>Breaking CAPTCHAs<br /><ul><li>Cracking CAPTCHAs through programs
  72. 72. Convert CAPTCHA into Grey scale.
  73. 73. Detect patterns in the image corresponding to the characters
  74. 74. Greg Mori & JitendraMalik have broken text CAPTCHAs </li></ul>Ex:- Easy Gimpy <br />
  75. 75. Contd…<br /><ul><li>To break this CAPTCHA
  76. 76. Segmentation
  77. 77. Locate possible letters in the image.
  78. 78. Construct graph of consisting letters.
  79. 79. Find out the possible words from the graph, use scores to rank
  80. 80. Roll = 11.94
  81. 81. Profit = 9.42 (better match) </li></li></ul><li>Contd…<br /><ul><li>Social engineering to break CAPTCHAs –
  82. 82. Spammer encounters a CAPTCHA
  83. 83. That CAPTCHA is copied to another site
  84. 84. Humans are baited, Ex:- free MP3s, free wallpapers, etc.
  85. 85. To get those MP3s or wallpapers, users are told to solve the copied CAPTCHA.
  86. 86. Then the solution is routed back to the spammer.</li></ul>Solution – Fix a time-to-live period for a<br />question.<br />
  87. 87. Issues with CAPTCHAs<br /><ul><li>Usability issue
  88. 88. W3C mandates web to be accessible to all people.
  89. 89. Some CAPTCHAs are in accessible to visually impaired, cognitively challenged people.
  90. 90. Compatibility issue
  91. 91. Java script may be needed to be activated in browsers.
  92. 92. Some may need Adobe Flash Plug-in.</li></li></ul><li>SUMMARY<br /><ul><li>CAPTCHAs are an effective way to counter bots & reduce spam.
  93. 93. They help advance AI knowledge.
  94. 94. Some issues with current implementations represent challenges for future improvements.</li></li></ul><li>ANY QUESTIONS ?<br />
  95. 95. THANK YOU <br />

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