By: Steven Baker

 What is a CAPTCHA?  History of CAPTCHA  Applications of CAPTCHAs  Accessibility  Examples of CAPTCHAs  reCAPTCHA  Vulnerabilities  Conclusion

 Completely Automated Public Turing test to tell Computers and Humans Apart  The goal:  To create an automated test that is easy for a human to pass but difficult for a computer  Optical character recognition (OCR) is a difficult problem for computers to solve  Take advantage of the fact that humans are good at recognizing patterns that computers have trouble with  The most common form of CAPTCHAs are images of text that are distorted: 

 Verification of a human in the loop, or identification via the Turing Test  Journal from 1996 by Moni Naor, an Israeli Computer Scientist  First work published detailing ideas behind modern day CAPTCHA  Mark D. Lillibridge, Martin Abadi, Krishna Bharat, and Andrei Z. Broder  Used CAPTCHAs for AltaVista in 1997 to prevent bots from adding to their search engine  Patented their process in 1998 although they didn’t use the term CAPTCHA  Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford  First to coin the term CAPTCHA in 2000 at CMU when they developed the first CAPTCHA used by Yahoo

 Protect website registration  Online Polls  Prevent comment spam on blogs  Prevent worms and spam  Search engine bots  Prevent dictionary attacks

 While you want to stop unwanted bots, you don’t want to keep legitimate users out  Section 508 in the US requires federal agencies to make information technology accessible to those with disabilities  Problem:  Visually impaired users will not be able to solve image based CAPTCHAs  Like OCR, speech recognition is also a difficult problem for computers to solve  Solution:  Provide an audio alternative for human verification

 Distorted text with audio option  Picture identification  Simple Math CAPTCHA  3D CAPTCHA

 Developed by Luis von Ahn, Ben Maurer, Colin McMillen, David Abraham, and Manuel Blum at CMU  Acquired by Google in September 2009  Assists in digitizing the text of books  Requires user to enter two words ▪ One is the test word that is known by the system ▪ The other is a word from the scan of a digitized text that a computer was unable to recognize  Used to digitize the archives of the New York Times and books from Google Books

 Images that use undistorted text or a consistent font are vulnerable  Image processing techniques are able to read the text if it is not distorted enough  Some companies sell CAPTCHA breaking services  Employ actual humans to solve CAPTCHAs as a job

 What is a CAPTCHA?  History of CAPTCHA  Applications of CAPTCHAs  Accessibility  Examples of CAPTCHAs  reCAPTCHA  Vulnerabilities  Any questions?

    different-types-of-captchas/ different-types-of-captchas/  CHAs.pdf CHAs.pdf