CAPTCHA = Completely Automated Public Turing test to tell Computers and Humans Apart
Information pertaining to CAPCTHAs
1. Carnegie-Melon Universitiy's CAPTCHA project page.
2. A Wikipedia reference page on CAPTCHAs.
Websites which use text-based CAPTCHAs to prevent denial--of-service attacks or mass-scale registration
1. Yahoo! Mail - Sign Up
2. PayPal - Sign Up.
3. MSN Hotmail account Sign Up.
4. Domain name owner search at Register.com.
5. Search for availability of tickets at Ticketmaster (including tickets to Penn State football games !)
Proposed Security Protocols which rely on CAPTCHAs
Countering Distributed Denial-of-service attacks: W. G. Morein, A. Stavrou, D. L. Cook, A. D. Keromytis, V. Mishra, and D. Rubenstein, "Using Graphic Turing Tests To Counter Automated DDoS Attacks Against Web Servers," Proc. ACM Conference on Communications and Computer Security, 2003. [PDF]
Preventing attacks on user-defined passwords: B. Pinkas and T. Sander, "Securing Passwords Against Dictionary Attacks,"
Proc. ACM Conference on Communications and Computer Security, 2002. [PDF]
Breaking conventional text-based CAPTCHAs
[A] Relevant Publications
1. K. Chellapilla and P. Y. Simard, "Using Machine Learning to Break Visual Human Interaction Proofs (HIPs)," Proc. NIPS, 2004. [PDF].
2. G. Moy, N. Jones, C. Harkless, and R. Potter, "Distortion Estimation Techniques in Solving Visual
CAPTCHAs," Proc. IEEE CVPR, 2004. [PDF]
3. G. Mori and J. Malik, "Recognizing Objects in Adversarial Clutter: Breaking a Visual CAPTCHA," Proc. CVPR, 2003. [PDF]
4. A. Thayananthan, B. Stenger, P. H. S. Torr, and R. Cipolla, "Shape Context and Chamfer Matching in Cluttered Scenes," Proc. IEEE CVPR, 2003. [PDF]
[B] Public-domain resources on breaking CAPTCHAs
1. PWNtcha - captcha decoder
2. The OCR Research Team (commercial).
Realistic Alternative
Natural image based CAPTCHAs - IMAGINATION demonstration page.
Presentation slides on IMAGINATION: [.PPT]