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Usability of CAPTCHAs Or usability issues in CAPTCHA design

Usability of CAPTCHAs Or usability issues in CAPTCHA design. Authors: Jeff Yan and Ahmad Salah El Ahmad Presented By: Kim Giglia CSC 682 11/19/2008. Definitions CAPTCHA – Completely Automated Public Turing Test to Tell Computers and Humans Apart Three main types of CAPTCHAs:

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Usability of CAPTCHAs Or usability issues in CAPTCHA design

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  1. Usability of CAPTCHAs Or usability issues in CAPTCHA design Authors: Jeff Yan and Ahmad Salah El Ahmad Presented By: Kim Giglia CSC 682 11/19/2008

  2. Definitions • CAPTCHA – Completely Automated Public Turing Test to Tell Computers and Humans Apart • Three main types of CAPTCHAs: • Text-based schemes • Sound-based schemes • Image-based schemes (www.captcha.net)

  3. Text-Based CAPTCHAs

  4. Sound-Based CAPTCHAs

  5. Image-Based CAPTCHAs • www.captcha.net – requires users to perform some sort of image recognition task

  6. Why CAPTCHAs? • Intuitive to users • Good security if deployed properly • Used for: • Preventing blogger spam • Protecting website registrations • Protecting e-mail addresses from scrapers • Securing Online Polls • Prevent dictionary password search attacks • Keep search engine bots out of certain web pages • Protect against e-mail worms and spam

  7. Usability of CAPTCHAs • Robustness and usability • What issues are involved in designing usable CAPTCHAs: learnability, efficiency, memorability, errors, and satisfaction • Authors focus on framework for evaluating CAPTCHAs from an accuracy, response time, and satisfaction issues point of view

  8. Category Usability Issue • Distortion method and level • Confusing characters • Friendly to foreigners? Framework for evaluating usability Distortion • Character Set • String length – fixed or random • Random chars or dictionary word • Offensive word Content • Font type and size • Image size • Use of Color • Integration with web pages Presentation

  9. Security issues and usability trade-offs with CAPTCHAs: • Size of a character set matters, but larger sets can present more confusing characters • String length matters, but longer or random strings may be harder for users • Suggest using a variable length string with the length info presented in the CAPTCHA • Random strings versus dictionary strings – authors state that emphasis is more on segmentation resistant design

  10. Security issues and usability trade-offs with CAPTCHAs: • Offensive words have no security implication, but clearly affect a user’s experience • Misuse of color can actually make your CAPTCHA less secure and way more difficult to read

  11. What is reCAPTCHA? • A free CAPTCHA service that helps to digitize books • Book pages are photographically scanned and then OCR is used to transform the images to text • Two words are given to a user: One answer is known and if user gets known text correct, other text answer is assumed correct • Note: about 60 million CAPTCHAs are solved daily, which is >150,000 hours of work daily

  12. Authors’ Conclusions • Text based CAPTCHAs can be difficult for foreigners (study not truly large enough to conclude this) • Length of strings has security and usability implications • Use of color has security and usability implications • Have developed a framework for examining usability issues (true, but they did not provide data as to which items are most important)

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