1. Breaking an Animated CAPTCHA Scheme
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo
University of Wollongong

2. About CAPTCHA
CAPTCHA: Completely Automated Public Turing test to Tell Computers and Humans Apart.
Other name: Human Interaction Proofs (HIPs).
Easily solvable by humans.
Cannot be solved by current computer programs.
Existing CAPTCHAs are mainly text-based on a static image.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

3. About CAPTCHA
Most of traditional text-based CAPTCHAs are known to be vulnerable against attacks.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

4. About CAPTCHA
Increase the security: distorting, blurring, rotation the text, overlaying of visual noise. But
Easy for humans hard for computers? Not guaranteed to exist.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

5. Animated CAPTCHA Static vs. Animation:
Animated CAPTCHAs has been proposed.
Assumption :
More usability: Animation makes increasing legibility for humans.
More security: Distributing the information required to solve the CAPTCHA challenge over multiple animation frames.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

6. Our questions Animated CAPTCHAs really provide more security ?
How to break animated CAPTCHA and design secure one ?
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

7. Breaking HelloCaptcha
Breaking a representative animated CAPTCHAs: HelloCaptcha
CAPTCHA provider: Affects many customer’s web sites if broken.
A variety of 84 different variations of 12 categories.
Flitter H-Mover Mass Flood Noisy Mosaic
Pop Up Roller Search light Smarties
Spread Fade Spring Swapper Text Flood
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

8. Outline Breaking HelloCaptcha. Results and Lessons learned.
Type Distinction.
Single Image Extraction.
By Pixel Delay Map (PDM).
By Catching Line (CL).
By Color Selection (CS).
Pre-Processing and Character Recognition.
Results and Lessons learned.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

9. Breaking HelloCaptcha
Type Distinction:
Number of frames
175
55-107
Number of blank frames 2 8
Maximum frame delay
4 ms
6ms
Background color
RGB(255,255,255)
RGB(92, 31, 92)
Results: Most of 84 types can correctly be distinguished 100%.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

10. Pixel Delay Map (PDM)
Feature: To get the human user’s attention, the text characters are displayed at certain fixed locations for longer periods of time
The PDM is an image resulting from the accumulation of the total amount of time that a pixel gets displayed in a color that is different from the background color.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

11. Pixel Delay Map (PDM) PDM and extracted static image.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

12. Pixel Delay Map (PDM) PDM on all frames.
PDMs constructed from consecutive 1/6 of the frames.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

13. Catching Line (CL) Character moving areas.
Selected frames and characters by “Catching line”.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

14. Color Selection (CS) Characters separated based on color.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo
Breaking an Animated CAPTCHA Scheme

15. Pre-Processing on extracted single image
Noise removal.
Refine by filling.
Shape removal.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

16. Character Recognition by OCR program
Use ABBYY FineReader 11.
Use existing embedded training database and/or own training set .
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

17. Experimental Results
8,400 animated CAPTCHA samples were collected from the HelloCaptcha website.
Accuracy of breaking (i.e. correctly recognizing all characters in the animated CAPTCHA challenges) ranges between 16% −100% of the time (wide accepted that more than 1% of the time is essentially broken).
Attacking time: 4 secs/challenge.
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

18. Lessons learned Delay periods: The number of frames. Frame 45 Frame 82
40ms ms ms
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

19. Lessons learned Character positions:
The important information is emphasized by displaying it for longer. That can be exploited using the PDM method.
PDM was used to break 61 of the 84 different types and can be affected types from other sources:
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

20. Lessons learned
Moving direction : Only move or scale in the vertical direction can vulnerable to attacks.
Use of color or luminance:  Less is best
Method of delivery: Gif, Flash or Video ?
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme

21. Question ?
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA Scheme