breaking an animated captcha scheme vu duc nguyen, yang-wai chow and willy susilo university of...
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Breaking an Animated CAPTCHA Scheme
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo
University of Wollongong
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About CAPTCHA
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated
CAPTCHA Scheme
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.
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About CAPTCHA
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Most of traditional text-based CAPTCHAs are known to be vulnerable against attacks.
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About CAPTCHA
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated
CAPTCHA Scheme
Increase the security: distorting, blurring, rotation the text, overlaying of visual noise. But
Easy for humans hard for computers? Not guaranteed to exist.
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Animated CAPTCHA
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated
CAPTCHA Scheme
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.
Static vs. Animation:
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Our questions
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Animated CAPTCHAs really provide more security ?
How to break animated CAPTCHA and design secure one ?
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Breaking HelloCaptcha
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated
CAPTCHA Scheme
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
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Outline
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Breaking HelloCaptcha. 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.
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Breaking HelloCaptcha
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Results: Most of 84 types can correctly be distinguished 100%.
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)
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Pixel Delay Map (PDM)
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
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.
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Pixel Delay Map (PDM)
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
PDM and extracted static image.
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Pixel Delay Map (PDM)
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
PDM on all frames.
PDMs constructed from consecutive 1/6 of the frames.
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Catching Line (CL)
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Character moving areas.
Selected frames and characters by “Catching line”.
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Color Selection (CS)
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo
Breaking an Animated CAPTCHA Scheme
Characters separated based on color.
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Pre-Processing on extracted single image
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Noise removal.
Refine by filling.
Shape removal.
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Character Recognition by OCR program
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Use ABBYY FineReader 11. Use existing embedded training database
and/or own training set .
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Experimental Results
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
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.
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Lessons learned
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
Delay periods:
40ms 40ms 1000ms
The number of frames.
Frame 45 Frame 82
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Lessons learned
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
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:
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Lessons learned
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme
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 ?
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Question ?
Vu Duc Nguyen, Yang-Wai Chow and Willy Susilo Breaking an Animated CAPTCHA
Scheme