ProxyBased Image Verification

1
Proxy-Based Image Verification

• Thomas Kotwal
• tkotwal_at_mit.edu
• MIT Laboratory for Computer Science
• January 9, 2002

2
System Layout

• Bob has access to an untrustworthy computer.

• He wishes to communicate with his proxy.

• He can trust what he sees on the screen, because
  his camera equipped K21 is monitoring the screen
  content.

3
Proxy-Based Approach

• Add an IR link between the K21 and untrustworthy
  computer
• This allows the K21 to send data (image) to the
  proxy
• Proxy can do large amounts of computation
• Proxy knows what the screen should display

4
Security Protocol
5
Performing a Secure Request
6
Image Verification Problem
Is this
the same as this?

• Steps to solution
• Undo distortions
• Compare content

• Obstacles
• Linear and non-linear distortions
• Decreased resolution
• Noise

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Step 1 Undo Image Distortion

• Undo lens distortion
• Model as radially symmetric distortion
• Non-linear process
• Undo linear distortions
• Corrects for affine (scaling, rotation,
  translation) and projection distortion
• Requires four known points in distorted image
• Undo other non-linear distortions
• Corrects for curvature of screen, etc.
• Least-squares with orthogonal polynomials
• May not be necessary

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a) Radially Symmetric Distortion
(x,y)
(u,v)
r
k constant
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b) Linear Distortion
(u,v)
(x,y)
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Step 2 Compare Content

• Assume content is text only
• Perform OCR on processed image
• Advantage proxy knows what the text should say
• To save computation time compare each character
  with what it should be, not every possible
  character
• Constrain font to facilitate OCR routine

11
Next Step Compare Images

• Generalize content to be anything, not just text
• Find areas of large difference between original
  and processed image
• Requires very accurate undistortion

12
Summary of Approach

• K21 takes a picture and uses its IR link to send
  the picture to the proxy

• Proxy corrects for image distortion and performs
  OCR

SELL IBM 300 SHARES TOTAL 300

• Proxy sends a confirmation back to the K21

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Why Proxy-based Approach?

• Advantages
• Proxy knows what the screen should display
• No image processing on the K21
• No calibration necessary
• Camera does not have to be immobile during
  session
• Translates easily to color
• Disadvantages
• Requires IR link between K21 and untrustworthy
  computer
• Possibly long verification time
• Image verification allows for possibility of error

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Summary / Status

• Q How do you use a computer that you dont
  trust?
• Our Answer Use visual authentication
• Pixel-Matching solution is running
• Proxy-based solution is under development

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Questions?