proxy-based image verification
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Proxy-Based Image Verification
Thomas Kotwaltkotwal@mit.edu
MIT Laboratory for Computer Science
January 9, 2002
System Layout
• He wishes to communicate with his proxy.
• Bob has access to an untrustworthy computer.
• He can trust what he sees on the screen, because his camera equipped K21 is monitoring the screen content.
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
Security Protocol
1. U.C. displays an image and the K21 takes a picture
2. K21 sends picture + nonce + MAC to proxy via U.C. using IR link
3. Proxy verifies the picture and sends nonce + MAC to K21 via U.C. using IR link
Performing a Secure Request
4. U.C. displays the image and the K21 takes a picture of it
5. K21 sends picture + nonce + MAC to the proxy using the IR link
6. Proxy verifies the picture and sends nonce + MAC to the K21 using the IR link
1. User enters a request
2. U.C. sends the request to the proxy
3. Proxy sends an image summarizing the request to the U.C.
Image Verification ProblemIs this… …the same as this?
Obstacles:• Linear and non-linear distortions• Decreased resolution• Noise
Steps to solution• Undo distortions• Compare content
Step 1: Undo Image Distortiona) Undo lens distortion
– Model as radially symmetric distortion– Non-linear process
b) Undo linear distortions– Corrects for affine (scaling, rotation, translation) and
projection distortion– Requires four known points in distorted image
c) Undo other non-linear distortions– Corrects for curvature of screen, etc.– Least-squares with orthogonal polynomials– May not be necessary
a) Radially Symmetric Distortion(x,y)
r(u,v)
2
2
1
1
kr
yv
kr
xu
+=
+= 22 yxr +=
k : constant
b) Linear Distortion
(x,y)(u,v)
[ ] [ ]
=
333231
232221
131211
11
aaa
aaa
aaa
vuyx
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
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
Summary of Approach1. K21 takes a picture and uses its IR link to send the
picture to the proxy
2. Proxy corrects for image distortion and performs OCR
SELL: IBM300 SHARESTOTAL:$300
3. Proxy sends a confirmation back to the K21
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
Summary / Status
Q: How do you use a computer that you don’t trust?
Our Answer: Use visual authentication
• Pixel-Matching solution is running
• Proxy-based solution is under development
Questions?
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