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New Attacks on Sari Image Authentication
System
Proceeding of SPIE
2004
Jinhai Wu1, Bin B. Zhu2, Shipeng Li, Fuzong Lin1State key Lab of Intelligent Technology and Systems, Beijing
2Microsoft Research Asia, Beijing
Reporter: YH Chung
Introduction
Image processing tools and widely available powerful computers have made image manipulations an easy task.
Checking an image’s authenticity becomes more and more important.
Sari System
Sari system means Self-Authentication and Recovery Image system, and it’s a widely studied image authentication schemes.
The SARI system exploits the fact that the same quantization table is applied to all the Discrete Cosine Transform blocks of an image in the JPEG.
Sari System
Check authenticity Parameters:
For the block pair p and q at the DCT frequency bin V.
Count
Measure the tolerance bound t
Histogram attack
When a histogram of DCT coefficients is stretched or shrunken, the relationship between any two DCT coefficients does not change.
Histogram attack steps: Collect all the DCT coefficient at a selected frequency bin
v. Calculate the histogram of these DCT coefficients.
For a JPEG compressed image, the histogram can b e the JPEG quantized integers of the DCT coefficients since any change will be quantized by the same JPEG quantization step.
Histogram attack
A fix to histogram attack To add the maximum and minimum DCT
coefficients of each selected frequency bin to the digital signature for an image.
It’s much more difficult to modify the histogram without changing any relationship of DCT coefficients and with the mean values of DCT values close enough to the mean values recorded in the digital signature.
Oracle attack
It is designed to find out the secret mapping function W.
In the SARI system , if two DCT coefficients from two blocks in the same pair at a selected frequency bin are modified by the same amount ,the corresponding feature bit does not change.
Two stage of oracle attack: Find the first pair of blocks Find the remaining pairs
Use the pair that previously found to ensure the sum of DCT values.
Oracle attack
Solution to fight oracle attack Making each pair has exactly one block in
common with one more pair. Shortcoming is increase the signature size.
Use two independent mapping functions W1 and W2 to generate two sets of the feature codes of the same size.