Steganalysis of Reversible Contrast Mapping Watermarking

Steganalysis of Reversible Contrast Mapping Watermarking Authors: Yeh-Shun Chen and Ran-Zan Wang Source: IEEE Signal Processing Letters, Vol. 16, No. 2, February 2009, pp. 125-128Reporter: Chia-Chun Wu (吳佳駿) Date: 2010/08/27

Outline • 1. Reversible Watermark • 2. Reversible Contrast Mapping • 3. Embedding • 4. Detection and Recovering • 5. Steganalysis • 6. Experimental results • 7. Conclusions

1. Reversible Watermark (1/4) Cover extraction Watermark Stego embedding Watermark Irreversible Watermarking Cover extraction Watermark Stego embedding Cover Watermark recover Reversible Watermarking

Internet 1. Reversible Watermark (2/4) • Irreversible Watermarking Stego image Cover image Watermark Watermark

1. Reversible Watermark (3/4) • Irreversible Watermarking Watermark Cover image xi xi+1 mi mi+1 mi mi+1 yi yi+1

Internet 1. Reversible Watermark (4/4) • Reversible Watermarking Stego image Cover image Cover image Watermark Watermark

Very Fast Watermarking by Reversible Contrast Mapping, Dinu Coltuc and Jean-Marc Chassery, IEEE Signal Processing Letters, Vol. 14, No. 4, April 2007, pp. 255-258. 2. Reversible Contrast Mapping (1/2) • RCM is a simple integer transform • RCM applies to pairs of pixels • Let be a pair of pixels in original image • Let be a pair of pixels in stego image • Let [0, L] be image graylevel range (ie. L = 255) • is the ceil function (the smallest integer greater than or equal to )

2. Reversible Contrast Mapping (2/2) • The forward RCM : • The inverse RCM : X’ and Y’ are both odd only if X and Y are both odd • If X and Y are not both odd • then inverse correct even if LSB bit set to 0

2. Reversible Contrast Mapping (2/2) • The forward RCM : • The inverse RCM : (7, 4)  (10, 1) (7, 9)  (5, 11) (10, 1)  (7, 4) (5, 11)  (7, 9) (10, 0)  (7, 4)

3. Embedding (1/2) Step1: Partition the image into pairs of pixels. Step2: For each pair Case 1: If and are not both odd pixel values. - transform the pair by the forward RCM - set the LSB of to “1” - embed data into the LSB of Case 2: If and are both odd pixel values. - transform the pair by the forward RCM - set the LSB of to “0” - embed data into the LSB of Case 3: If - set the LSB of to “0” and save the true value

Case 1 Case 2 10 12 11 13 embed “0” embed “0” embed “1” embed “1” Case 1 Case 3 10 13 11 21 embed “0” embed “1” save the LSB of X

4. Detection and Recovering (1/2) Step1: Partition the image into pairs of pixels. Step2: For each pair Case 1: If the LSB of is “1” - extract the LSB of - set the LSBs of to “0” - transform the inverse RCM Case 2: If the LSB of is “0” and belongs to - extract the LSB of - set the LSBs of to “1” - transform the inverse RCM

4. Detection and Recovering (2/2) Case 3: If the LSB of is “0” and the pair with the LSBs set to “1” does not belong to - replacing the LSB of with the corresponding true value extracted from the watermark sequence

5. Steganalysis (1/2) Case 1: If and are not both odd pixel values. - set the LSB of to “1” - embed data into the LSB of Case 2: If and are both odd pixel values. - set the LSB of to “0” - embed data into the LSB of Case 3: If - set the LSB of to “0” and save the true value LSB(X, Y) = (0, 0), (0, 1) and (1, 0)  LSB(X', Y') = (1, 1) or (1, 0) LSB(X, Y) = (1, 1)  LSB(X', Y') = (0, 0) or (0, 1) LSB(X, Y) = (0, 0), (0, 1) , (1, 0) and (1, 1)  LSB(X', Y') = (0, 0) or (0, 1) 14

5. Steganalysis (2/2) 15

Mean value of the PLSB(1)-PLSB(0): -0.013306 Standard derivation of the PLSB(1)-PLSB(0): 0.03736 6. Experimental results (1/3) Fig. 1. Distribution of the PLSB(1)-PLSB(0) values of RCM watermarking stego-images for various embedding ratios. (962 test images)

6. Experimental results (2/3) Fig. 2. Detection result of the proposed method for various threshold values. (1516 test images)

5. Experimental results (3/3) Table I Mean Estimated Embedding Ratios of Hidden Messages

6. Conclusions • This paper presented a simple method for cracking RCM watermarking.

Steganalysis of Reversible Contrast Mapping Watermarking