Watermarking Scheme Capable of Resisting Sensitivity Attack Source: IEEE signal processing letters, vol. 14, no. 2, February. 2007, pp. 125-128 Author: Xinpeng Zhang and Shuozhong Wang Speaker: Shu-Chuan Ko Date: 2009.12.1

Outline Introduction Sensitivity attack Watermark detector Embedding algorithm Detection algorithm Security analysis Experimental results Conclusions

Introduction This letter proposes a watermarking scheme capable of defeating the sensitivity attack a novel tailor-made embedding algorithm a corresponding detection mechanism are designed to “mislead” the attackers: get a “fake” signal

Sensitivity attack(1/3) Attacker possesses a watermarked image and an available detector Remove or change the embedded without causing serious distortion Original image Watermarked image Sensitivity attack  watermark detector Watermark embedding Watermark Not watermarked

Sensitivity attack(2/3) Decision boundary : between the “Watermarked” and “Not watermarked” regions is a hyper-plane in a multidimensional space. Not watermarked Watermarked Decision boundary

Sensitivity attack(3/3) : perpendicular to the decision boundary : shows how sensitive the detector is to modification in the direction of each vector : is either +1 or -1 indicates addition or subtraction Subtracts from the watermarked copy with an increasing strength until the detector reports that no watermark is present. Thus, the embedded watermark is removed.

Public watermark detector Output 1 (Watermarked) > Threshold Detection function F ≤ Threshold Test image Output 0 (Not watermarked) Black box Watermark detector : provide adequate information about an embedded watermark

Embedding algorithm(1/6) C2,3 C2,0 C1,0 C0,0 C2,2 C2,1 C1,2 C1,1 C0,2 C0,1 DWT 3-levels IDWT Watermarked image Watermark

Embedding algorithm(2/6)

Embedding algorithm(3/6)

Embedding algorithm(4/6) Generate( )data-groups in a pseudo-random manner The number of elements in each data group is equal to that of the host DWT coefficients, and all elements in the groups : mutually independent and satisfy a standard Gaussian distribution with zero mean and unit standard deviation

Embedding algorithm(5/6) : modified DWT coefficients where All modified DWT coefficients are inversely transformed to yield a watermarked copy

Embedding algorithm(6/6) According to the central limit theorem is very close to its mean 1 All are approximately 0 Standard deviation of

Detection algorithm (1/2) If output “ Watermarked ” output “ Not watermarked ”

Detection algorithm (2/2) If test image contains watermark, all close to 0   If test image contains no watermark   M  189 avoid excessive distortion

Security analysis Attacker does not know and is impossible to estimate by measuring the sensitivities of The detection function of the attacked signal always greater than 0 ( F > 0). This means the sensitivity attack cannot remove the embedded watermark.

Experimental results A 960 1280 still image captured Detection function F Original -1.7104 Watermarked 1.5104 A 960 1280 still image captured by a digital camera was used as the original test signal. The system parameter M=189

Conclusions The proposed watermarking scheme is capable of defeating the sensitivity attack. The corresponding detection mechanism can mislead the attackers. The output of detector cannot be used to remove watermark with low distortion.