Cryptanalysis of Correlation-Based Watermarking Schemes Using Single Watermarked Copy Author: Tanmoy Kanti Das and Subhamoy Maitra From IEEE SIGNAL PEOCESSING LETTERS, April 2004 Presented by 詹益誌 6/8/2004

Outline Introduction How to Get Convinced That the Attack is Successful. Exact Cryptoanalytic Attack. Experimental results. Conclusions

Introduction Most of the existing digital watermarking techniques are based on correlation between “some information stored in the watermarked copy” and “related information retrieved from attacked watermarked copy”. They show how to remove this correlation to mount a cipher text-only cryptanalytic attack on these watermarking schemes.

How to Get Convinced That the Attack is Successful Theorem 1: consider two datasets v 1,…,v t and u 1,…,u t that are uncorrelated.T he mean and standard deviation of the dataset are approximately and

How to Get Convinced That the Attack is Successful Corollary 1: consider two datasets v 1,…,v t and u 1,…,u t selected at random from a standard normal distribution. The mean and standard deviation of the data u 1 -v 1,…,u t -v t are approximately

How to Get Convinced That the Attack is Successful Neither I d nor s (i) is known to the attacker, but some knowledge about statistical distribution of s (i) is known.

Exact Cryptoanalytic Attack 1.A single watermarked copy I (i) is available. Push I (i) in a stack ST of image. 2.Take the topmost image from the stack ST and consider it as I #. 3.The maximum t values of I d # are identified. DCT polynomial are formed. 4.The coefficients of the DCT polynomial are changed in a small range to create a population of several DCT polynomials and respective images are considered.

Exact Cryptoanalytic Attack 5.From the population, images are selected which are visually indistinguishable from I (i). Moreover, we analyze s (i,#) as mentioned above. If the mean and standard deviation of s (I,#) is close to 0.1 and, respectively, then we select I d # as an attacked one. 6.If required number of images are available, the terminate; otherwise go to step 2.

Experimental results

Experimental results

Conclusions Support the theoretical concepts based on statistical criteria.