Method for protecting digital content against minority collusion attacks
View Patent ↗A method and system of detecting colluders conducting a collusion attack including a minority-type collusion attack on a digital product includes the generation of codewords used as watermarks in the digital product. The inner code of the codewords is generated using permutations of rows in a Hadamard matrix and concatenating them together. A typical outer code of the codeword is the Reed Solomon code. An adaptive detector is able to accurately detect one of three or more colluders of a minority-type attack. Prior art schemes using an error correcting code-based watermarking mechanism with an inner code fail to detect colluders with a minority-type collusion attack which includes three colluders.
1. A method performed by a computer system, the method comprising:
generating a square matrix whose rows are mutually orthogonal;
permuting rows of the square matrix to generate a plurality of permuted square matrices;
aggregating the plurality of permuted square matrices to produce aggregated rows; and
using the computer system, applying one of the aggregated rows as a codeword in a digital watermark to a digital product, wherein said digital watermark is immune to minority attacks of three colluders.
2. The method of claim 1 , further comprising:
displaying, using a computer system, one of the aggregated rows, wherein the displayed aggregated row represents a codeword for a digital watermark.
3. The method of claim 1 , further comprising:
substituting the codeword into an outer code of the digital watermark.
4. The method of claim 3 , further comprising:
randomly permuting bits of the codeword after the substituting step but before applying the digital watermark to a digital product.
5. The method of claim 3 , wherein the outer code is an Error Correcting Code.
6. The method of claim 5 , wherein the outer code is a Reed Solomon code.
7. The method of claim 1 , wherein the square matrix is a Hadamard matrix.
8. The method of claim 1 , wherein the permuting step is accomplished using one of a fixed number of permutations, an exhaustive number of permutations, and a random number of permutations.
9. The method of claim 1 , wherein permuting rows of the square matrix comprises randomly permuting the rows of the square matrix.
10. A method for detecting watermarking codewords, using a computing system, the method comprising:
extracting watermark information from a digital product;
inputting said extracted watermark information into an adaptive detector that switches between a hard detector and a soft detector; and,
using the computing system, generating detected watermarking codewords from the output of said adaptive detector, wherein said adaptive detector calculates a vague symbol count based on said input watermark information and switches to said soft detector if said vague symbol count is higher than a pre-determined threshold, and to said hard detector if said vague symbol count is lower than equal to a pre-determined threshold.
11. The method as in claim 10 , wherein said digital product is exposed to a collusion attack.
12. The method as in claim 10 , wherein said soft detector extracts codewords through correlation from said watermark information.
13. The method as in claim 10 , further comprising extracting suspicious symbols from said watermark information for each symbol position of the codeword using said hard detector.
14. The method as in claim 13 , further comprising
calculating the number of suspicious symbols for each codeword;
identifying one or more codewords having the largest number of suspicious symbols; and
outputting said identified one or more codewords.
15. The method as in claim 10 further comprising a metric statistic for user i at symbol j, T i (j) , which is a function of the difference between the detection statistic of user i and the median of the detection statistics of all users.
16. An apparatus for watermarking comprising a computer system configured to:
generate a square matrix whose rows are mutually orthogonal;
permute rows of the square matrix to generate a plurality of permuted square matrices;
aggregate the plurality of permuted square matrices to produce aggregated rows; and
apply one of the aggregated rows as a codeword in a digital watermark to a digital product, wherein said digital watermark is immune to minority attacks of three colluders.
17. The apparatus of claim 16 , further configured to:
display, one of the aggregated rows, wherein the displayed aggregated row represents a codeword for a digital watermark.
18. The apparatus of claim 16 , further configured to:
substitute the codeword into an outer code of the digital watermark.
19. The apparatus of claim 18 , wherein the outer code is an Error Correcting Code.
20. The apparatus of claim 19 , wherein the outer code is a Reed Solomon code.
21. The apparatus of claim 18 , further configured to:
randomly permute bits of the codeword after the substituting step but before applying the digital watermark to a digital product.
22. The apparatus of claim 16 , wherein the square matrix is a Hadamard matrix.
23. The apparatus of claim 16 , wherein it is configured to permute by using one of a fixed number of permutations, an exhaustive number of permutations, and a random number of permutations.
24. The apparatus of claim 16 , wherein permuting rows of the square matrix comprises randomly permuting the rows of the square matrix.
25. An apparatus for detecting watermarking codewords comprising a computing system configured to:
receive watermark information from a digital product;
input said extracted watermark information into an adaptive detector that switches between a hard detector and a soft detector; and,
generate detected watermarking codewords from the output of said adaptive detector, wherein said adaptive detector calculates a vague symbol count based on said input watermark information and switches to said soft detector if said vague symbol count is higher than a pre-determined threshold, and to said hard detector if said vague symbol count is lower than equal to a pre-determined threshold.
26. The apparatus of claim 25 , wherein said digital product is exposed to a collusion attack.
27. The apparatus of claim 25 , wherein said soft detector extracts codewords through correlation from said watermark information.
28. The apparatus of claim 25 , further configured to:
extract suspicious symbols from said watermark information for each symbol position of the codeword using said hard detector.
29. The apparatus of claim 28 , further configured to:
calculate the number of suspicious symbols for each codeword;
identify one or more codewords having the largest number of suspicious symbols; and
output said identified one or more codewords.
30. The apparatus of claim 25 further configured to:
applying a metric statistic for user i at symbol j, T i (j) , which is a function of the difference between the detection statistic of user i and the median of the detection statistics of all users.