Abstract
For the Tardos traitor tracing scheme, we show that by combining the symbol-symmetric accusation function of Škorić et al. with the improved analysis of Blayer and Tassa we get further improvements. Our construction gives codes that are up to four times shorter than Blayer and Tassa’s, and up to two times shorter than the codes from Škorić et al. Asymptotically, we achieve the theoretical optimal codelength for Tardos’ distribution function and the symmetric score function. For large coalitions, our codelengths are asymptotically about 4.93% of Tardos’ original codelengths, which also improves upon results from Nuida et al.
Highlights
Watermarking digital content allows distributors of copyrighted digital data to embed socalled fingerprints into their data in such a way that each copy of the data can be uniquely identified
Since by the Central Limit Theorem these accusation scores will converge to normal distributions for asymptotically large c, this provides a lower bound on the codelength, when using the arcsine distribution function and the symmetric score function
First we present the construction of the Tardos traitor tracing scheme, as in [6], where we use auxiliary variables d, dz, dδ for the codelength, accusation offset Z and cutoff parameter δ respectively
Summary
Watermarking digital content allows distributors of copyrighted digital data to embed socalled fingerprints into their data in such a way that each copy of the data can be uniquely identified. Assuming that besides the watermarks all copies are the same, this allows coalitions to detect part of the watermark By editing this data, they can create a forged copy, which contains the same digital content as their original copies, but has a forged fingerprint that cannot be traced back to them directly. After assigning codewords to users and distributing the watermarked copies, a subset C ⊆ U of c users (called colluders or pirates) may form a coalition to create a forged copy. After the coalition has created a forged copy, we assume the distributor intercepts it and extracts the forgery y from the data He runs some tracing algorithm σ on the forgery, to get a subset σ (y) ⊆ U of users that are accused. We sometimes say that a scheme is secure, to denote that it is sound and complete for certain (implicit) parameters ε1 and ε2
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