Wide-sense 2-frameproof codes.

Various kinds of fingerprinting codes and their related combinatorial structures are extensively studied for protecting copyrighted materials. This paper concentrates on one specialised fingerprinting code named wide-sense frameproof codes in order to prevent innocent users from being framed. Let Q be a finite alphabet of size q. Given a t-subset X = { x 1 , ... , x t } ⊆ Q n , a position i is called undetectable for X if the values of the words of X match in their ith position: x i 1 = ⋯ = x i t . The wide-sense descendant set of X is defined by wdesc (X) = { y ∈ Q n : y i = x i 1 , i ∈ U (X) } , where U(X) is the set of undetectable positions for X. A code C ⊆ Q n is called a wide-sense t-frameproof code if wdesc (X) ∩ C = X for all X ⊆ C with | X | ≤ t . The paper improves the upper bounds on the sizes of wide-sense 2-frameproof codes by applying techniques on non 2-covering Sperner families and intersecting families in extremal set theory. [ABSTRACT FROM AUTHOR]