A set of semi-fragile watermarksW_F\tau = {V1, V2,.., V_v} can be used as building blocks for constructing any digital fingerprint. One such family, is obtained by modulating the sign bits alone of selective DCT-AC coefficients, where, each V_j represents the positions of a disjoint subset of modulated coefficients. We show that a linear collusion of K \le n sign fingerprinted images neatly translates into a majority bit vote of K corresponding binary strings and in this binary output, '1' represents survival of a particular block V_j and '0' its erasure. By design, each block V_j is unperturbed by specific collusion patterns and so by facilitating complementary coverage of each newly added block V_j+1, a compact anti-collusion code (ACC) for tracking linear collusions can be constructed.