Resistance to ambiguity attack is an important requirement for a secure digital rights management (DRM) system. In this paper,we revisit the non-ambiguity of a blind watermarking based on the compu-tational indistinguishability between pseudo random sequence generator (PRSG) sequence ensemble and truly random sequence ensemble. Ambiguity attacker on a watermarking scheme,which uses a PRSG sequence as watermark,is viewed as an attacker who tries to attack a noisy PRSG sequence. We propose and prove the security theorem for binary noisy PRSG sequence and security theorem for gen-eral noisy PRSG sequence. It is shown that with the proper choice of the detection threshold Th = an1/2 (a is a normalized detection threshold; n is the length of a PRSG sequence) and n 1.39×m/a2 (m is the key length),the success probability of an ambiguity attack and the missed detection probability can both be made negligibly small thus non-ambiguity and robustness can be achieved simultaneously for both practical quantization-based and blind spread spectrum (SS) watermarking schemes. These analytical resolutions may be used in designing practical non-invertible watermarking schemes and measuring the non-ambiguity of the schemes.
