Embedding and detecting spread-spectrum watermarks under estimation attacks

Spread-spectrum (SS) watermarking has been one of the oldest methodologies for hiding data in multimedia signals. Recently, it has been demonstrated that simple block repetition codes provide strong robustness of such watermarks to noise addition and limited arbitrary geometric transformations. In order to further investigate the security of such watermarks, we explore in this paper their robustness with respect to watermark estimation attacks. In such attacks, the adversary has knowledge of the watermark algorithm, except the secret keys. We present a modification of the traditional SS watermark detector that forces the adversary to increase the amount of noise to be proportional to the signal amplitude, in order to remove an SS watermark.