Danger of low-dimensional watermarking subspaces

The security issue has been neglected for a long time in digital watermarking. Recent results for video watermarking have pointed out that existing watermarking schemes were not secure i.e. hostile intelligence succeeds in removing the hidden watermarks. In particular, for a given secret key, many watermarking schemes embed watermarks which lie in the same low-dimensional subspace whatever the host data is. In this article, it will be shown that this subspace can be quite easily estimated with an efficient Principal Component Analysis (PCA). For storage convenience, an online Expectation-Maximization (EM) algorithm will be considered. Once this watermarking subspace has been estimated, an attacker only has to project incoming data onto the orthogonal of this subspace to remove the watermark.