Digital watermarking studies have always been driven by the improvement of robustness. Most of articles of this field deal with this criterion, presenting more and more impressive experimental assessments. Some key events in this quest are the use of spread spectrum, the invention of resynchronization schemes, the discovery of side information channel, and the formulation of the opponent actions as a game. On the contrary, security received little attention in the watermarking community. This paper presents a comprehensive overview of this recent topic. We list the typical applications which requires a secure watermarking technique. For each context, a threat analysis is purposed. This presentation allows us to illustrate all the certainties the community has on the sub ject, browsing all key papers. The end of the paper is devoted to what remains not clear, intuitions and future studies.