Using electronic watermarks as copyright protection for still images requires robustness against geometrical attacks. In this paper we propose a watermarking scheme that is robust to rotation and scaling distortions. The watermark detection is performed in a 1-D invariant signature whereas the embedding process is performed adding a watermark signal in the DFT domain. This embedding procedure allows the watermarking signal to be shaped in the frequency domain. This shaping is determined solving a game opposing the watermarker and the attacker. Statistically significant roc curve test results under several attacks are presented.