Guilloches (preventing modification) and watermarking (preventing substitution) technologies are combined to protect a medium, such as a card or a passport. The major idea stems from their dual role for medium authentication: the presence of one is needed to check the validity of the other one. By nature, the guilloche network is visible in the image and easily detectable by image processing. This network is therefore used to generate reference points as elements of synchronization to retrieve watermark information originally embedded in the medium that has submitted an RST transformation. These reference points are by nature stronger than any other means involving invisible templates in the spatial or Fourier space.