1. A method for designing a 3D modeled object comprising the steps of: by a processor: providing at least one original surface of the 3D modeled object defined by a field of one or more points of the surface and an original field of normal vectors normal to the original surface; modifying the original surface by modifying a geometry of the field of one or more points such that at least one of the modified points is not on the original surface, thereby obtaining a modified surface characterized by a modified set of normal vectors normal to the modified surface; and creating a watermarked version of the 3D modeled object defined by the modified surface and the original field of normal vectors.