To improve the security of watermark

a three-dimensional (3D) object was employed as the watermark to embed in a host

and a new technique of multiple watermark embedding was proposed based on Computer-generated Hologram (CGH). The tomography was used to obtain the Fresnel hologram of the 3D object. Then

the Arnold transformation was used to encrypt the hologram by scrambling the image

and the scrambled image was taken as the watermark to be embedded in. Furthermore

the Discrete Cosine Transformation (DCT) was chosen as the embedding and extracting algorithm and the Arnold transformation times and the different diffraction distances of every layer were regarded as encryption keys. Numerical and optical experimental results show when the keys are all correct

the reconstructed result of hologram extracted from the watermark is accord with that direct reconstructed hologram

and can display the different information at different layers of the 3D object ; otherwise

nothing useful in for mation could be distinguished when the keys are all incorrect. These results prove the effectiveness and security of proposed method

and show that the method can allow the watermark to be three-dimensional and multiplicity.