We propose an optical hiding method based on visual cryptography. In the hiding process, we convert the secret information into a set of fabricated phase-keys, which are completely independent of each other, intensity-detected-proof and image-covered, leading to the high security. During the extraction process, the covered phase-keys are illuminated with laser beams and then incoherently superimposed to extract the hidden information directly by human vision, without complicated optical implementations and any additional computation, resulting in the convenience of extraction. Also, the phase-keys are manufactured as the diffractive optical elements that are robust to the attacks, such as the blocking and the phase-noise. Optical experiments verify that the high security, the easy extraction and the strong robustness are all obtainable in the visual-cryptography-based optical hiding.

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We propose to realize visual cryptography in an indirect way with the help of diffractive optics, the pure-amplitude keys being substituted with the phase-only keys that are invisible for common-used intensity detectors, leading to the significantly enhanced security and improved usability for practice. Three typical realizations are provided by using static or dynamic diffractive optical elements, and the new concept of invisible visual cryptography (IVC) may be established. This concept is demonstrated by a compact opto-electronic system with only one spatial light modulator within time-delayed exposure, maintaining the fast speed of decryption and no requirement to the calculation of decryption in conventional visual cryptography. Further, IVC shows the immunity to noises and good information capacity, which is partly inherited from diffractive optics. IVC might suggest a new strategy for visual cryptography with the respective of diverse realizations, such as opto-electronic or even bio-chemical or other methods.

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