Two basic articles on visual cryptography:
- Kumar, P., Agarwal, S., & Shivani, S. Halftone Visual Cryptography with Pixel Expansion through Error Diffusion. International Journal of Information & Computation Technology. ISSN, 0974-2239.
- Naor, Moni, and Adi Shamir. “Visual cryptography.” Advances in Cryptologyâ€”EUROCRYPT’94. Springer Berlin Heidelberg, 1995.
One interesting thing about this schema is that can be applied for stenography and can be simply implemented physically using only shadows.
See too: One Time Pad Using Gimp.