Visual cryptography is a new secure technology that distributes a secret image by separating it into a number of shares using different visual cryptography schemes (VCS). Early VCS schemes mainly focused on black-and-white secret images. Using a black & white VCS normally degrade the image quality and also suffered from a major drawback of pixel expansion, which means that the size of each secret share is several times larger than that of the original image. The black & white VCS were extended to the colour VCS. The main problem with the colour VCS is larger pixel expansion when number of colours increases. So in order to avoid these drawbacks a scheme called Black&White-Colour Visual Cryptography Scheme (B&W-C VCS) is used here. This scheme exploits the colour model to split a b & w image into coloured shares using a smaller pixel expansion and improved contrast. For any of the VCS, the main issue to be considered is authentication, i. e. , there are no means by which one can ensure that the secret recovered is genuine. It would be advantageous to check the fidelity of the shares before they are used to reconstruct the secret. For this purpose, this paper proposes a watermark method to authenticate coloured shares thereby ensuring a successful secret recovery. Unlike other watermarking techniques, the pattern is not directly embedded into the share, but an authentication matrix is generated which checks the authenticity of the shares before they are used for secret recovery.

1. M. Naor, A. Shamir," Visual cryptography", in: EUROCRYPT'94, LNCS, vol. 950, Springer-Verlag, 1995,pp. 1–12
2. R. De Prisco and A. De Santis, "Color Visual Cryptography Schemes for Black and White Secret Images", Theoretical Computer Science, vol. 510, Elsevier, 28 October 2013, pp. 63- 86.
3. R. -H. Hwang, "A Digital Image Copyright Protection Scheme Based on Visual cryptography," Tamkang Journal of science and Engineering, Vol. 3, No. 2, 2002, pp. 97-106.
4. Adel Hammad Abusitta " A Visual Cryptography Based Digital Image Copyright Protection" J. Information Security, 2012,vol. 3, 96-104
5. S. Cimato, R. D. Prisco, and A. D. Santis, "Colored visual cryptography without color darkening",Theoretical Computer Science, Elsevier, 374:261–276, 2007.
6. G. Ateniese, C. Blundo, A. D. Santis, and D. R. Stinson,"Visual cryptography for general access structures",Inf. Comput. , Elsevier ,129(2):86–106, 1996.
7. A. Adhikari, T. K. Dutta, and B. Roy,"A new black and white visual cryptographic scheme for general access structures", In Progress in Cryptology – INDOCRYPT2004, pages 399–413, 2004. Lecture Notes in Computer Science, Vol. 3348.
8. C. Blundo and A. D. Santis,"Visual cryptography schemes with perfect reconstruction of black pixels", Computers and Graphics, Elsevier, 22(4):449–455, August 1998.
9. P. A. Eisen and D. R. Stinson ,"Threshold visual cryptography schemes with specifiedwhiteness levels of reconstructed pixels",Des. Codes Cryptography, Elsevier, 25(1):15–61, 2002.
10. E. Verheul and H. van Tilborg ,"Construction and properties of k out of n visual secret sharing schemes", Designs Codes Cryptogr, Elsevier, (11):179–196, 1997.
11. Y. C. Hou and S. F. Tu. ,"A visual cryptographic technique for chromatic images using multi-pixel encoding method", Journal of Research and Practice in Information Technology, 37(2):179–191, May 2005.
12. J. Weir and W. Q . Yan, "Sharing Multiple Secrets Using Visual Cryptography", Proceedings of the IEEE International Symposium on Circuits and Systems, Taipei,24-27 May 2009,pp. 509-512 .
13. C. N. Yang, C. A. Laih, "New colored visual secret sharing schemes", Des. Codes Cryptogr. , Elsevier ,20 (2000)325-335
14. Mahmoud A Hassan and Mohammed A Khalili, "Self Watermarking based on Visual Cryptography", Proceedings of World Academy of Science, Engineering and Technology 8:159-162, October 2005.
15. Young-Chang Hou and Pei-Min Chen, "An Asymmetric Watermarking Scheme based on Visual Cryptography", WCCC-ICSP 5th International Conference on Signal Processing Proceedings, 2:992-995, 2000

Secret Sharing Visual Cryptography Threshold Schemes B&w Secret Images Color Shares watermarking