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Visual Cryptography

Visual cryptography is a popular solution for image encryption. Using secret sharing concepts, the encryption procedure encrypts a secret image into the so-called shares which are noise-like secure images which can be transmitted or distributed over an untrusted communication channel. Using the properties of the human visual system to force the recognition of a secret message from overlapping shares, the secret image is decrypted without additional computations and any knowledge of cryptography. 

Visual cryptographic solutions operate on binary or binarized inputs. Therefore, natural (continuous-tone) images must be first converted into halftone images by using the density of the net dots to simulate the original gray or color levels in the target binary representation. Then, the halftone version of the input image is used instead of the original secret image to produce the shares. The decrypted image is obtained by stacking the shares together. Because binary data can be displayed either as frosted or transparent when printed on transparencies or viewed on the screen, overlapping shares that contain seemingly random information can reveal the secret image without additional computations or any knowledge of cryptographic keys. However, due to the nature of the algorithm, the decrypted image is darker, contains a number of visual impairments, and most of visual cryptography solutions increase the spatial resolution of the secret image. In addition, the requirement for inputs of the binary or dithered nature only limits the applicability of visual cryptography.

 

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  Secret image   Halftone   Share 1   Share 2   Decrypted image  

Most of the existing secret sharing schemes are generalized within the so-called {k,n}-threshold framework that confidentially divides the content of a secret message into n shares in the way that requires the presence of at least k, for kn, shares for the secret message reconstruction, Thus, the framework can use any of n!/(k!(nk)!) possible combinations of k shares to recover the secret message, whereas the use of k−1 or less shares should not reveal the secret message.

 

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  Secret image   Halftone   Share 1   Share 2   Decrypted image  

References:

 
bullet R. Lukac and K.N. Plataniotis, Secure Color Imaging. In Color Image Processing: Methods and Applications, (eds.) R. Lukac and K.N. Plataniotis, CRC Press / Taylor & Francis, pp. 185-202, October 2006. ISBN 0-8493-9774-X
bulletR. Lukac, K.N. Plataniotis, and C.N. Yang, Image Secret Sharing. In Encyclopedia of Multimedia, (ed.) B. Furth, Springer-Verlag, pp. 328-335, January 2006. Including short articles: Compression in Image Secret Sharing (pp. 86-87), Halftoning-Based Visual Secret Sharing (pp. 363-364), Image Watermarking Using Visual Cryptography (pp. 337-338), Private-Key Cryptosystem (pp. 712-714), Threshold Schemes with Minimum Pixel Expansion (pp. 849-850), Visual Cryptography (pp. 950-951). ISBN 0-387-24395-X
bulletR. Lukac and K.N. Plataniotis, "Bit-Level Based Secret Sharing for Image Encryption," Pattern Recognition, vol. 38, no. 5, pp. 767-772, May 2005.
bulletR. Lukac and K.N. Plataniotis, "Image Representation Based Secret Sharing," Communications of the CCISA (Chinese Cryptology & Information Security Association), Special Issue on Image Secret Sharing and Quantum Cryptography, vol. 11, no. 2, pp. 103-114, April 2005.

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Last update: 10/15/06

2006 Rastislav Lukac