The basic model was then extended to (k, n) threshold cryptography where any ‘k’ or more shares will reveal the secret image. The construction of out of ‘k’ ‘n’ visual secret sharing is similar to the basic model with one difference. That is in basic model the threshold value is n where 2.1.2 Visual cryptography for general access structures In (k,n) Basic model any shares will decode the secret image ‘k’ which reduces security level. To overcome this issue the basic model is extended to general access structures by G. Ateniese, C. Blundo, A. De Santis, and D. R. Stinson [2], where an access structure is a specification of all qualified and forbidden subsets of shares . Any subset of or more qualified shares can ‘n’ ‘k’ decrypt the secret image but no information can be obtained by stacking lesser number of qualified shares or by stacking disqualified shares. Construction of k out of n threshold visual cryptography scheme for general access structure is better with respect to pixel expansion than Basic Model 2.1.3 Visual cryptography for gray level images Previous efforts in visual cryptography were restricted to binary images which is insufficient in real time applications. Chang- ChouLin, proposed visual cryptography for gray level images by dithering techniques. Instead of using gray sub pixels directly to constructed shares, a dithering technique is used to convert gray level images into approximate binary images. Then existing visual cryptography schemes for binary images are applied to accomplish the work of creating shares. The effect of this scheme is still satisfactory in the aspects of increase in relative size and decoded image quality, even when the number of gray levels in the original image still reaches 256. 2.1.4 Recursive Threshold visual cryptography The (k,n) visual cryptography needs shares to reconstruct ‘k’ the secret image. Each share consists at most [1/k] bits of secrets. This approach suffers from inefficiency in terms of number of bits of secret conveyed per bit of
The basic model was then extended to (k, n) threshold cryptography where any ‘k’ or more shares will reveal the secret image. The construction of ‘k’ out of ‘n’ visual secret sharing is similar to the basic model with one difference. That is in basic model the threshold value is n where 2.1.2 Visual cryptography for general access structures In (k,n) Basic model any ‘k’ shares will decode the secret image which reduces security level. To overcome this issue the basic model is extended to general access structures by G. Ateniese, C. Blundo, A. De Santis, and D. R. Stinson [2], where an access structure is a specification of all qualified and forbidden subsets of ‘n’ shares . Any subset of ‘k’ or more qualified shares can decrypt the secret image but no information can be obtained by stacking lesser number of qualified shares or by stacking disqualified shares. Construction of k out of n threshold visual cryptography scheme for general access structure is better with respect to pixel expansion than Basic Model 2.1.3 Visual cryptography for gray level images Previous efforts in visual cryptography were restricted to binary images which is insufficient in real time applications. Chang- ChouLin, proposed visual cryptography for gray level images by dithering techniques. Instead of using gray sub pixels directly to constructed shares, a dithering technique is used to convert gray level images into approximate binary images. Then existing visual cryptography schemes for binary images are applied to accomplish the work of creating shares. The effect of this scheme is still satisfactory in the aspects of increase in relative size and decoded image quality, even when the number of gray levels in the original image still reaches 256. 2.1.4 Recursive Threshold visual cryptography The (k,n) visual cryptography needs ‘k’ shares to reconstruct the secret image. Each share consists at most [1/k] bits of secrets. This approach suffers from inefficiency in terms of number of bits of secret conveyed per bit of
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