The Internet has become a popular communication network where the distribution of multimedia content, confidential data such as military information, financial documents, etc.
has become a common practice. But, over Internet the information is viewed to many users. Hence, now the security of visual information has become more and more important in many real applications. To fulfill such an increasing demand of security, many security providing tools are there in this scenario and Visual cryptography is one of them. This was introduced by Naor and Shamir in 1979 to provide confidentiality and security when secret visual information is transmitted through unsecured communication channels. Using secret sharing concepts, the encryption procedure encrypts a secret image into number of shares (printed on transparencies) which can be distributed among group members or transmitted or distributed over number of an untrusted communication channels such that only stacking of these shares can reveal the information otherwise not. Then the idea of secret sharing was also separately proposed by Adi Shamir and G. Blakley in 1979.
In 1983 another method of secret sharing was proposed by Asmuth and Bloom. Shamir’s scheme is based on Polynomial Interpolation. Blakley secret sharing is based on hyper plane geometry. Asmuth-Bloom secret sharing scheme is based on Chinese Remainder theorem.
Then there are many methods to protect sensitive data in image and Steganography is also one of them. This method hides a secret message in a innocent cover medium which could be a digital image,video, audio; etc. but, the weakness of this technique is that an entire secret data is kept in a single cover medium and if this cover medium get lost or corrupted then that hidden data may also get lost or corrupted. In 2004, Lin and Tsai proposed a method that used Steganography for generation of meaningful shares with secret image sharing. This scheme also used polynomial-based secret sharing approach proposed by Shamir which leads to high computational complexity. As the decryption process is done by human visual system, secret information can be retrieved by anyone if the person gets at least k number of shares. So that simple visual cryptography became very insecure 1.