Analysis and design of tamperproof and contrastenhanced secret sharing based on visual cryptography schemes

Abstract

The concept of secret sharing was independently introduced by Shamir and Blakley in 1979. Secret sharing becomes indispensable whenever secret information needs to be kept collectively by a group of participants in such a way that only a qualified subgroup is able to reconstruct the secret. An example of such a scheme is a k-out-of-n threshold secret sharing, in which there are n participants holding their shares of the secret and every k (k and#8804; n) participants can collectively recreate the secret, while k-1 participants cannot get any information about the secret. The need for secret sharing arises if the storage system is not reliable and secure. Secret sharing is also useful if the owner of the secret does not trust any single person. This concept was first applied to numbers; but in 1990s, researchers extended it to images. Visual cryptography implements secret sharing of images. In 1995, Naor and Shamir invented a new type of secret sharing scheme, called visual cryptography (VC). Visual cryptography is a kind of secret image sharing scheme that uses the human visual system to perform the decryption computation. A visual cryptography scheme (VCS) allows confidential messages to be encrypted into k-out-of-n secret sharing schemes. VC is interesting because decryption can be done with no prior knowledge of cryptography and can be performed without any cryptographic computations. Visual cryptography scheme eliminates complex computation problems in the decryption process and the secret images can be restored by stacking operation. This property makes visual cryptography useful for low computation requirements. Contrast is one of the most important parameters of visual cryptography schemes. Usually, the reconstructed secret image will be darker than the original secret image. This thesis presents three different methods to improve the contrast of visual cryptography schemes.