Today security is the main concern about the transmission of data.
We need a cryptosystem which ensures that our data will be secure during the transmission over the network. In secured communication the information is converted from the intelligible to unintelligible structure using certain coding operation at the transmitter. There are some techniques are used for making the data secure during conveying information over the network one of these are known as encryption and decryption.
The encrypted form of the information is then transmitted through the insecure channel to the destination 4.Previously, Naor and Shamir introduced secret sharing approach in 1979. Afterwards, Asmuth and Bloom proposed another secret sharing algorithm. Shamir’s secret sharing scheme is based on Lagrange’s Polynomial Interpolation theorem. This scheme divides a secret data image into n number of shares share1, share2…
…share n and these n shares are Xeroxed onto n transparencies, respectively, and distributed amongst n participants, one for each participant. Such that: i) By superimposing any k or more shares among share i transparencies together can reveal the secret information.
ii) Less than k shares reveals no information about the secret share. This technique is called (k, n) threshold secret sharing. The (k, n) secret sharing comes from the concept that k pointsare necessary to define a polynomial of degree (k?1) 5.
Blakley Secret sharing scheme is based on hyper plane geometry. It is known that non-parallel planes intersect at a single specific point. This secret sharing scheme says that: i) Secret is a single point in m-dimensional space. ii) Share corresponds to a hyper plane. Iii) Intersection of thresholdplanes gives the secret. iv) Less than threshold planes will not reveal secret 6.
In Asmuth-Bloom secret sharing scheme shares are created on the basis of Chinese Remainder theorem. In this, shares are generated by reduction modulo operation and the secret is recovered by solving the system of congruence using the Chinese Remainder Theorem. Previously, visual cryptography was restricted to binary images and because of this; it became inefficient in real time applications 7. Chang-ChouLin, Wen-Hsiang Tsai proposed visual cryptography for gray level images by dithering techniques. A dithering technique is used to convert gray level images into approximate binary images.
Then shares are created by applying existing visual cryptography schemes for binary images. The limitation in this is that all generated shares are random patterns carrying no visual information, look like noisy images. 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 8.
Halftone Visual Cryptography was proposed by Zhi Zhou, Gonzalo R. Arce, and Giovanni Di Crescenzo.Abstract Visual cryptography encodes a secret binary image (SI) into n shares of random binary patterns. If the shares are xeroxed onto transparencies, the secret image can be visually decoded by superimposing a qualified subset of transparencies, but no secret information can be obtained from the superposition of a forbidden subset. The binary patterns of the n shares, however, have no visual meaning and hinder the objectives of visual cryptography. Extended visual cryptography was proposed to construct meaningful binary images as shares using hypergraph colourings, but the visual quality is poor 9.
