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# Visual cryptography

 ✅ Paper Type: Free Essay ✅ Subject: It Research ✅ Wordcount: 2469 words ✅ Published: 1st Jan 2015

### Introduction

One of the popular solutions for image encryption is visual cryptography. Secret sharing concepts are used to encrypt a secret image into the shares (customized versions of the original image) which can be transmitted over an undeveloped communication channel and by overlapping shares, the secret message is obtained

Visual cryptographic solutions are based on binary or binaries inputs. Due to this, natural images must be first transformed into halftone (this is a technique which uses different density of dots to display the image) images to simulate the original gray or colour levels in the target binary image

To create the shares, the halftone version of the input image is used instead of the original image. The decrypted image is made by stacking the shares together. Because binary data can be displayed in an apparent form when printed on transparencies or viewed on the screen and overlapping shares that contain apparently random information can expose the secret image. However, the decrypted image is darker, contains a number of visual damages due to the algorithm that is used for the process and the resolution of secret image is increased by most of visual cryptography solutions (Lukac at el,2005) and(Lukac 2006).

### 1.1 Objectives

The base of protected communication among users using insecure channel is known as cryptography. In order for cryptographic security to take place, an algorithm is involved for the practice of encrypting and decrypting messages

In its basic form, one image is split into two separate images called shares. When the shares are stacked on top of each other, the unique image is retrieved

A general n out of n visual cryptography scheme involves n participants in which a secret image is split into n random looking shares and all n shares are required to recover the original image.

### 1.2 Aims

The important idea behind Visual Cryptography is to encrypt information in the form of images in a secure and flawless way

The information must be encrypted before it is sent, so that it cannot be illegally read or modified in the middle of transmission. Suppose 4 persons have deposited their money in a bank account. These persons obviously do not trust each other. In particular, they do not want a single member of themselves to withdraw the money. However, they assume that withdrawing money by two members of the group is not considered a conspiracy; rather it is considered to have received authorizations

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Therefore, they decided to set the bank code (with a trusted computer) into 4 partitions so that any two or more partitions can be used to reconstruct the code. Since the person’s representatives will not have a computer with them to decode the bank code when they come to withdraw the money, they want to be able to decode visually: each thief gets a transparency

The transparency should yield no information about the bank code (even implicitly). However, by taking any two transparencies, stacking them together and aligning them, the secret number should pop out. How can this be done? (Naor and Shamir, 1994) proposed a solution and introduced a simple but perfectly secure way that allows secret sharing without any cryptographic computation, which they termed as Visual Cryptography Scheme. The simplest Visual Cryptography Scheme is given by the following setup. A secret image consists of a collection of black and white pixels where each pixel is treated independently

To encode the secret, the original image is split into n customized versions (referred as shares). To decode the image, a subset S of those n shares is picked and display each of them onto a transparency

In this project, a new visual cryptography scheme is intended for shadow images. The proposed scheme will be flexible, easily implemented, and more secure for shadow images. Moreover, shadow image structure is augmented to reduce the chances for an attacker to guess the secret.

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