VISUAL CRYPTOGRAPHY

[summer project pal]

Visual Cryptographic Schemes
A Seminar Report
by
Jerry Vijayan
M105109
Department of Computer Science & Engineering
College of Engineering Trivandrum
Kerala - 695016
2010-11

Abstract
Visual cryptographic scheme (VCS) is a kind of cryptographic scheme which allows the
encryption of text documents and text images. The secret image is encrypted into n shares and
the decryption of the secret image requires neither the knowledge of cryptography nor complex
computation. Someone with all the n shares could decrypt the message by overlapping each
of the shares over one another. There are mainly two approaches to visual cryptography
as described by Naor and Shamir, namely (k; n) Threshold VCS and (n; n) Threshold VCS.
They also extended their model for continuous toned images and also to Extended Visual
cryptographic schemes where the existence of secret message is hidden by making each share
look as though a meaningful image. Later, Colour visual cryptography became an interesting
research topic after the formal introduction of visual cryptography by Moni Naor and Adi
Shamir in 1994.

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1 Introduction
Visual cryptography deals with the encryption of material, such as, images, documents, etc.
in a manner such that it is easy for a user to decipher the secret message by using the human
visual system, i.e., eyes. The basic model consists of a printed page of the cipher text and a
printed transparency. The original message can be revealed by placing the transparency over
the cipher text page. The simplicity of the system allows anyone without any knowledge of
cryptography or complex decrypting methods to use it very easily. This basic method of visual
cryptography is similar to the use of a One-time Pad, i.e., each cipher page is decrypted with
a dierent transparency. Another advantage of this method is the ability to encrypt several
secret messages into a single cipher text.
Several models can be used to implement visual crytography. A few have been discussed in
this paper. These include :
1. Basic Model
2. Extended Visual Cryptographic Model
3. Half-tone Visual Cryptographic Model
4. Colour Visual Cryptographic Model
5
2 Basic Model
This simple model assumes that the message consists of a collection of black & white pix-
els, each of which are handled separately. Each pixel in the original image appears in n
shares/modied versions, called transparencies. Each share or transparency is a collection
of m black & white sub-pixels. The resulting structure is dened by an n  m Boolean matrix
Si = [sij ];where sij = 1 if the jth subpixel in the ith transparency is black.
The transparencies are stacked together to get the combined share, where the subpixels are
represented as the Boolean OR of the rows. The grey level of the combined shares is directly
proportional to the Hamming weight of the OR-ed vector V . It is interpreted as black if
H(V )  d & as white if H(V )  d