VISUAL CRYPYTOGRAPHY USING (2,2) ENCODING SCHEME

SHASHIKIRAN B. S

OVERVIEW

INTRODUCTION ARCHITECTURE IMPLEMENTATION ADVANTAGES ISSUES APPLICATIONS EXTENSIONS CONCLUSION REFERENCES

INTRODUCTION

What is Cryptography ?

Plain Text Encryption Cipher Text

Plain Text Decryption Channel

TYPES OF CRYPTOGRAPHY:

SECRET SHARING

Divide data D into n shares D can be constructed from any k shares out of n Complete knowledge of k-1 shares reveals no

information about D Written (k, n): k of n shares is necessary to reveal secret

data.

SECRET SHARING EXAMPLE

6 thieves share a bank account They don’t trust one another They assume there will be no collusion between more

than 2 of them The thieves split up the password for the account in such a way that: Any 3 or more thieves working together can have access

to account, but NOT < 3.

VISUAL CRYPTOGRAPHY

What is Visual Cryptography ? Visual cryptography is a cryptographic technique which

allows visual information (pictures, text, etc.) to be encrypted in such a way that the decryption can be performed by the human visual system, without the aid of computers.

Visual cryptography was pioneered by Moni Naor and Adi Shamir in 1994

ARCHITECTURE

Plaintext (in form of image)

Encryption (creating shares)

Channel (Fax, Email)

Decryption (Human Visual System)

Plain text

Encryption

Key Encrypted Message

Decryption

Secret Image ( plain text )

Share1

Stacking the share reveals the secret

Share2

Encryption Decryption

IMPLEMENTATION2 out of 2 Scheme (2 subpixels)

A pixel P is split into two sub pixels in each of the two shares.• If P is white, then a coin toss is used to randomly choose one of the first two rows in the figure above.

• If P is black, then a coin toss is used to randomly choose

one of the last two rows in the figure above. Then the pixel P is encrypted as two sub pixels in each of

the two shares, as determined by the chosen row in the figure. Every pixel is encrypted using a new coin toss.

Now let's consider what happens when we superimpose the two shares (here we refer to the last column of the figure). Consider one pixel P in the image.

• If P is black, then we get two black sub pixels when we superimpose the two shares;

• If P is white, then we get one black sub pixel and one white sub pixel when we superimpose the two shares.

Thus, we could say that the reconstructed pixel (consisting of two sub pixels) has a grey level of 1 if P is black, and a grey level of 1/2 if P is white. There will be a 50% loss of contrast in the reconstructed image, but it should still be visible

COMPUTER REPRESENTATION OF PIXELS

Visual Cryptography scheme represented in computer using n x m Basis matrices.

Original Pixel

share1 s1= s0=

share2

overlaid Image

2 out of 2 Scheme (2 subpixels)

Example:

2 out of 2 Scheme (2 subpixels)

The two subpixels per pixel variant can distort the aspect ratio of the original image

+

2 OUT OF 2 SCHEME (4 SUBPIXELS)

Two-level plaintext image (black/white) Each pixel encoded as

a 2x2 cell in two shares (key and cipher)

Each share has 2 black, 2 transparent subpixels When stacked, shares combine to

Solid black Half black (seen as gray)

2 OUT OF 2 SCHEME (4 SUBPIXELS)

Choose random cell in key (6 choices)

Choose complementary (for black) or matching (for white) cell in cipher

Key conveys no information by itself Neither does cipher To view, stack and align

2 out of 2 Scheme (4 subpixels)

Horizontal shares Vertical shares Diagonal shares

2 out of 2 Scheme (4 subpixels)

2 out of 2 Scheme (4 subpixels)

share1

share2

stack

pixel

4

1

0

5

random

0 1 2 3 4 5 0 1 2 3 4 5

EXAMPLE :

ADVANTAGES

Simple to implement Encryption don’t required any NP-Hard problem dependency Decryption algorithm not required (Use a human Visual System).

So a person unknown to cryptography can decrypt the message. We can send cipher text through FAX or E-MAIL Infinite Computation Power can’t predict the message.

ISSUES

The encryption does require increased disk space , but with the falling cost of this resource and the option of compressing files, resource and the option of compressing files, this aspect of the approach is not significant.

The n, rather than 1, files (shares) are transmitted. This takes more time but in contrast to the other cryptographic algorithms that are computationally intense, this time is minimal.

APPLICATIONS

Bank customer identification Bank sends customer a set of transparencies (key) in

advance Bank web site displays cipher Customer applies overlay, reads transaction key Customer enters transaction key

Verifiable Receipts in Electronic Voting Anti spam-bot measure

EXTENSIONS K of N secret sharing

N shares Any K combine to reveal message Any less can’t

Gray level Could encode three levels in 2x2 Can encode four levels in 3x3

Color

CONCLUSION Among various advantages of Visual Cryptography Schemes is the property that VCS decoding relies purely on human visual system, which leads to a lot of interesting applications in private and public sectors of our society. Visual Cryptography is used with short messages, therefore giving the cryptanalyst little to work with. As with any analysis techniques, having little cipher text inhibits the effectiveness of a technique being used to break an encryption .

Since Visual Cryptography uses short message, public keys can be encrypted using this method. Visual Cryptography has proved that security can be attained with even simple encryption schemes.

REFERENCES Naor and Shamir, Visual Cryptography, in Advances in

Cryptology - Eurocrypt ‘94

The Visual Cryptography Projectwww.esat.kuleuven.ac.be/~rijmen/vc/

http://www-sec.uni-regensburg.de/vc/

file:///D:/seminar/visual.html