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Visual Cryptography is a symmetric key 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 encrypts the visual information to be sent at the sender in to two shares in (2, 2) model, in this two shares one act as a key for decryption and another as encrypted message (transparency), at the receiver end no computation is required it just required the stacking of the key and the encrypted message. The decryption can be performed by the human visual system. This technique can be extended for (k, n) model, gray level and for color images
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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
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