A Novel Key Distribution Scheme f or a Visual Crypto System

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 03 | Mar -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 2537
A Novel Key Distribution Scheme for a Visual Crypto System
Kalyan Das1, Aromita Sen2
1Department of Information Technology, St. Thomas College of Engineering and Technology, Kolkata.India
2Department of Computer Science and Engineering, St. Thomas College of Engineering and Technology Kolkata
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Abstract - With the high speed advancement of Computer
network and internet, has hiked the chance of data snooped
during the time of transmission. In this issue, Cryptography
plays a vital role, which hides the actual information using
some secret key and thereby converts it into an alternative
equivalent multimedia file like image, video or audio in such a
way that only the intended recipient can retrieve back the
original data. Here in this paper, a novel secret sharing
technique has been suggested, which comprises of two main
parts. The first part for establishing shared secret key (one
time key), using asymmetric key cryptography and then
transmit the confidential data using that shared secret key,
ensuring the confidentiality and integrity. Here for the
encoding purpose Modulus Operation is used to increase the
difficulty for the inverse function.
Key Words: Visual Cryptography, Encryption, Decryption,
Key Distribution, Pixel, Data Security, Cryptanalysis
1. INTRODUCTION
Cryptography is a Greek word. It literally means hidden
writing it's the science and the art of making secret codes.
Encryption is the process oftakinga messagetransformingit
by replacing letters and changing it around until it looks like
something totally not like a message that nobody can read
you can send it to your destinationandthentheycandecrypt
it and get back the original data or the message that was
there. Until a few decades ago, the information wascollected
by an organization and stored on physical files. But now,
information storage become electronic and thus
implementation of their confidentiality, integrity and
availability become more different and challenging.
To deal with all those security issues, various image secret
sharing schemes have been developed which gave rise to
new technologies in the area of Image Cryptography
requiring less computation andlessstorage. Cryptography,a
word with Greek origin, means “secret writing”. It refers to
encryption and decryption of secret messages using secret
keys.
VCS is a cryptographic technique that allows for the
encryption of visual information such thatdecryptioncan be
performed using the human visual system. We can achieve
this by one of the following access structure schemes.
• (2, 2) Threshold VCS scheme-Thisisthesimplestthreshold
scheme that encrypts a secret message in two different
shares such that it reveals the secret image when they are
overlaid.
• (n, n) Threshold VCS scheme- In this scheme the secret
message is encrypted into n shares, such that when all of the
n shares are combined, it will reveal the secret message.
• (k, n) Threshold VCS scheme- This scheme encrypts the
secret image into n shares such that when any group of at
least k shares are overlaid the secret image will be revealed.
The question on key exchange was one of the first problems
addressed by a cryptographic protocol that was prior to the
invention of Public Key Cryptography. In case of Symmetric
Key Cryptography, the point is to agree on a key that two
parties can use for encryption, in such a way that the
eavesdropper cannot attain the key.
Our prime objective is to suggest a strong protected
transmission technique which ensures secure potential
message transmission, from one end to another over a local
area network, using image processing based cryptography
techniques.
1.1 Preliminaries
•Cryptography: Cryptography is the study and practice of
protecting information by data encoding and transformation
techniques. The primary goal of cryptography is to conceal
data to protect it against unauthorized third-party access by
applying encryption. The more theoretical or mathematical
effort is required for an unauthorized third party to recover
data, the stronger is the encryption.
•Encryption: In cryptography, encryption is the process of
transforming information (referred to as plaintext) using an
algorithm (called cipher) to make it unreadable to anyone
except those possessing special knowledge, usually referred
to as a key.
•Cipher: An encrypted message is known as cipher text.
•Decryption: Decryption is the inverse of encryption; the
process of taking cipher text (encrypted data) and a
cryptographic key, andproducingcleartext(theoriginalnon-
encrypted data)
•Symmetric Key Cryptography: It is also referred to by
another term like: Secret Key Cryptography. In this scheme,
only one key is used and the same key is used for both
encryption and decryption of messages. Obviously, both the

parties must agree upon the key before any transmission
begins and nobody else should know about it.
Basically, at the sender’s end the key transforms the plain
text message into cipher text form. At t the receiver’send,the
same key is used to decrypt the encrypted message, thus
retrieving the original message out of it. Example: DES (Data
Encryption Standard)
•Asymmetric Key Cryptography: In Asymmetric Key
Cryptography, also called as Public Key Cryptography, two
different keys (which form a key pair) are used. One key is
used for encryption and only the other corresponding key
must be used for decryption. No other key can decrypt the
message – not even the original (i.e. the first) key can be used
for encryption.
•Pixel: In digital imaging, a pixel, or picture element is a
physical point in a raster image, or the smallest addressable
element in a display device. The address of a pixel
corresponds to its physical coordinates. Each pixel is a
sample of an original image; more samples typically provide
more accurate representations of the original. The intensity
of each pixel is variable. In color image systems, a color is
typically represented by three or four component intensities
such as red, green, and blue, or cyan, magenta, yellow, and
black.
•Modular Arithmetic: The division relationship (a= q*n +r)
has two inputs (a and n) and two outputs (q and r). In
modular arithmetic, we only deal with the remainder r. We
don’t care about the quotient. The modulo operator (mod)
takes an integer (a) from theset Z andapositivemodulus(n).
The operator creates r that is a mod n =r
2. EXISTING WORK
Diffie-Hellman Key Exchange Algorithm
Suppose two participants Alice and Bob are allowed to
communicate over a line which is tapped. So any message
they pass canl be intercepted and misused by Eve, who is
always listening to that channel. The trick is to agree on a
secret numerical key withoutthe knowledge of the intruder..
This trick is done using colors so howcouldAlice&Bobagree
on a secret color without Eve finding it out. The key
agreement protocol named as Diffie-Hellman Key Exchnage
Algorithm(1976)[2] was the first method for establishing a
shared secret over an unsecured communication channel.
The trick is based on two facts. One it's easy to mix two
colors together to make a third color and to give it a mixed
color, but it's hard to reverse it in order to find the exact
original colors. This is the basis for a lock is easy in one
direction and hard in the reversedirection.Thisisknownasa
one-way function.
First they publicly agree on a starting color say yellow.
Next Alice and Bob randomly select their own private colors
and mix them into the public yellow in order to disguisetheir
privatecolor. Now both of them keepstheirprivatecolorand
sends the mixtureto the other one. Now the heart of the trick
is, Alice and Bob add their privatecolorstotheotherperson's
mixturethat hasarrived andget the sharedsecretcolor.Now
Eve is unable to determine this color since she needs one of
the private colors to do so.
Figure1. Illustration of the Diffie–Hellman Key
Exchange
Now to do the same with numbers we need a numerical
procedure which is easy in one direction and hard in the
reverse direction, which brings us to modular arithmetic
which is known as clock arithmetic. For example to find
46mod 12 we take a rope of length 46 units and wrap it
around a clock of 12 units which is called the modulus and
wherever the road ends is the solution.
Now to make this work we need a prime modulus such as
17 instead of 12 then wefind the primitiverootof17whichis
a number that has no factors in common in this case 3 and it
has this important property that when raised two different
exponents the solution distributes uniformly around the
clock three is knownas the generator. So if weraised3toany
exponent X the solution is equally likely to be any integer
between 0 and 17. Now we have our one-way function easy
to perform but hard to reverse, we have to do trial and error
in order to find the matching.
The steps are as follows:
1. Alice chooses a large random integer x within the range of
0<=x<=p-1 and sends Bob X=gx mod p
2. Bob chooses a large random integer y within the range of
0<=y<=q-1 and sends Alice Y=gy mod p
3. Alice computes k=Yx mod p
4. Bob computes k=Xy mod p
k is the symmetric key. k is equal to gxy mod n . Both have
received the same value without Bob knowing the value of x
and without Alice knowing the value of y.
Alice and Bob want to be able to generate a key to use for
subsequent message exchange. The key generatingexchange
can take place over an unsecured channel that allows
eavesdropping. The ingredients to the protocolare: p,alarge
prime and g, a primitive element of Zn. These two numbers
do not need to be confidential. They can be sent through the
internet.

Exponent is easy with small numbers but if we use a
prime modulus, which is hundreds of digits long, it starts to
get seriously hard even if you had access to all the
computational power on earth it could take thousands of
years or more to find the answer. So the strength of a one-
way function is based on the time needed to reverse it.
Security Analysis
The Diffie-Hellman Scheme does not provide authentication
of any kind. It only allows 2 anonymous parties to share a
common secret. But for all Alice knows, she could be shaking
hands with the devil (instead of Bob). Theevedoesn’tneedto
find out the value of x and y to attack the protocol. She can
fool Alice and Bob by creating two keys: one between herself
and Alice, and another between herself and Bob. This is why
we need at least one party to be authenticated. Thus the
Diffie-Hellman algorithm fails to provide security over man-
in-the-middle attack.
To overcome this problem, we have suggested a new
technique forkey transmission over publicchannelsecurely,
without letting others know about it, by not sharing the
information directly, but creating a key together.
3. PROPOSED METHOD
Most of the visual cryptographic scheme uses a set of values
as a key or a dictionary which has been used during the
substitution technique. For this it is better to use a secure
technique than direct transmission of the content. That’s
why we have introduced a new technique for key exchange
as a reflection of the Diffie-HellmanKeyExchangealgorithm.
One of the reasons that intruders can be successful is that
most of the information they acquire from a system is in a
form that they can read and comprehend. One solution to
this problem is to hide information in digital media and use
of key for further security. Here we have used cover images
(colored) for the hiding part.
In proposed technique a new secret sharing
technique has been suggested where we have used the
concept of asymmetric key cryptography for transmitting a
secret symmetric key to both the parties, which they will be
using for further communication (encryption & decryption)
without letting others know about their original secretkeys.
Here initially a key value is decided between both the
parties, which need not to be confidential and known as the
public key. Then both the parties Alice andBobhavedecided
their own secret keys which are only known to them. Then
they have applied modulus operation to both their secret
keys using the public key. And after that the computed
values are sent over the secure network. Then both of them
again need to apply some mathematical operation over the
received data and their secret keys in order to generate the
symmetric key which they will be using for their future
communication.
The proposed scheme has been considered secured against
the large number of arithmetic operations need to be
performed in order to generate the key.
After some experiments over various data sets, we have
found that in all the cases, both the parties are retrieving the
same key, which is ready for future use. As here weareusing
modulus operation which is not reversible and we are not
sending the secret keys directly, we are limiting the chance
of eavesdropping during the transmission.
4. METHODOLOGY
In additive model or RGB model, every color image is
composed of pixels where each pixel is a series of bits
composed of RGB values with 24bit depth. Each value is in
the range of 0-255 i.e. Red ranges from 0-255, Green ranges
from 0-255 and Blue ranges from 0-255. When all these
three values for RGB are combined we get a color which
defines the pixel of the image.
While calculating the intensity values,ifwegeta valuewhich
is less than “0” i.e. negative value, we have considered those
as value “0” and all other intensity values in unit8 format.
Figure2. Flow chart representation of the proposed scheme
Brief description of each step
Alice and Bob are the sender and receiver of the
potential messages who need to agree on a similar key for
transmission of secrets using symmetric key cryptography
algorithms, in such a way thattheeavesdroppercan'tgetany
knowledge about the key. As here we are using visual
cryptography for the key transmission, so we have
considered the key to be an image.
•Step 1: Firstly Alice and bob publicly decided a key whichis
a non-zero positive number and a color imagewhichisof the
same size that of the secret key.
•Step 2: Then Alice and Bob privately decided their own
private keys, let them to be A and B.
•Step 3: Now they will apply the encrypt function 1 on their
private keys along with the public image. Let the outputs to
be A' and B' respectively.
•Step 4: Now they will exchange the keys among them.
•Step 5: Then the encrypt function 2 will be applied on their
own sent data and received data, i.e. Alice will operate on A
and B’ & Bob will operate on B and A'.
•After the final operation they both will be holding the same
key.

For the encryption functions, each time we will use the
modulus operation with help of the public key.
5. IMPLEMENTATION DETAILS
The above mentioned schemeisimplementedinto“MATLAB
R2009a and IrfanView have been used as the basic image
editing software. This technique can work for both color
images as well as gray scale images. After some experiments
over various data sets, we have found that in all the cases
both the parties are retrieving the same key, which is ready
for future use.
6. EXPERIMENTAL RESULTS
Below is a set of Output Generated for different key values
and public shared image as ‘peppers.jpg’.All theimagesused
are in 512*512*24bit size and the average elapsed timeisas
follows:
Encryption 1 for Alice: Elapsed time is 0.110362 seconds.
Encryption 1 for Bob: Elapsed time is 0.112347 seconds.
Encryption 2 for Alice: Elapsed time is 0.007116 seconds.
Encryption 2 for bob: Elapsed time is 0.006040 seconds.
Table 1. Resultant Output of the Proposed Scheme
Sl
No
Publ
ic
Key
Secret
Image
of
Alice
Secret
Image
of Bob
Alice’s
Encrypte
d Image
Bob’s
Encrypte
d Image
Generate
d Secret
image
1. Key=
47
2. Key=
58
3. Key=
96
4. Key=
112
Quality Measurement
In this process we have taken some well known benchmark
images for the experimental purpose.
For performance calculation we need to use SSIM or PSNR
index. Here we have used the PSNR index for measuring the
quality between two images. In PSNR quality measurement
one of the images are compared provided the other image is
regarded as of perfect quality. The formulae used are given
below:
…….. Eq1
………Eq2
[X is the original image and Y is the output image; I is the
dynamic range of pixel values normally 255; (M,N) are the
dimensions of the image]
The quality measures are calculated between the original
image and the encrypted images. Table2 shows the quality
measures of the images after encryption process.
Table2. Experimental results for Key Generation process
Sl.
No.
PSNR index for Alice’s
Images
PSNR index for Bob’s
Images
Encrypted
Image 1
Encrypted
Image 2
Encrypted
Image 1
Encrypted
Image 2
1 8.1683
9.1421
11.6839}
7.2935
8.2051
10.5477
13.7294
12.7471
11.0015
12.2402
11.3551
9.8864
2 10.8872
11.6542
12.1304
9.7806
10.4960
10.8304
13.9658
13.1101
11.3852
12.5978
11.7273
9.9410
3 12.1567
14.3039
15.5573
9.5964
11.3913
11.9212
16.9360
15.8081
13.0325
12.6188
11.9538
10.0645
4 11.5731
15.3968
16.6778
9.7383
13.3323
14.8573
14.7120
14.2854
12.6960
12.7470
12.2081
10.6767
7. CONCLUSIONS
We have applied the proposed method in several image sets
with different values for the public key and retrieved
different results. In all the cases the generated secret key is
far different from the original private message which
ensures the confidentiality in the scheme.
As conclusion it can be said that, the proposed key sharing
scheme is undoubtedly fine and fantastic to use. In our
proposed algorithm the same secret key is retrieved in both
the sides. The same technique can be used on binary or gray
scale images also without any change in the algorithm. Till
now we haven’t considered the noise included in the image
during transmission and changes included due to that, but
we will be checking those facts in the near future and work
on that matter as per needed. This project can be extended
further to implement key agreement for a group without
increasing the bandwidth requirement in a securedmanner.

ACKNOWLEDGEMENT
We are very grateful to all authors inreference section.Their
methods, algorithms, conceptual techniquesareveryhelpful
for our research
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