A Novel Visual Cryptographic Steganography by Mohit Goel

					                                            International Journal of Computer, Electronics & Electrical Engineering
                                                                            (ISSN: 2249 - 9997)Volume 2– Issue 2

          A Novel Visual Cryptographic Steganography
                                             Mohit Kumar Goel *, Dr. Neelu Jain #
                    Dept. of Electronics and Electrical Comm., PEC University of Technology, Chandigarh
                    Dept. of Electronics and Electrical Comm., PEC University of Technology, Chandigarh

Abstract— With development in technologies, the amount of data being exchanged on internet is increasing exponentially. The security
of information can be achieved by cryptography and steganography. Cryptography hides the contents of message by converting it to an
unreadable cipher. Steganography hides the existence of message by embedding data in some other digital media like image or audio
files. The paper proposes a security system which is combination of both the techniques. In proposed system data is firstly encrypted
using RSA encryption algorithm and then embedded in an image using DCT based steganographic method. The experimental result
shows that proposed system has better PSNR value in comparison to other techniques like LSB, Modulus arithmetic steganography. It
also maintainstaisfactory security as secret message can’t be extracted without knowing the decoding algorithm.

Keywords- steganography; RSA encryption; data hiding; discrete cosine transform.

                                                                       conceal the secret messages within some image, music or
                      I.   INTRODUCTION                                audio file so that it is not visible to others. Image
     Due to increasing the technologies, security systems are          steganography schemes can be divided into two categories:
very popular in many areas. The information comes in                   Spatial Domain and Frequency Domain.
numerous forms and requires secure communication. Such
secure communication ranges from bank transactions,                    A. Spatial domain steganography
corporate communications and credit card purchases.                        Spatial domain techniques embed messages in the
Encryption and steganography are the preferred techniques              intensity of the pixels directly [6][7][8]. Least Significant
for protecting the transmitted data. Encryption hides the              Bit (LSB) is the first most widely used spatial domain
contents of the message, but cannot hide the message data              steganography technique. It embeds the bits of a message in
itself. However, encryption clearly marks a message as                 the LSB of the image pixels [9][10]. But the problem with
containing “interesting” information, and the encrypted                this technique is that if the image is compressed then the
message becomes subject to attack. The commonly used                   embedded data may be lost. Thus, there is a fear for loss of
encryption schemes include DES (Data Encryption                        data that may have sensitive information [11]. LSB has been
Standard) [1], AES (Advanced Encryption Standard) [2] and              improved by using a Pseudo Random Number Generator
RSA [3]. DES, an encryption standard that was used by                  (PRNG) and a secret key in order to have private access to
many national governments, successfully withstood attacks              the embedded information [12]. The embedding process
for many years. However, E. Biham and A. Shamir mention                starts with deriving a seed for a PRNG from the user
a cryptanalytic attack that can break DES in only a few                password and generating a random walk through the cover
minutes [4]. Another example of a broken encryption                    image that makes the steganalysis hard. Another recent
algorithm is WEP. WEP was designed to provide                          improvement based on random distribution of the message
confidentiality to users on wireless networks. A.                      was introduced by M. Bani Younes and A. Jantan [13]. In
Stubblefield illustrates how WEP can be broken within                  this method they utilize an encryption key to hide
hours [5]. DES and WEP are examples of two encryption                  information about horizontal and vertical blocks where the
algorithms that were thought to be secure at the time of their         secret message bits are randomly concealed. SSB-4
design, but were broken in the future when attackers had               steganography approach introduced by Rodrigues, Rios and
more powerful computational resources. So, in many cases               Puech is about changing the 4th bit of a pixel in the original
it is desirable to send information without being notice by            image according to the bit message. Then modify the other
anyone that information has been sent. Steganography                   bits (1st, 2nd, 3rd and/or 5th) to minimize the difference

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                                             International Journal of Computer, Electronics & Electrical Engineering
                                                                             (ISSN: 2249 - 9997)Volume 2– Issue 2

between the changed pixel value and the original one [14].            3) Hash     Functions:        Uses    a     mathematical
The 4th digit is a significant bit and if the image is                   transformation    to        irreversibly    “encrypt”
compressed the embedded information is not destroyed [15].               information.
Tu C. and Tran T D. argued that the difference must be            A. RSA encryption algorithm
equal or less than four (i.e., ±4) [16]. The 4th bit was chosen
because it satisfies that changing of ±4 units in the channel         RSA is a Public key cryptography named after its
color value is imperceptible to human eyes, and it is the         inventors: Ronald Rivest, Adi Shamir and Leonard Adleman.
most significant bit which provides the minimum change in         RSA can be used for encryption as well as for authentication
the pixel values. Modulus arithmetic steganography                [3]. An example of Alice and Bob, who want to use
proposed by Sayuthi Jaafar and Azizah A Manaf has                 asymmetric RSA algorithm for secure communication is
calculated last four bits of each pixel by mod-16 operation.      shown in fig. 1. For encryption purpose, Alice would encrypt
Then these bits are replaced with data bits [8]. In this the      the message using Bob’s Public key and send the cipher text
amount of the data that can be embedded is more but stego         to Bob. Upon receiving the cipher text, Bob, who is owner of
image has less PSNR value than LSB and SSB-4                      corresponding private key, can then decrypt the message
techniques.                                                       with his private key. For authentication purposes, Alice
                                                                  would encrypt (or sign) the message using her own private
B. Frequency domain steganography                                 key. Other people such as Bob can verify the authenticity of
                                                                  the message by using Alice’s Public key, which is the only
    In frequency domain, images are first transformed and
                                                                  key that matches the signing private key.
then the message is embedded in the image [17][18][19].
When the data is embedded in frequency domain, the hidden
data resides in more robust areas, spread across the entire
image, and provides better resistance against statistical
attacks. There are many techniques used to transform image
from spatial domain to frequency domain. The most
common frequency domain method usually used in image
processing is the 2D discrete cosine transform [20][21]. In
this technique the image is divided into 8×8 blocks and
DCT transformation on each block is performed. The data
bits are embedded in the low frequency coefficients of DCT.
SSB-4 & DCT steganography proposed by Nedal M. S.
Kafri and Hani Y Suleiman uses DCT approach with SSB-4
technique [21].
    Steganography with cryptography can be combined so
that, even if an attacker does realize that a message is sent,
he would still have to decode it [26]. Piyush Marwaha and
Paresh Marwaha use DES encryption and LSB
steganography for data security [25]. In this paper we
propose a method which uses RSA encryption and LSB-
DCT steganography for data security.

                                                                                     Figure 1. RSA Encryption
    In cryptography, the message is scrambled to make it
meaningless and unintelligible unless the decryption key is       The steps for RSA algorithm are:
available. It makes no attempt to disguise or hide the                1) Select two prime numbers p, q.
encoded message. Basically, cryptography offers the ability           2) Calculate n= p × q and (n)= (p-1)(q-1)
of transmitting information between persons in a way that
                                                                      3) Select integer ‘e’ such that
prevents a third party from reading it. Cryptography can
also provide authentication for verifying the identity of                     gcd ( (n),e)=1; 1<e < (n)
someone or something. There are several ways of                       4) Calculate d such that d × e=1mod( (n))
classifying cryptographic algorithms. The three types of              5) Now Public key (PU) is {e, n} and Private
algorithms are:                                                          Key (PR) is {d, n}.
                                                                      6) At sender side, message (M) to be sent is
    1) Secret Key Cryptography: Uses a single key for                    converted into cipher text (C) as follows:
       both encryption and decryption.                                                 C= Me mod n                       (1)
    2) Public Key Cryptography: Uses one key for                      7) At receiver side, cipher text is converted to original
       encryption and another for decryption.                            message as follows:
                                                                                        M= Cd mod n                     (2)

                                                                         www.ijceee.org                         Page 40
                                                    International Journal of Computer, Electronics & Electrical Engineering
                                                                                    (ISSN: 2249 - 9997)Volume 2– Issue 2

            III.        LSB-DCT STEGANOGRAPHY                            DCT is performed on each block. Then scan the DCT block
                                                                         in zigzag way and extract the embedded data.
   LSB-DCT steganography image (I) is divided into 8x8
blocks and two dimensional (2-D) is performed on each                                   IV.      PROPOSED METHOD
block. The 2-d DCT is calculated as follow:
                                                                             The challenge in this work was to find a way to
         1          7 7
                                       π(2x +1)u     π(2y +1)            camouflage a secret message in an image without
F(u, v) = C(u)C(v)         f (x, y)cos           cos            (3)      perceptible degrading the image quality and to provide
         4         x=0 y=0                16            16
                                                                         better resistance against the steganalysis process. The data is
for x=0,..., 7 and y=0,..,7                                              first converted into cipher text using RSA encryption and
                                                                         then hided into lower frequency components of image using
                   1 / 2 for k = 0                                       LSB-DCT steganography.
where C ( k ) =
                   1    otherwise
                                                                         A. Embedding algorithm
    In DCT block lower frequency cofficents are at upper                    Steps of embedding algorithm are given as follow:
left positions and high frequency coefficients are lower right
positions. Now image is compressed by quantization.                      Input: An M×N size cover image and data to be concealed.
Quantization is achieved by dividing each element in the
DCT coefficient block by the corresponding value in the                  Output: Stego image.
standard quantization matrix shown in fig. 2 and the result is               1)   Encrypt the plain text using encryption key.
rounded to the nearest integer. As eye is not able to discern                2)   Divide the cover image into 8×8 blocks.
the change in high frequency components so these can be                      3)   Perform 2-D DCT on each block.
compressed to larger extent. Lower right side components of                  4)   Perform quantization on each block.
quantization matrix are of high value so that after                          5)   Perform zigzag scan to convert 8×8 block into one
quantization high frequency components become zero.                               dimensional array.
                        16 11 10 16     24   40   51    61
                                                                             6)   Replace the LSB of DCT coefficients with data
                        12 12 14 19     26   58   60    55
                        14 13 16 24     40   57   69    56
                                                                             7)   Convert 1-D zigzag array back to 8×8 block.
                                                                             8)   Perform Inverse DCT on each block.
                        14 17 22 29     51   87   80    62
                   Q=                                                        9)   Combine all the blocks to form stego image.
                        18 22 37 56     68 109 103      77
                        24 35 55 64     81 104 113      92
                        49 64 78 87 103 121 120 101
                        72 92 95 98 112 100 103         99
                        Figure 2. Quantization Matrix

    Although the DCT coefficients have been decorrelated
by DCT transform to some extent, DCT coefficients in the
same block are still not independent, which is called as
intra-block correlation [16]. While neglecting the impact of
block edge, the general trend in magnitude of the block
coefficients in each block is non-increasing along zigzag
scan order. After block DCT coefficients are arranged by
zigzag scan pattern, dependencies among neighboring
coefficients in both horizontal and vertical directions can be
conveniently investigated [23]. Now data is embedded in
one dimensional zigzag array
    a) If data bit is ‘0’, then make the DCT coefficient
       even or,
    b) If the data bit is ‘1’, then make the DCT coefficient
    After embedding data zigzag array is again converted
into 8×8 block. These blocks are dequantized and inverse
                                                                                              Figure 3. Proposed Method
DCT is performed. The entire 8×8 blocks are combined to
form the stego image which is then sent to receiver.
                                                                         B. Extraction algorithm
   At the receiver side the stego-image is received in spatial              Steps for extraction algorithm are given as follows:
domain. Now stego image is divided into 8×8 blocks and
                                                                         Input: An M×N size Stego image.

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                                                               International Journal of Computer, Electronics & Electrical Engineering
                                                                                               (ISSN: 2249 - 9997)Volume 2– Issue 2

Output: Secret message.                                                                   (c) Original Flower.jpg               (d) Stego Flower.jpg

    1) Divide the stego image into 8×8 blocks.
    2) Perform 2-D DCT on each block.
    3) Perform quantization on each block.
    4) Perform zigzag scan to convert 8×8 block into one
       dimensional array.
    5) Check the DCT coefficient.
       a) If DCT coefficient is even then data bit is 0 or,
       b) If DCT coefficient is odd then data bit is 1.
                                                                                           (e) Original Building.jpg         (f) Stego Building.jpg
    6) Concatenate the bits to obtain cipher message.
    7) Decrypt the cipher text using decryption keys and
       display original message on screen.

              V.         EXPERIMENTAL RESULTS
    Since the visual detection of stego images is depending
on the nature of the image [24] so, varieties of image
categories are utilized in the experiments. The experimental
image data set consists of 100 JPEG images, which were                                       (g) Original Tree.jpg             (h) Stego Tree.jpg
taken by digital camera. We focused on short messages with
length of 3000 bits because they are the most challenging to                         Figure 4. Original Images and Stego Images using DCT steganography
detect [24]. Comparative analysis of LSB, Modulus                                       The comparative analysis of PSNR value of different
arithmetic (mod-16), and proposed method has been done                              steanography technique, is given in table 1, shows that
on the basis of Peak signal to noise ratio (PSNR). To                               proposed steganography method has better image quality of
calculate PSNR, first MSE is calculated as follows:                                 stego image than other techniques.
              1 m −1 n −1                            2                              Table 1. Comparative analysis of PSNR values of different steganography
     MSE =                I (i , j ) − K ( i , j )                       ( 4)                                      techniques
             mn i =0 j =0
                                                                                                                         PSNR Value
    Where MSE is the Mean Squared Error of Original
image (I) and stego image (K). Thereafter PSNR value is                                        Image
                                                                                                                          Modulus      RSA & LSB-
calculated as follow:                                                                                                     (mod-16)        DCT
                          MAX i2             MAX i                                           Human.jpg         52.10       49.23
   PSNR = 10. log10              = 20. log10                               (5)
                          MSE                 MSE                                                                                         56.36
                                                                                             Flower.jpg        53.54       50.53
    Where, MAXi is the maximum pixel value of the image.                                                                                  54.59
In other words MAXi = 2b − 1, where b is the bit depth of                                   Building.jpg       52.43       48.77
the original image. PSNR computes the peak signal to noise                                                                                55.57
ratio, in decibels, between two images. This ratio is used as                                 Tree.jpg         53.46       50.46
quality measurement between two images.

                                                                                                         V.      CONCLUSION
                                                                                        In this paper we used mixed approach cryptography and
                                                                                    steganography is used for data security. By using RSA
                                                                                    encryption, ASCII codes corresponding to characters of
                                                                                    plain text are converted into 16 bits encrypted codes. Hence
                                                                                    it becomes difficult to get original text without knowing
                                                                                    decryption keys. Then cipher data is hided into cover image.
                                                                                    Average PSNR value of 55 is obtained for 100 images using
     (a) Original Human.jpg                              (b) Stego Human.jpg        proposed method. The obtained experimental results
                                                                                    indicate that, the proposed method is a good and acceptable
                                                                                    scheme for data security. Furthermore, by embedding
                                                                                    information in the least significant bits of the DCT domain,
                                                                                    the hidden message resides in more robust areas, spread
                                                                                    across the entire stego image, and provides better resistance
                                                                                    against statistical attacks than other techniques. The future
                                                                                    work may focus on the improvement and further
                                                                                    development in this technique.

                                                                                            www.ijceee.org                             Page 42
                                                       International Journal of Computer, Electronics & Electrical Engineering
                                                                                       (ISSN: 2249 - 9997)Volume 2– Issue 2

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