International Journal of Computer, Electronics & Electrical Engineering

(ISSN: 2249 - 9997)Volume 2– Issue 2

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Mohit Kumar Goel *, Dr. Neelu Jain

#

* Dept. of Electronics and Electrical Comm., PEC University of Technology, Chandigarh

mohitgoel4u@gmail.com

# Dept. of Electronics and Electrical Comm., PEC University of Technology, Chandigarh

neelujain@pec.ac.in

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.

I. INTRODUCTION

Due to increasing the technologies, security systems are very popular in many areas. The information comes in numerous forms and requires secure communication. Such secure communication ranges from bank transactions, corporate communications and credit card purchases. Encryption and steganography are the preferred techniques for protecting the transmitted data. Encryption hides the contents of the message, but cannot hide the message data itself. However, encryption clearly marks a message as containing “interesting” information, and the encrypted message becomes subject to attack. The commonly used encryption schemes include DES (Data Encryption Standard) [1], AES (Advanced Encryption Standard) [2] and RSA [3]. DES, an encryption standard that was used by many national governments, successfully withstood attacks for many years. However, E. Biham and A. Shamir mention a cryptanalytic attack that can break DES in only a few minutes [4]. Another example of a broken encryption algorithm is WEP. WEP was designed to provide confidentiality to users on wireless networks. A. Stubblefield illustrates how WEP can be broken within hours [5]. DES and WEP are examples of two encryption algorithms that were thought to be secure at the time of their design, but were broken in the future when attackers had more powerful computational resources. So, in many cases it is desirable to send information without being notice by anyone that information has been sent. Steganography

conceal the secret messages within some image, music or audio file so that it is not visible to others. Image steganography schemes can be divided into two categories: Spatial Domain and Frequency Domain.

A. Spatial domain steganography

Spatial domain techniques embed messages in the intensity of the pixels directly [6][7][8]. Least Significant Bit (LSB) is the first most widely used spatial domain steganography technique. It embeds the bits of a message in the LSB of the image pixels [9][10]. But the problem with this technique is that if the image is compressed then the embedded data may be lost. Thus, there is a fear for loss of data that may have sensitive information [11]. LSB has been improved by using a Pseudo Random Number Generator (PRNG) and a secret key in order to have private access to the embedded information [12]. The embedding process starts with deriving a seed for a PRNG from the user password and generating a random walk through the cover image that makes the steganalysis hard. Another recent improvement based on random distribution of the message was introduced by M. Bani Younes and A. Jantan [13]. In this method they utilize an encryption key to hide information about horizontal and vertical blocks where the secret message bits are randomly concealed. SSB-4 steganography approach introduced by Rodrigues, Rios and Puech is about changing the 4th bit of a pixel in the original image according to the bit message. Then modify the other bits (1st, 2nd, 3rd and/or 5th) to minimize the difference

A Novel Visual Cryptographic Steganography

Technique

International Journal of Computer, Electronics & Electrical Engineering

(ISSN: 2249 - 9997)Volume 2– Issue 2

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between the changed pixel value and the original one [14]. The 4th digit is a significant bit and if the image is compressed the embedded information is not destroyed [15]. Tu C. and Tran T D. argued that the difference must be 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 color value is imperceptible to human eyes, and it is the most significant bit which provides the minimum change in the pixel values. Modulus arithmetic steganography proposed by Sayuthi Jaafar and Azizah A Manaf has calculated last four bits of each pixel by mod-16 operation. Then these bits are replaced with data bits [8]. In this the amount of the data that can be embedded is more but stego image has less PSNR value than LSB and SSB-4 techniques.

B. Frequency domain steganography

In frequency domain, images are first transformed and 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.

II. BACKGROUND OF CRYPTOGRAPHY

In cryptography, the message is scrambled to make it meaningless and unintelligible unless the decryption key is available. It makes no attempt to disguise or hide the encoded message. Basically, cryptography offers the ability of transmitting information between persons in a way that prevents a third party from reading it. Cryptography can also provide authentication for verifying the identity of someone or something. There are several ways of classifying cryptographic algorithms. The three types of algorithms are:

1) Secret Key Cryptography: Uses a single key for both encryption and decryption.

2) Public Key Cryptography: Uses one key for encryption and another for decryption.

3) Hash Functions: Uses a mathematical transformation to irreversibly “encrypt” information.

A. RSA encryption algorithm

RSA is a Public key cryptography named after its inventors: Ronald Rivest, Adi Shamir and Leonard Adleman. RSA can be used for encryption as well as for authentication [3]. An example of Alice and Bob, who want to use asymmetric RSA algorithm for secure communication is shown in fig. 1. For encryption purpose, Alice would encrypt the message using Bob’s Public key and send the cipher text to Bob. Upon receiving the cipher text, Bob, who is owner of corresponding private key, can then decrypt the message with his private key. For authentication purposes, Alice would encrypt (or sign) the message using her own private key. Other people such as Bob can verify the authenticity of the message by using Alice’s Public key, which is the only key that matches the signing private key.

Figure 1. RSA Encryption

The steps for RSA algorithm are:

1) Select two prime numbers p, q. 2) Calculate n= p × q and �(n)= (p-1)(q-1) 3) Select integer ‘e’ such that

gcd (� (n),e)=1; 1<e < �(n) 4) Calculate d such that d × e=1mod(� (n)) 5) Now Public key (PU) is {e, n} and Private

Key (PR) is {d, n}. 6) At sender side, message (M) to be sent is

converted into cipher text (C) as follows: C= Me mod n (1)

7) At receiver side, cipher text is converted to original message as follows:

M= Cd mod n (2)

International Journal of Computer, Electronics & Electrical Engineering

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III. LSB-DCT STEGANOGRAPHY

LSB-DCT steganography image (I) is divided into 8x8 blocks and two dimensional (2-D) is performed on each block. The 2-d DCT is calculated as follow:

)3(16

)12(cos

16

)12(cos),()()(

4

1),(

7

0

7

0��

���

� +

��

���

� += ��

= =

yuxyxfvCuCvuF

x y

ππ

for x=0,..., 7 and y=0,..,7

� =

=

otherwise

kforkCwhere

1

02/1)(

In DCT block lower frequency cofficents are at upper left positions and high frequency coefficients are lower right positions. Now image is compressed by quantization. Quantization is achieved by dividing each element in the DCT coefficient block by the corresponding value in the standard quantization matrix shown in fig. 2 and the result is rounded to the nearest integer. As eye is not able to discern the change in high frequency components so these can be compressed to larger extent. Lower right side components of quantization matrix are of high value so that after quantization high frequency components become zero.

����������

�

�

����������

�

�

=

9910310011298959272

10112012110387786449

921131048164553524

771031096856372218

6280875129221714

5669574024161314

5560582619141212

6151402416101116

Q

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 odd

After embedding data zigzag array is again converted into 8×8 block. These blocks are dequantized and inverse DCT is performed. The entire 8×8 blocks are combined to form the stego image which is then sent to receiver.

At the receiver side the stego-image is received in spatial domain. Now stego image is divided into 8×8 blocks and

DCT is performed on each block. Then scan the DCT block in zigzag way and extract the embedded data.

IV. PROPOSED METHOD

The challenge in this work was to find a way to camouflage a secret message in an image without perceptible degrading the image quality and to provide better resistance against the steganalysis process. The data is first converted into cipher text using RSA encryption and then hided into lower frequency components of image using LSB-DCT steganography.

A. Embedding algorithm

Steps of embedding algorithm are given as follow:

Input: An M×N size cover image and data to be concealed.

Output: Stego image.

1) Encrypt the plain text using encryption key. 2) Divide the cover image into 8×8 blocks. 3) Perform 2-D DCT on each block. 4) Perform quantization on each block. 5) Perform zigzag scan to convert 8×8 block into one

dimensional array. 6) Replace the LSB of DCT coefficients with data

bits. 7) Convert 1-D zigzag array back to 8×8 block. 8) Perform Inverse DCT on each block. 9) Combine all the blocks to form stego image.

Figure 3. Proposed Method

B. Extraction algorithm

Steps for extraction algorithm are given as follows:

Input: An M×N size Stego image.

International Journal of Computer, Electronics & Electrical Engineering

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Output: Secret message.

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.

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 taken by digital camera. We focused on short messages with length of 3000 bits because they are the most challenging to detect [24]. Comparative analysis of LSB, Modulus arithmetic (mod-16), and proposed method has been done on the basis of Peak signal to noise ratio (PSNR). To calculate PSNR, first MSE is calculated as follows:

)4(),(),(1 2

1

0

1

0

��−

=

−

=

−=

m

i

n

j

jiKjiImn

MSE

Where MSE is the Mean Squared Error of Original image (I) and stego image (K). Thereafter PSNR value is calculated as follow:

)5(log.20log.10 10

2

10 ��

��

�=��

�

���

�=

MSE

MAX

MSE

MAXPSNR ii

Where, MAXi is the maximum pixel value of the image. In other words MAXi = 2b − 1, where b is the bit depth of the original image. PSNR computes the peak signal to noise ratio, in decibels, between two images. This ratio is used as quality measurement between two images.

(a) Original Human.jpg (b) Stego Human.jpg

(c) Original Flower.jpg (d) Stego Flower.jpg

(e) Original Building.jpg (f) Stego Building.jpg

(g) Original Tree.jpg (h) Stego Tree.jpg

Figure 4. Original Images and Stego Images using DCT steganography

The comparative analysis of PSNR value of different steanography technique, is given in table 1, shows that proposed steganography method has better image quality of stego image than other techniques.

Table 1. Comparative analysis of PSNR values of different steganography

techniques

PSNR Value

Image LSB

Modulus

(mod-16)

RSA & LSB-

DCT

Human.jpg 52.10 49.23 55.87

Flower.jpg 53.54 50.53 56.36

Building.jpg 52.43 48.77 54.59

Tree.jpg 53.46 50.46 55.57

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 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.

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VI. REFERENCES

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[19] Iwata M., Miyake K. and Shiozaki, “Digital Steganography Utilizing Features of JPEG Images”, IEICE Transfusion Fundamentals, E87-A, 4, pp. 929 – 936, 2004.

[20] M. Kharrazi, H. Sencar and N. Memon, “Performance study of common image steganography and steganalysis techniques,” Communications of the SPIE and IS&T, 15, No.4, pp. 1017-9909, Oct-Dec., 2006.

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[23] Zhiping Zhou and Maomao Hui, “Steganalysis for Markov Feature of Difference Array in DCT Domain”, Proceedings of Sixth International Conference on Fuzzy Systems and Knowledge Discovery, pp. 581 - 584 , Aug. 2009.

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