Image Steganography Combined with DES Encryption Pre-processing Yang Ren-er, Zheng Zhiwei, Tao Shun, Ding Shilei

College of Information Science and EngineeringNingbo University, Ningbo 315211, China

AbstractIn order to improve the security of steganography, this paper studied image steganography combined with pre-processing of DES encryption. When transmitting the secret information, firstly, encrypt the information intended to hide by DES encryption is encrypted, and then is written in the image through the LSB steganography. Encryption algorithm improves the lowest matching performance between the image and the secret information by changing the statistical characteristics of the secret information to enhance the anti-detection of the image steganography. The experimental results showed that the anti-detection robustness of image steganography combined with pre-processing of DES encryption is found much better than the way using LSB steganography algorithms directly.

Keywords-Information hiding; Steganography; DES encryption; High security

I. INTRODUCTIONSteganography is an information hiding technique

developed in recent years. It is a procedure that makes use of human perceptive sense of visual or auralredundancy to digital multimedia, and that embeds the secret information in the public media to transfer digital media carrying confidential information to achieve covert communications[1]. Imperceptibility is an important feature of steganography, also called transparency or anti-detection performance whichincludes visual imperceptibility and statistical imperceptibility. The imperceptibility of steganography is enhanced by improving the method of steganography[2],or selecting an appropriate steganographic carrier[3], or improving the matching relationship between carrier image and secret information.Steganography is different from traditional encryption,which is theoretically based on cryptography to hide communication contents by encrypting plaintext, but it is difficult to mask the fact that communication occurred,and ciphertext transmission will easily arouse the attackers suspicions, and then it could be intercepted, attacked or cracked. Steganography complements thedeficiency of concealment of the encryption. Through transmitting communication contents hidden secretly inpublic media, it covers up the facts of realcommunication purpose and that communication occurred. It is not a competitive relation between steganography and encryption, but they can reinforceeach other, to further ensure the safety of covertcommunication[4]. This paper studied image steganography based on encryption, which combines the traditional encryption and information steganography. It not only makes the secret information transmission"incomprehensible" and "invisible", but makes the steganographic system have a higher anti-detection performance.

II. IMAGE STEGANOGRAPHY COMBINED WITH ENCRYPTION

A. DES Encryption DES uses a 56-bit key and an additional 8-bit parity bit, resulting in the largest 64-bit packet size. This is an iterative block cipher, using the technology known as the Feistel, which will encrypt the text block half. The use of sub-key pair, half of which application circulatory function, and then the output with the other half to " exclusive or" operator followed by the exchange of the two and a half, this process will continue, but in the end a cycle of non-exchange. DES uses 16 cycles, using xor,replacement, substitution, four basic arithmetic shift operations[5]. After the hidden information is encrypted, not only the invisibility of information is increased, but also the statistical characteristics of the information and the correlation between information are changed in the same time, which makes the information become 0-1bitstream, which is in the probability distribution. Inorder to verify the characteristics of encryption algorithm, download three English document of 32768 characters from the Internet, and inspect the 0 -1 distribution in the document before and after encryption. From table 1,before the encryption, the number of 0 and 1 is not balanced because of correlation between characters in the natural document, but after the encryption, binary stream of 0 s and 1 s close to equal probability distribution in the document.

Table 1 the 0 -1 Distribution in the Document before and after Encryption

document

before the encryption

after the encryption

the number of 0

the number of 1

the number of 0

the number of 1

1 143674 118470 130931 1312132 143698 118446 131218 1309263 141719 120425 131069 1310754 142757 119387 131665 130479

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Figure 1.Frequency of sequences of 0 and 1 in the tour

B. LSB Steganography Combined with Digital Image

The LSB (Least Significant Bit) which hides the secret information to the least significant bit of carrier information, is the most classic and simpleststeganography algorithm, which also features of a large capacity and concealment. It is still widely considered most practical and applied up to today.

The LSB steganography uses the secret information instead of the last bit of the image pixels value, which is equivalent to superimposing a weak signal upon the original carrier signal, and therefore its difficult to realize visually. But during the embedding process, if the lowest bit of secret information is different from that ofpixel value, we need to change the lowest bit of pixel value, namely change the pixel value 2i to 2i+1 or 2i+1 to 2i. The parity value transform in the LSB steganography produces parity asymmetry. Fridirich et al. proposed the method of using this asymmetrical parity value transform of LSB to go on RS attack[7], and Westfield et al. proposed Chi-square test[8] to attack LSB steganography algorithm. If steganographic method can minimize the distortion of the histogram, it can fundamentally resist the attack of analysis methods aforementioned. So in this paper, we propose the steganography combined with encryption, encrypting the information intended to hide before the steganalysis with encryption pre-processing, and the imperceptibility of the steganography algorithm can be improved by reducing the distortion of the histogram.

C. Image Steganography Combined withEncryption

In order to improve the imperceptibility of steganography algorithm, this paper studied the steganography combined with encryption algorithm, which reach the goal by changing the matching relationship between carrier image and secret

information. At the transmitting terminal, we firstly choose the appropriate image as the carrier, and then encrypt text information or documents designed to hide by DES encryption followed by using the LSB steganography algorithm to hide the encrypted information in the image; at the receiving terminal, firstly extract the encrypted information from the carrier image by the algorithm which is contrary to steganography algorithm, and then recover the hidden information by DES decryption algorithm.

the secretinformation

carrierimage

secret key secret image

the secretinformation

DESEncryption

decryptionalgorithm

LSBimbedding

Extraction algorithm

Figure 2.the flow chart of Image Steganography Combined with Encryption

D. The Evaluation Method of Steganography Algorithm

1) Histogram Difference and the Sum of Histogram absolute Difference

We denote )(nh as the image gray histograms function. For the gray level image of 8 bits,

]255,0[n . And )(nh is a sequence which represents the number of each pixel values in the image, namely

1

0

1

0)),(,()(

W

i

H

jjipnnh (1)

Where

vuvu

ifif

vu

,0,1

),(

And )( jip indicates the images pixel value at ),( ji , W is the images width, H is the images height.

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We denote )(nh as the histogram of the original

image and )(' nh as the encryption image, ]255,0[n . Then, ][nhm denote Histogram difference, namely ][][][ nhnhnHm (2)

255

0|)(')(|

iihihHae (3)

The absolute difference of histogram expresses how the image changes before and after the steganography. The smaller Hae is, the better imperceptibility of the steganography is.

2) Relative Entropy of the Image before and after Steganography (KL divergence) Relative entropy of the image before and after steganography or the KL divergence is used to measure the security of the steganographic system put forward by Cachin[6].The decrease of the KL divergence shows the improvement of the steganographic systems safety performance. If the K_L divergence is zero, the steganographic system is absolutely safe, namely

0,)||(

PsPcD is absolutely safe

2 5 5

0 ][][l o g][)||(

n s

cc np

npnpPsPcD (4)

Where ][npc is gray levels probability distribution of

carrier image, ][nps is that of steganasis image.

III. EXPERIMENTAL RESULTS AND ANALYSIS Simulation experiments embed information that needs to hide and the information after DES encryption respectively to 512 x 512 pixel image 1-3 by LSB steganography algorithm. It is showed in Figure 3.

Figure3. the experimental picture

From the experimental results in Figure 3, we can seethe figure on the left, if the information is unencrypted before the LSB algorithm , the difference between carrier image and the steganasis image is large, if not, we can see it from the figure on the right, the difference is small. The smaller the histogram difference is, the better the performance of steganographic system is. From table 1 and its experimental results, we can also see that after the encryption, the histogram modification and KL divergence decrease, suggesting that after the encryption, the anti-interference performance of the steganography gets better. Thus, steganography system combined with encryption is superior to direct steganalysis.

Figure 4. the histogram difference before and after steganography in two ways

Table 2. steganasis image effects without encryption

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Table 3. steganasis image effects with encryption

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IV. CONCLUSIONSThis paper mainly probes the steganography carried by

image, which combines DES encryption with LSB algorithm, and implements dual protect of hidden information, making the hidden information

318325325325325

incomprehensible and invisible. Besides, the DES encryption algorithm changes the statistical characteristics of the secret information, and improvesthe lowest matching degree of its bitstream and carrier image, leading to a better imperceptibility of steganographic algorithm.

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