Stego Ofdm Blend For Secure Communication Computer Science Essay

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Abstract- Gone are the days when machines were the world's "prime movers".  Today, the "prime mover" of the world is information, particularly, secret information.  It is also a commodity whose value perishes with time.  This dual-quality of power and perish-ability makes information to be the wealth, most difficult to protect on the earth.  Nevertheless, in order to protect the secret information, technology has given us methods which provide security, though not a fool-proof one. Steganography is one such method which incorporates security by hiding the very existence of the secret by camouflaging it inside an image.  In addition to the enormous security provided by the steganographic algorithms, if we implement some security in the very methodology of transmission and reception of an image, it would prove to be a useful venture. Such a useful venture is proposed in this paper, where four different users can transmit secret messages through a single stego image by taking advantage of the OFDM-QPSK duo, thereby imparting multiple-access and improving security in an otherwise normal stego image. Comparison of BER in AWGN and Random Noise channel has also been done in order to quantify the effectiveness of the proposed technique.

INTRODUCTION

With the profuse proliferation in technology, communication and the mass media, more and more information is transmitted over overt channels. When critically confidential information has to be communicated, a need for impregnable covert channel arises. Over the decades, steganographic techniques were used to hide the critical data in cover images, audios and videos, thereby ensuring the legitimacy of the transmitted data[1-3]. These techniques, though invulnerable, were tedious and also introduced a considerable time lag between data encryption, followed by embedding and then transmission hence, behooved a novel method. Fortunately, the advent of OFDM ameliorated the effective use of time and bandwidth. This paper proposes the use of the physical layer of the Orthogonal Frequency Division Multiplexing (OFDM) to embed data. Here the critical data is transmitted along with the base band signal thereby obviating the arduous task of encryption before transmission.

The choice OFDM proves itself as an efficient one by virtue of the following facts.

This technique makes efficient use of the spectrum by allowing orthogonal overlap and also reduces ISI.

By dividing the channel into narrowband flat fading sub channels, OFDM is more resistant to frequency selective fading than single carrier systems are. i.e. robustness to frequency selective fading channels

This technique eliminates ICI through use of a cyclic prefix.

Using adequate channel coding and interleaving one can recover symbols lost due to the frequency selectivity of the channel.

Channel equalization becomes simpler than by using adaptive equalization techniques with single carrier systems.

It is possible to use maximum likelihood decoding with reasonable complexity.

OFDM is computationally efficient by using FFT and IFFT functions.

This technique is less sensitive to sample timing offsets than single carrier

This technique provides good protection against co channel interference and impulsive parasitic noise.

A few challenges posed by this technique are as follows:

It is more sensitive to ICI (inter carrier interference) which is due to frequency offset. But it could be eliminated by cyclic prefix.

Peak to average power ratio(PAPR) is high 

Bandwidth and power loss can be significant due to guard interval.

High power transmitter amplifiers need linearization

Low noise receiver amplifiers need large dynamic range

The use of this technique warrants smooth secret sharing by the following access technologies:

DAB - OFDM forms the basis for the Digital Audio Broadcasting (DAB).

ADSL - OFDM forms the basis for the global (asymmetric digital subscriber line) standard.

Wireless Local Area Networks - wireless point-to-point and point-to-multipoint configurations using OFDM technology.

The IEEE 802.11 working group published IEEE 802.11a, which outlines the use of OFDM in the 5.8 GHz band.

OFDM is also used in the digital cellular mobile communication.

Information security could be achieved through cryptography [4] or with steganography [3]. The latter rather hides the data whereas the former encrypts the data. Steganography provides much more security when compared to cryptography because there is no chance of any unintended user to know that a message is being sent whereas in Cryptography there will always be a suspicion that a secret message is being sent. Hence, cryptographic communications are more prone to be hacked or suppressed.  Another classification in information security called watermarking.

Watermarking is generally used for authentication and copyrights protection [3]. It can also be used to mark a digital file so that it is intended to be visible (visible watermarking) or invisible (invisible marking) to its creator. The main purpose of watermarking is to prevent the illegal copying or claim of ownership of digital media. But steganography typically relates to clandestine point-to-point communication between two parties. Thus, steganographic methods are usually not robust against modification of the data (integrity has to be maintained), or have only limited robustness and protect the embedded information against technical modifications that may occur during transit and storage in system, worsened in the case of format conversion (.bmp files to .jpeg), compression (stego covers could not withstand lossy compression), or even in digital-to-analog conversion.

In any normal information security scenario, Cryptography is used as a pioneer of the other two data hiding techniques. Data is encrypted in both the techniques in order to increase the randomness of the embedding to avoid the statistics-based attacks and to protect the hidden data in watermarking. Commonly it is a practice to first encrypt the data and then use any one of the embedded techniques.

Fan Tie-sheng et al. [5] improved the hiding capacity of a method proposed by Chen and Wornell [6].  This was an information hiding method in OFDM which enabled QAM and QIM to combine orthogonally. The user identification ability through convolutional based information hiding schemes [7] has been proposed in the physical layer of the communication channel. J.-S. Pan et al. [8] proposed a watermarking scheme through multiple description coding procedure, All of these information hiding methods are different with the traditional Watermarking schemes, which embed the information during the transmission rather than embed the information into the source digital contents. Furthermore, there are only few works till now, which focus on the embedding of multiple user information into OFDM system. In [8], the authors proposed an OFDM based information hiding scheme, which embeds the information by constellation dithering and baud dithering, and the information hiding procedure is after the signal mapping procedure. In [9] the authors proposed a method to embed the secret information into the physical layer of the baseband Orthogonal Frequency Division Multiplexing (OFDM) wireless networks by adopting QPSK modulation and tested the result in AWGN channel. Another similar approach has been proposed in [10] where the author employed BPSK modulation.

Motivated by these schemes, this paper proposes a novel OFDM based multiuser information hiding scheme, which embeds the information during the signal mapping procedure from each user. In the basic block of OFDM system, initially the serial baseband signal is transformed into parallel baseband. Each user chooses a unique phase and this will be their secret key (kerchoff law satisfied). Then, in each of the subcarriers, one baseband signal is mapped into one complex number. In the proposed methodology, the baseband signal is mapped into one of the two complex numbers, according to the information to be embedded by each user. The experimental results indicate that in the presence of AWGN channel and random noise channel, the proposed information hiding method for multi-user could work effectively along with OFDM system.

This paper is organized as follows. Section II offers a narration on the basic building blocks of the OFDM system and examine the possibility of information hiding scheme, followed by information hiding and extracting procedure for four users. Section III gives the experimental result of the proposed method. A conclusion and future research direction has been given in Section IV.

Multi User OFDM Stego Method

OFDM is a multicarrier modulation scheme [9, 10] in which the available frequency band, divided into several channels is used to transmit a narrow band data by independently modifying its carrier. OFDM uses a large number of overlapping orthogonal subcarriers thereby facilitating simultaneous trans-reception of data independently. The OFDM allows high data rate communication over extremely hostile wireless channels at a relatively low complexity as the entire bandwidth is subdivided into several narrow band channels operating at lower data rates.

Fig 1. Basic Block diagram of the OFDM system

High data rate serial input is given to the S-P converter which converts them in to parallel slow data outputs. The Signal mapper utilizes QPSK modulation scheme which converts each baseband signal in to a complex number. IFFT produces orthogonal subcarriers which converts the parallel slow stream in to parallel slow time signals. Orthogonality can be achieved by carefully selecting the sub-carrier frequencies. In this implementation each sub-carrier is orthogonal to the other sub-carriers. One of the ways to achieve this is to select sub-carrier frequencies such that they are harmonics are orthogonal to each other.

Parallel to serial converter converts the parallel time signal in to serial data. A guard time is introduced in every OFDM symbol where cyclic prefix of the OFDM symbol is added which eliminates ICI whereas overlapping subcarriers introduces high spectral efficiency. It is possible to have overlapping sub channels in the frequency domain, thus increasing the transmission rate.   D/A converter converts the Digital signal in to Analog signal and then, analog signal is transmitted through the channel.

In the case of multiple access in OFDM, each user is allocated with a phase value and the phase value should be such that it should not coincide with the normal phase value utilized by the modulation scheme employed in the signal mapper or with the phase value allocated to other users.

Result and discussion

In the proposed scheme, the information is embedded during the signal mapping, which lies between the Serial-to-parallel converter and IFFT. Generally, one baseband signal is mapped into one complex number. In our scheme, in order to embed the extra information, the baseband is mapped into one of the two complex numbers.  Assume the symbol need to be transmitted is m, the bit number per symbol is 2, the mapped phase is ɸ and the information sequence is W = {wk}, wk = {0,1}. If there is no information to be embedded, according to the value of the symbol, the phase can be mapped and shown in Fig 2.

If there is information to be embedded, the mapping procedure can be changed as follows for user 1: With additional user information from user 1 alone, for ɸ = 5.625shown in Fig 3

Fig. 3 with additional information from user 1 phase ɸ = 5.625

If there is information to be embedded, the mapping procedure can be changed as for user 2: With additional user information from user 2 alone, for ɸ =11.25.

Fig. 4 with additional information from user 2 phase ɸ = 11.25

If there is information to be embedded, the mapping procedure can be changed as for user 3: With additional user information from user 3 alone, for ɸ =16.875

Fig. 5 with additional information from user 3 phase ɸ = 16.875

If there is information to be embedded, the mapping procedure can be changed as for user 4: With additional user information from user 4 alone, for ɸ =22.5.

Fig. 6 with additional information from user phase 4 ɸ =22.5

In this present implementation the regular information is gray image of size 56 Ã- 25 pixels and the user's secret information is text file all the sample values are given in Fig 7.

Fig 7(a) Input Cover Image.

Fig 7(b) Output Stego Image.

User 1 Input Text : Padmapriya ɸ =5.625

User 2 Input Text : Amirtharajan ɸ =11.25

User 3 Input Text : John Bosco Balaguru ɸ =16.875

User 4 Input Text : Thenmozhi ɸ =22.50

The reverse process has been followed to obtain the user's secret information. Without the user phase and special demodulator to indentify and detect it is really difficult to extract the user's secret information. Each user's phase is deciding the secret information retrieval hence this present methodology supports multi user environment. But care should be taken to assign the phase values to prevent any interference in phase. The proposed methodology works fine for image transmission using OFDM scheme and the additional user's information needs the same phase used for embedding the additional information.

The effectiveness of the stego process proposed has been studied by estimating the MSE and PSNR metrics for the stego image in AWGN channel and random noise found to be 11.7064, 37.45 dB and 10.98, 37.7248 respectively.

Error metrics

Peak Signal to Noise Ratio (PSNR)

The PSNR is calculated using the equation,

where Imax is the intensity value of each pixel which is equal to 255 for 8 bit gray scale images. Higher the value of PSNR better the image quality

Mean Square Error (MSE)

The MSE is calculated by using the equation,

where M and N denote the total number of pixels in the horizontal and the vertical dimensions of the image Xi, j represents the pixels in the original cover image and Yi, j, represents the pixels of the stego-image

The results are depicted in Fig 8. It shows that the BER performance comparison of the proposed scheme in both AWGN and Random noise channels. It is inferred from the figure 7 that the performance of the scheme using AWGN channel is better with lower bit error rate with increased PSNR.

Fig 8. BER comparison in AWGN and Random channel.

Conclusion

When QPSK modulation technology is combined with OFDM, it enables the effective transmission of the carrier signal, used to transmit primal information along with the embedded and conceited data from all the four user's, maintaining the same data rate, thereby using Quadrature phase shift keying from four different users, which is successfully decrypted on the receiving end with respect to corresponding changes in phase. The additional phase change introduced enables only the targeted to user to decrypt the data when the decryption otherwise a secret remains a secret itself.

Acknowledgment

The authors wish to thank Dr.R.Varadharajan, Professor / ECE and for his guidance and support.

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