Developing a Cryptosystem for Multimedia Data

: Announcement The multimedia information is not secure in the wireless systems compared to that of wired systems. So, there is a necessity to encrypt the information and transmit over wireless medium and then receive the encrypted information from the wireless medium to recover the original information using a decryption method. In this paper, we propose a new method of cryptographic algorithm by combining both the symmetric key mono alphabetic shift ciphering and the RSA (Rivest, Shamir, and Adleman) algorithm which can be more applicable to the future generation of wireless systems. We have developed an algorithm for generating the RSA secret keys. The simulation is implemented in MATLAB. The security performance is studied for multimedia data files such as text, audio and image. Since, RSA is based on modular exponentiation, our experimental results show that the proposed algorithm is very efficient and secure for practical implementation as a multimedia cryptosystem.


Introduction
In the world of Internet, security of information is a must. The cryptography has become more important and critical issue in wireless communications [1]. Unless the system is able to provide some mechanisms to ensure security requirements, the system will have problems to be accepted. The wireless medium is an open air radio environment where any hacker can receive the information. But the multimedia information is not secured in the wireless environment compared to that of wired environment. The hacker has to get the information from wired system by physically attacking the medium. Since the wireless medium is open for access to any intruder or interceptor who can receive the information just with a radio receiver, the wireless information must be encrypted so that the information is secured [2]. Therefore, cryptographic techniques are always employed to protect critical and confidential information against malicious attack from the intruders. So that we can create a privacy to encrypt our message as the sender site and decrypt it at the receiver site. Specifically, the security of information can be applied at the ISO-OSI model of Internet, either at the application layer (Software) or at the data link layer (Hardware) [3]. As the number of hackers (code breakers) increasing, we need to find a more sophisticated secured cryptosystem for our information such that we can spend less time to encrypt/decrypt our information, and managing the keys used for cryptosystems. At the same time we must make the hackers not to get our information. The cryptosystem gives a support to the VoIP (Voice over Internet Protocol) and in turn it finds an application in DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting) [4]. These days, more expert cryptosystems have developed from the two basic classifications: the Symmetric Key Cryptography (SKC) and the Public Key Cryptography (PKC). In this paper, we propose an efficient cryptosystem by combining both the symmetric key mono alphabetic shift ciphering and the public key RSA (Rivest, Shamir, and Adleman) ciphering which can be more appropriate to the upcoming generation of multimedia wireless systems. We have considered the advantages of both SKC and PKC and eliminated the drawbacks of both SKC and PKC [5], [6]. Cryptography algorithms for multimedia (that is images and video) is not so simple. The DES, AES, RES algorithms are not proper for color images and video, which are 3D arrays of data [7]. The paper is organized as following: next section introduces our proposed cryptosystem (SHIFT/RSA), then we give a detailed report of the of our experimental work and the related results. Finally, we conclude the paper.

THE PROPOSED CRYPTOSYSTEM
The basic idea of our proposed cryptosystem is using the combination of both RSA and Shift cipher algorithms. The shift ciphering is a symmetric key cryptography algorithm using a shared key for both encryption (converting plain text to cipher text) and decryption (converting cipher text to plain text) and here the ASCII characters are substituted as numbers from 0 to 127 and they are shifted according to a key and the shifted numbers are transmitted. At the receiver the original numbers are obtained again by shifting using a key which is shared along with the transmitter key. The RSA algorithm is a common public-key cryptography algorithm. RSA uses two exponents, e and d, where e is public and d is private. Encryption and decryption use modular exponentiation which is feasible in polynomial time using fast exponentiation algorithm. However, modular logarithm is as hard as factoring the modulus, for which there is no polynomial time yet. The general idea behind the procedure used in RSA Here:  First, we choose two large prime numbers p and q such that n= p × q.  Compute phi = (p -1) × (q -1).  Choose e (less than n) such that e and phi are relatively prime (having no common factor other than 1).
 Choose d such that (e × d) mod phi = 1.  The public key is the encryption key is [e, n]  The private key is the decryption key is [d, n].  Compute the cipher text: C = P e mod n  Compute the plaintext: P = C d mod n. The encryption/decryption process comprises the following steps: Transmitter Step 1: Initialize all network parameters Step 2: Generate data

RSA/SHIFT Encryption
Step 3: Encrypt the data Step 4: Transmit the signal as frames

Receiver
Step 5: Receive the signal frames

RSA/SHIFT Decryption
Step 6: Receive and decrypt the data Step 7: Plot the results.

Text Encryption/Decryption Example
The input text file shown below is the plain text (text information in a file) and the characters are converted into numbers and then they are encrypted using our proposed algorithm to get the cipher text and then the cipher text is transmitted over the medium.

Input Text File:
Hello Sir, Good Morning! How are you?
I will come at 10 AM!
The cipher text file shown below is the cipher text (text information in a file) which was transmitted over the medium. Since the encryption/decryption method is based on modular exponentiation, it is very hard for the hacker to get the information from this cipher file as factoring the modulus for which there is no polynomial algorithm yet [1].

Cipher Text File:
The output file shown below is the plain text (text information in a file) obtained after the cipher text was decrypted using our proposed algorithm to get the numbers and then the numbers are converted back to characters of plain text.

Output Text File:
Hello Sir, Good Morning! How are you?
I will come at 10 AM!
The above text files:  Input: Transmitter  Output: Receiver  Cipher: Wireless medium represent the results of RSA/SHIFT secured wireless system. Since our encryption algorithm is based on modular exponentiation, and there no polynomial algorithm to factor the used modulus, we believe that the results are practical and secure in wireless systems.

Voice Encryption/Decryption Example
In figure 2 below, the speech signal waveforms shown. The top waveform indicates the plain text and the middle waveform is the cipher text. The decrypted output speech waveform is shown in the bottom. The input speech signal is sampled, compressed and then they are quantized. The quantized samples are encrypted using our algorithm. It is very hard for the hacker to get the original speech signal from the ciphered speech signal. The cipher speech is decrypted to get the plain speech signal after proper decompression and Digital-to-Analog Conversion.  Here, we read the image and convert the image into pixels and the pixels are encrypted using our algorithm to get the cipher image.

Conclusion
In this paper, we have proposed a new cryptosystem for encrypting/decrypting multimedia information such as text, audio, and image in less time and at the same time it is very hard for the hackers to get the information which is being transmitted in the wireless medium. The results are extremely convincing and are coincident with the practical existing multimedia wireless applications. Since, our method is based on RSA, the security of our system is based on the idea that the modulus is so large that it is infeasible to factor it in a reasonable time. So, the ease of implementation using recent techniques indicates that this represents a suitable model for next-generation (4G and beyond) wireless communications, in particular for Orthogonal Frequency Division Multiplexing (OFDM) related communications [9], [10]. Issues in videos: video data is not directly encrypted or decrypted. First, video data is converted into a number of image frames and then cryptography algorithm is applied on individual image frames.