With the advent of communication, security has become one of the most important criterions. There lies a behavior between rigid regularity and randomness based on pure chance. It is called a chaotic system, or chaos for short. Chaos is all around us. Chaos has attracted much attention in the field of Cryptography due to its deterministic nature and its sensitivity to initial values. Chaos has remained in darkness for quite sometime owing to its problems in synchronization. This paper has successfully recreated chaotic sequence in the analog domain.
We intend to build a secure digital communication system based on chaotic modulation, cryptography. The proposed system consists of a Chaotic Modulator (CM), a Chaotic Secure Transmitter (CST), a Chaotic Secure Receiver (CSR) and Chaotic Demodulator (CDM) .Chaotic systems require synchronization for practical applications as they are sensitive to parameter changes and initial conditions. The sender and receiver generate the chaotic key using a synchronous system, typically using a Chua's circuit, where the same set of pseudo random keys is generated simultaneously. This system provides security by enforcing the need for synchronization between the transmitter and receiver apart from other parameters used. Hardware circuits are capable of producing outputs which closely resemble random behavior. On the other hand, software implementations help in improving the speed of operation and diversity of key. The encryption model uses a software generated synchronization signal for the purpose of ensuring synchronous output between the transmitter and the receiver. This removes the need for a separate channel for synchronization, at the same time improving the security of the model to a great extent as we use the surrounding disturbances around us for encryption.
This encryption model is well suited for a Wide Area Network (WAN) structure owing to its software key diversity. The hard key and soft key, in a way, secure each other. That is, a hacker cannot attempt different hard keys without knowing the soft key, since he will not be able to generate the key stream correctly. In order to attempt to break the soft key, the hacker must necessarily know the hard key and vice versa. To a hacker who probes the data from the public channel, the hardware information is also a secret, proving breaking of the system to be a real challenge.
Keywords: Cryptography, Chaotic system, Synchronization, Chua's circuit
Since the pioneering work of Pectoral and Carroll in the field of chaos control in 1991, digital communication techniques based on chaotic systems have been the subject of intensive study. A literature review reveals a large number of related studies, including chaotic coding and chaotic modulation/demodulation.Our planet is being increasingly flooded by signals flowing across it in various forms. The proliferation of the Internet and Cellular Mobile Technology in recent years is responsible for this trend. Advancements in the fields of Microprocessors and data storage are rendering the job of a cryptanalyst easier and also less time consuming. This has necessitated the development of robust cryptographic techniques for ensuring information security. A chaos based cryptographic scheme will be an ideal candidate for this task. The design of communication schemes that are based on chaotic dynamics was studied during the last decade, while the study of traditional communication has began a century ago. Chaos based communication schemes have various properties that make them ideal candidates for implementing secure communication with relatively simple hardware.
II. SIMPLE ANALOG HARDWARE IMPLEMENTATION
Implementation of a communication scheme that transmits a broadband signal at high frequency, and where the waveform is non-periodic, can be implemented, in some cases, using chaotic dynamics hardware that is simpler than its traditional frequency hopping or spread spectrum counterpart. Chaotic communication enables implementation of both encryption and broadband modulation using a simple analog circuit. In traditional communications schemes encryption requires digitization of data, therefore separate circuits are required in order to implement encryption and broadband modulation. Some IEEE papers deals with
A IC chip of Chua's chaotic circuit that operates around 160 KHz and it was implemented on a 2.5mm x 2.8mm silicon area.
A communication scheme based on Chaos Shift Keying (CSK) modulation based on synchronization of a simple Chua's circuit.
III. ANALOG ENCRYPTION
Traditional encryption schemes are defined over integer number fields and are implemented using digital hardware. Chaotic encryption schemes can be implemented using analog components. Encryption of continuous analog waveforms (speech, video, etc.) using traditional encryption schemes would require digitization of the data. Digitization and encryption at high bit rates may require more complicated hardware. Encryption using continuous chaotic dynamics will eliminate the need to digitize the data and a single analog circuit can be used to both encrypt and modulate the data on a broadband carrier.
IV.COMMUNICATION SCHEMES BASED ON CHAOS SYNCHRONIZATION
A large number of communication schemes that are based on chaos synchronization have been proposed during the last decade. In this section, the phenomena of chaos synchronization will be reviewed, followed by a description of several communication schemes that are based on chaos synchronization.
V. CHAOS BASED COMMUNICATION SCHEMES
Two schemes have been developed to shift narrowband communications to a wideband configuration. In the first scheme, known as direct sequence (DS) spread spectrum (SS) communications, a pseudorandom sequence is utilized to spread the transmitted signal. In such a scheme, the effectiveness of multiple accesses is ensured by assigning these sequences such that they are orthogonal to each other. It is noted that this scheme lies at the core of Code Division Multiple Access (CDMA) communication systems. However, when the number of users accessing the communication system is very large, maintaining the synchronization of these sequences throughout the entire system becomes problematic. It is known that chaotic signals are random-like, deterministic and wideband. In chaotic communication systems, replacing the sample functions (sine and cosine) with appropriate chaotic signals permits the advantage of wideband communications in multi-path propagation systems to be realized. The second method proposed for shifting narrowband communications to a wideband configuration involves the use of certain pieces of the chaotic signals to represent the digital data which is to be transmitted. The Chaos Shift Keying (CSK) and Differential Chaos Shift Keying (DCSK) modulation schemes are two well-known examples of this particular approach. When using a coherent receiver, achieving chaotic synchronization between transmitter and receiver is a fundamental issue. The preliminary configuration of a chaotic synchronization scheme consists of a drive chaotic system and a driven chaotic system, which are coupled uni-directionally. Chaotic signal masking, communication by signal reconstruction, communications-based active-passive decomposition, digital communication by chaotic switching, and digital communication based on in-phase and anti-phase synchronization are particular examples in the field of chaotic communications. However, neither the additive masking method nor the chaotic switching methods were very secure. Since a low-dimensional chaotic attractor has a comparatively simple geometric structure, its description in terms of various measures is possible, and hence an intruder may well be able to acquire the hidden text. A common feature of these systems is the utilization of the state variables of the chaotic systems (other than the transmitted state) as keys in the encryption and decryption algorithms. Examples of these systems include the application of a low-dimensional Chua's circuit as a chaotic system and the active-passive decomposition approach. In these systems, an independent low-dimensional chaotic system sends a driving signal to two chaotic slave systems, which are then synchronized and used to generate confidential keys. However, these systems have the drawback that two signals must be sent, namely the driving signal for chaotic synchronization purposes, and the cipher text which is to be transmitted in the public channel. The current paper proposes a communication system that attempts to overcome the above limitation by incorporating a new synchronization technique. In conventional digital communication systems, the data to be transmitted must first be mapped into a weighted sum of analog sample functions (e.g. sine and cosine) and then transmitted in the public channel via RF carriers. It is noted that these sample functions yield signals which are both periodic and narrowband in nature. Two schemes have been developed to shift narrowband communications to a wideband configuration. In the first scheme, known as direct sequence (DS) spread spectrum (SS) communications, a pseudorandom sequence is utilized to spread the transmitted signal. The second method proposed for shifting narrowband communications to a wideband configuration involves the use of certain pieces of the chaotic signals to represent the digital data which is to be transmitted. The broadband nature of the chaotic signal makes it a suitable candidate for performing this task.
VI. DESCRIPTION OF BLOCK DIAGRAM
Fig.1 presents a block diagram of the proposed communication system. It can be seen that the system comprises of four modules, namely the Chaotic Modulator (CM), the Chaotic Secure Transmitter (CST), the Chaotic Secure Receiver (CSR) and the Chaotic Demodulator (CDM).We intend to achieve synchronization between the CST and the CSR by using a drive-driven configuration. This enables the chaotic key used for encryption to be regenerated at the receiver.
Proposed Secure Communication System
VII. CHAOTIC MODULATION
The Chaos Shift Keying (CSK) and Differential Chaos Shift Keying (DCSK) modulation schemes can be employed for spread the transmitted signal. DCSK is a popular method for transmitting binary information using a chaotic signal as a carrier. It is non co herent in nature and does not require synchronization between the transmitter and the receiver. In DCSK, a reference chaotic waveform x(t) is transmitted during the first half of each bit period of the input data stream. If the bit to be transmitted is a "1," the same waveform is transmitted again during the second half of the bit period, while if the bit is a "0," its additive inverse -x(t) is transmitted. In the normal DCSK receiver, the signal is delayed by half a bit period and correlated with the undelayed signal to get the decision variable for producing the output data stream. M-DCSK is a generalization of the DCSK scheme described above. The reference waveform of DCSK (which is typically transmitted during the first half of the bit period) is broken up into time slots distributed all over the bit duration. Each time slot is 2i-chips long and contains i chips forming the reference waveform R, followed by the same waveform multiplied by the data value being transmitted (we call this the data waveform T). k and i are assumed to be integers. A single bit is composed of k time slots with different reference and data waveforms.
VIII. APPLICATION OF THE ENCRYPTION MODEL
This encryption model is well suited for a Wide Area Network (WAN) structure owing to its software key diversity. In a WAN architecture, all users in the 34 network can have the same hardware circuit, but different soft keys. This way user can communicate independently with security provided for each user. To a hacker who probes the data from the public channel, the hardware information is also a secret, proving breaking of the system to be a real challenge. The hard key and soft key, in a way, secure each other. That is, a hacker cannot attempt different hard keys without knowing the soft key, since he will not be able to generate the key stream correctly. In order to attempt to break the soft key, the hacker must necessarily know the hard key and vice versa. The encryption model is independent of the channel, and can be implemented for all type of data. It can also be applied in a real-time environment for multimedia data.
The theory of chaos and chaotic signal generators have been studied. A lorentz attractor and 1D chaotic generator were simulated using MATLAB. Also the recently proposed chaotic modulation schemes have been studied and a simulation of DCPK and M-DCSK schemes are being worked on. While this is just a beginning, future work will include the study of different encryption and decryption algorithms followed by the implementation and performance evaluation of the proposed digital communication system.
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