07-12-2012, 04:55 PM
A Study on Efficient Chaotic Image Encryption Schemes
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Introduction
Motivation and Objective
In recent years, audio-visual information sharing has become more prevalent under the rapid development of Internet. Real-time multimedia applications are also made possible with the advancement of mobile communication technologies. However, in open networks, there is a potential risk of making sensitive information such as military and medical images vulnerable to unauthorized interceptions. The development of robust cryptographic schemes is thus essential to the provision of multimedia security. For textual information, it can be satisfied with the direct application of many well-established encryption schemes such as Data Encryption Scheme (DES)[1], International Data Encryption Algorithm (IDEA) [2] and Advanced Encryption Scheme (AES) [3]. However, the case of multimedia information in real-time communication is different and hard to be accomplished by traditional schemes.
This is because the intrinsic properties of audio-visual information such as bulk data capacity, strong pixel correlation and high redundancy, lower the encryption performance.
Since traditional encryption schemes are not fit for modern multimedia requirement, many researches have been devoted to investigate better solutions for image and video encryptions. In particular, application of chaos theory in multimedia encryption is one of the important research directions. The field of chaotic cryptography has undergone tremendous growth over the past few decades. The primary motivation of employing chaotic systems is its simplicity in form and complexity in dynamics. According to the classification of chaotic systems, the security application of chaos can be divided into analog chaotic secure communications utilizing continuous dynamical systems [4, 5] and digital chaotic cryptosystems utilizing discrete dynamical systems [6 - 8]. For today’s computer technology, the way realizing chaos in digital domain is more vital to security application running in finite precision machines.
In response to the aforementioned challenges in protecting multimedia content, the objective of this research work is specially oriented towards analyzing chaos-based image encryption schemes. Many existing schemes under this category are found to merely achieve moderate or even low security. Only a few of them [9 - 11] promise to achieve sufficient security, but without maintaining a satisfactory speed performance. Our work is to modify and optimize some existing chaotic image encryption schemes so as to uplift the efficiency required for real-time operation purpose. In this regards, two enhancement measures in the system efficiency have been proposed to the main components of typical chaos-based image cryptosystems: chaotic confusion and pixel diffusion processes. The superior results of numerical and security analysis justify the feasibility of such proposed schemes in real-time communication environment.
Outline of the Thesis
Chapter 2 covers the fundamentals and terminologies of cryptography, including the issues of private-key cryptography and public-key cryptography. Note that the chaotic image encryption schemes under study fall into the category of private-key cryptosystems. In order to have a clear background for the remaining chapters, the type of ciphers and mode of operations in private-key cryptosystems will explicitly be highlighted. In addition, some modern cryptographic standards such as DES, AES and RSA will be discussed.
Chapter 3 introduces an overview of chaotic cryptography. The illustration of chaos theory will start with some widely studied one-dimensional (1D) and two-dimensional (2D) chaotic maps. Given the backgrounds of chaotic properties, the similarities and differences between chaotic maps and cryptosystems will then be analyzed. Based on the above established relationships, a more detail description on existing chaotic image encryption schemes will be given together with the issue of design considerations and list of particular cryptanalysis.
Chapter 4 presents a modified approach to the confusion process in typical chaotic image cryptosystems through a special review on an image encryption scheme using 2D chaotic standard map. The principle of this approach including the encryption and decryption procedures will be explained in detail. The security evaluations on the proposed scheme will be provided after the design principle.
In Chapter 5, our attention turns to the effectiveness of the diffusion process which is another important component in image cryptosystems. The goal is to investigate a light weight replacement for the concerned process which commonly requires real-valued computation and consequent integer quantization. The problems will be elaborated by two practical examples of existing schemes based on 1D logistic map. With suitably use of table lookup techniques, a new diffusion approach will be proposed. The corresponding image encryption scheme together with security consideration will be provided.
Finally, we conclude our work in this thesis and give some remarks on future research in Chapter 6.
Fundamentals of Cryptography
In this chapter, the basic principles of cryptography will be introduced as a foundation for the remaining chapters of this thesis. In Section 2.1, the background of cryptography and some terminologies will be covered. The issues of private-key cryptography will be presented in Section 2.2, while the introduction of public-key cryptography will be provided in Section 2.3. A summary will finally be given in Section 2.
Background
Confidential communication has long been a common practice in the social life. However, as information can be communicated electronically, it is exposed in public domain and unavoidably resulted in interceptions. A scientific approach to respond the demands on achieving the sense of security is cryptography. The term cryptosystem, also called cipher, is often used in cryptography. Intuitively, its meaning is clear enough which refers to an encryption system. The central
idea of encryption is to transform the message in which its original information can only be reconstructed by a designated recipient. By definition, a message in its original form is known as plaintext P and the information concealed in an unintelligible form is known as ciphertext C. The encryption process consists of an algorithm and a key. It is generally described as C = E(P, ke), where ke is the encryption key and E( ) is the encryption algorithm. Therefore, the ciphertext C can be transmitted over public channels without exposing the information it represents. Similarly, a corresponding decryption process is the reverse of encryption which is based on the ciphertext C with decryption key kd for the reconstruction of the original plaintext: P = D(C, kd), where D( ) = E-1( ). The principle of encryption process is depicted in Figure 2.1. As an illustration, Caesar cipher is chosen which is the simplest and most classical cipher attributed to Julius Caesar [12].