Research On Encryption Algorithms And Key Agreement Protocols Based On Digitalized Chaos

It is more convenient to deposit information with the invention of digital media. The transmission and application of digital information gain great further development in the Internet environment. The security of digital information is of great importance therefore in this background. Because cryptography is the theoretical foundation of information security, research on cryptography in depth provides a crucial resolution to information security.Chaos, which is a product of nonlinear science, found applications in cryptography, and became a hot spot in information security. Chaotic cryptography has attracted more and more research attentions in the last two decades for the ubiquity of chaos and its analogies with traditional ciphers. A great number of ciphers based on chaos have been proposed, and the security analyses on these ciphers have also been provided by many researchers. Based on the current status of chaotic cryptography, this thesis is dedicated to further research on the design and analysis of document and image encryption algorithms utilizing digitalized chaos, as well as key agreement protocols based on chaotic maps.The main work of this thesis includes:(1) Chaos theory and chaotic cryptography are introduced. First, the state of the art in modern cryptography is stated. Then, the relationship between chaos theory and cryptography is compared. Last, the detailed descriptions of various chaotic ciphers are given, including chaotic stream ciphers, chaotic block ciphers, chaotic public-key ciphers, other chaotic ciphers, chaotic image encryption, and key agreement based on chaos.(2) The cryptosystems proposed by Pareek et al. are analyzed and improved. First, a generalized description of Pareek's cryptosystems is given. Then, their inherent weaknesses as well as some redundancies that contribute little to the security are presented. Based on these analyses, an improved cryptosystem is proposed. In the improved scheme, all the existing weaknesses of Pareek's cryptosystems are eliminated by different approaches. The redundant operations are also abandoned to simplify and accelerate the cipher.(3) A novel document encryption algorithms based on TDCM is proposed. By analyzing the feature of multi-dimensional chaotic systems and the flowchart of document encryption, the feasibility of employing multi-dimensional chaotic maps for document encryption is validated. A novel symmetrical cryptosystem based on discretized TDCM is proposed then. Three prevalent discretized TDCMs widely used in the existing image encryption schemes, i.e. standard map, generalized cat map, and generalized baker map, are adopted in the cryptosystem. Furthermore, it also gets rid of the problem of float-point number manipulation in traditional document encryption, as well as the origin problem in image encryption.(4) The concept, advantages, and state of the art of selective image encryption are introduced. The current research on selective encryption is classified into several main directions, and a brief introduction of these directions is given with performance remark, respectively. Based on these statements, a new selective image encryption method utilizing spatiotemporal chaotic systems is proposed. First, the feasibility of selective image encryption on bitplane is investigated. The effectiveness of selectively encrypting several significant bits is discussed and validated. Then, a selective image encryption algorithm based on CML is proposed to encrypt gray-level and GRB images, respectively. In order to achieve a good tradeoff between security and efficiency, the selection criteria and the corresponding security are also discussed. It is concluded that only selectively encrypting 50% of the whole image data can gain an acceptable security.(5) The security of key agreement protocol based on Chebyshev map is discussed. First, key agreement protocols based on Chebyshev map is introduced. A key agreement protocol based on Chebyshev map proposed by Xiao et al. and its improved version are introduced and analyzed. Then, the security of the improved protocol is discussed in detail. Due to the weaknesses in password authentication procedures, two attacks can be launched to recover the secret key utilized to encrypt the Chebyshev map. It is furthermore pointed out with proof that any attempt along this line is redundant.(6) Conclusions, as well as future work, are given at the end of this thesis.