Xiaochun Cheng

Topology control of ad hoc wireless networks for energy efficiency

In ad hoc wireless networks, to compute the transmission power of each wireless node such that the resulting network is connected and the total energy consumption is minimized is defined as a Minimum Energy Network Connectivity (MENC) problem, which is an NP-complete problem. In this paper, we consider the approximated solutions for the MENC problem in ad hoc wireless networks. We present a theorem that reveals the relation between the energy consumption of an optimal solution and that of a spanning tree and propose an optimization algorithm that can improve the result of any spanning tree-based topology. Two polynomial time approximation heuristics are provided in the paper that can be used to compute the power assignment of wireless nodes in both static and low mobility ad hoc wireless networks. The two heuristics are implemented and the numerical results verify the theoretical analysis.

Numeric characteristics of generalized M-set with its asymptote

The Mandelbrot set (M-set) formulated by fc(z)=z^2+c is important in chaos theory. It has been extended into generalized M set (k-M set) fc(z)=z^k+c by using different exponent k. The escape time algorithm is used to draw fractals, which need preset thresholds and a maximum iteration times as input. This paper presents and proves the upper and lower bounds of generalized M set thresholds. A new method is also introduced to draw k-M set when k is a rational number. This method uses asymptotes of k-M set to approach k-M set from the outside. We prove that k-M set is the limit of these asymptotes. Finally, some experiments are presented to validate our methods.

A cooperative particle swarm optimizer with statistical variable interdependence learning

Cooperative optimization algorithms, such as the cooperative coevolutionary genetic algorithm (CCGA) and the cooperative particle swarm optimization (CPSO) algorithm, have already been used with success to solve many optimization problems. One of the most important issues in cooperative optimization algorithms is the task of decomposition. Decomposition decision regarding variable interdependencies plays a significant role in the algorithms performance. Algorithms that do not consider variable interdependencies often lose their effectiveness and advantages when applied to solve nonseparable problems. In this paper, we propose a cooperative particle swarm optimizer with statistical variable interdependence learning (CPSO-SL). A statistical model is proposed to explore the interdependencies among variables. With these interdependencies, the algorithm partitions large scale problems into overlapping small scale subproblems. Moreover, a CPSO framework is proposed to optimize the subproblems cooperatively. Finally, theoretical analysis is presented for further understanding of the proposed CPSO-SL. Simulated experiments were conducted on 10 classical benchmarks, 10 rotated classical benchmarks, and 10 CEC2005 benchmarks. The results demonstrate the performance of CPSO-SL in solving both separable and nonseparable problems, as compared with the performance of other recent cooperative optimization algorithms.

Fractal property of generalized M-set with rational number exponent

Dynamic systems described by fc(z)=z^2+c is called Mandelbrot set (M-set), which is important for fractal and chaos theories due to its simple expression and complex structure. fc(z)=z^k+c is called generalized M set (k-M set). This paper proposes a new theory to compute the higher and lower bounds of generalized M set while exponent k is rational, and proves relevant properties, such as that generalized M set could cover whole complex number plane when k 1), and that k-M set can be divided into |p-q| isomorphic parts.

Degradation and encryption for outsourced PNG images in cloud storage

In cloud storage environment, users frequently store images and retrieve them by personal device, including mobile phones. However, the content of outsourced images will be leaked to the cloud server, which is not trusted by users. To protect the privacy of sensitive images, this paper proposes format-compliant degradation and encryption method for Portable Network Graphics PNG. More specifically, for PNG degradation, improved prefix method and noise generation methods are developed. Furthermore, for PNG encryption, a modified generalised-Feistel method is developed. Finally, our security analysis demonstrates that the proposed scheme is secure. Our experiment results also show that the scheme is efficient and practical.

A NOVEL FAST FRACTAL IMAGE COMPRESSION METHOD BASED ON DISTANCE CLUSTERING IN HIGH DIMENSIONAL SPHERE SURFACE

Fractal encoding method becomes an effective image compression method because of its high compression ratio and short decompressing time. But one problem of known fractal compression method is its high computational complexity and consequent long compressing time. To address this issue, in this paper, distance clustering in high dimensional sphere surface is applied to speed up the fractal compression method. Firstly, as a preprocessing strategy, an image is divided into blocks, which are mapped on high dimensional sphere surface. Secondly, a novel image matching method is presented based on distance clustering on high dimensional sphere surface. Then, the correctness and effectiveness properties of the mentioned method are analyzed. Finally, experimental results validate the positive performance gain of the method.

Efficient Identity-based Broadcast Encryption without Random Oracles

We propose a new efficient identity-based broadcast encryption scheme without random oracles and prove that it achieves selective identity, chosen plaintext security. Our scheme is constructed based on bilinear Diffie-Hellman inversion assumption and it is a good efficient hybrid encryption scheme, which achieves O (1) -size ciphertexts, public parameters and constant size private keys. In our scheme, either ciphertexts or public parameters has no relation with the number of receivers, moreover, both the encryption and decryption only require one pairing computation. Compared with other identity-based broadcast encryption schemes, our scheme has comparable properties, but with a better efficiency.

Multi-user Searchable Encryption with Coarser-Grained Access Control in Hybrid Cloud

In consideration of feasibility, searchable encryption schemes in multi-user setting have to handle the problem of dynamical user injection and revocation, especially to make sure that user revocation will not cause security issues, such as secret key leakage. Recently, fine-grained access control using trusted third party is proposed to resolve this issue, however, it increases the management complexity for maintaining massive authentication information of users. In this paper, we for the first time present new concept of coarse-grained access control and use it to construct a multi-user searchable encryption model in hybrid cloud. In our construction, two typical schemes are used, one is broadcast encryption (BE) scheme to simplify access control, and the other is single-user searchable encryption scheme, which can support two-phase operation and be secure when untrusted server colludes with the adversary. Furthermore, we implement such a practical scheme using an improved searchable symmetric encryption scheme, and security analysis shows that our scheme is secure.

Ubiquitous E-learning System for Dynamic Mini-courseware Assemblying and Delivering to Mobile Terminals

From 2000, mobile Learning (m-Learning) and ubiquitous Learning (u-Learning) has been the hottest research topic in e-learning, and now, integrating ubiquitous learning into mainstream of education and train has been the direction in the area, which demand new generational elearning system. The paper introduces our research efforts in this direction. Based on the key concepts, such as ubiquitous learning object, mini-courseware, a new generational ubiquitous e-learning system is designed, which can be used for new requirements in m-Learning and u-Learning environments. In the system, learning resource related to a course is encapsulated into different ubiquitous learning objects, and mini-courseware can be assembled dynamically with learning resource extracted from these ubiquitous learning objects, accordingly, a mini-courseware player is designed for the situation. Based on these work, a resource based ubiquitous e-Learning system is designed considering pedagogical requirements under m-Learning and u-Learning environment.

Detection of Application Layer DDoS Attacks with Clustering and Bayes Factors

One of the attacks observed against HTTP protocol is HTTP-GET attack using sequences of requests to limit accessibility of web servers. This attack has been researched in this report, and a novel detection technique has been developed to tackle it. In general, the technique uses entropy-based clustering and application of Bayes factors to distinguish among legitimate and attacking sequences. It has been presented that the introduced method allows for formation of recent patterns of behaviours observed at a web server, that remain unknown to the attackers. Subsequently, Bayes factors are introduced to measure anomaly of web sessions. The method performs reasonably well, against strategy and scope varying attackers.