Lianhua Chi

Fast Graph Stream Classification Using Discriminative Clique Hashing

As many data mining applications involve networked data with dynamically increasing volumes, graph stream classification has recently extracted significant research interest. The aim of graph stream classification is to learn a discriminative model from a stream of graphs represented by sets of edges on a complex network. In this paper, we propose a fast graph stream classification method using DIscriminative Clique Hashing (DICH). The main idea is to employ a fast algorithm to decompose a compressed graph into a number of cliques to sequentially extract clique-patterns over the graph stream as features. Two random hashing schemes are employed to compress the original edge set of the graph stream and map the unlimitedly increasing clique-patterns onto a fixed-size feature space, respectively. The hashed cliques are used to update an “in-memory” fixed-size pattern-class table, which will be finally used to construct a rule-based classifier. DICH essentially speeds up the discriminative clique-pattern mining process and solves the unlimited clique-pattern expanding problem in graph stream mining. Experimental results on two real-world graph stream data sets demonstrate that DICH can clearly outperform the compared state-of-the-art method in both classification accuracy and training efficiency.

Nested Subtree Hash Kernels for Large-Scale Graph Classification over Streams

Most studies on graph classification focus on designing fast and effective kernels. Several fast subtree kernels have achieved a linear time-complexity w.r.t. the number of edges under the condition that a common feature space (e.g., a subtree pattern list) is needed to represent all graphs. This will be infeasible when graphs are presented in a stream with rapidly emerging subtree patterns. In this case, computing a kernel matrix for graphs over the entire stream is difficult since the graphs in the expired chunks cannot be projected onto the unlimitedly expanding feature space again. This leads to a big trouble for graph classification over streams - Different portions of graphs have different feature spaces. In this paper, we aim to enable large-scale graph classification over streams using the classical ensemble learning framework, which requires the data in different chunks to be in the same feature space. To this end, we propose a Nested Subtree Hashing (NSH) algorithm to recursively project the multi-resolution subtree patterns of different chunks onto a set of common low-dimensional feature spaces. We theoretically analyze the derived NSH kernel and obtain a number of favorable properties: 1) The NSH kernel is an unbiased and highly concentrated estimator of the fast subtree kernel. 2) The bound of convergence rate tends to be tighter as the NSH algorithm steps into a higher resolution. 3) The NSH kernel is robust in tolerating concept drift between chunks over a stream. We also empirically test the NSH kernel on both a large-scale synthetic graph data set and a real-world chemical compounds data set for anticancer activity prediction. The experimental results validate that the NSH kernel is indeed efficient and robust for graph classification over streams.

Lightweight key management on sensitive data in the cloud

As cloud servers may not be trusted, sensitive data have to be transmitted and stored in an encrypted form. Major challenges for users are from the management (storage, update, protection, backup, and recoverability) of keys that can help users to decrypt authorized data available on the servers. In this paper, we propose a versatile approach for extremely lightweight key management, which is one of the most basic security tasks in cloud systems. In the multiple data owners scenario, each user only needs to manage a single key by our approach. With the help of the single key and a set of public information stored on the server, users can decrypt all authorized data from different data owners. Specifically, our paper proposes a novel access control model, proves the correctness and security, and analyzes the complexity of the model. Experimental results show that our approach significantly outperforms the single-layer derivation encryption and double-layer derivation encryption on the lightweight performance. Copyright

Hashing Techniques: A Survey and Taxonomy

With the rapid development of information storage and networking technologies, quintillion bytes of data are generated every day from social networks, business transactions, sensors, and many other domains. The increasing data volumes impose significant challenges to traditional data analysis tools in storing, processing, and analyzing these extremely large-scale data. For decades, hashing has been one of the most effective tools commonly used to compress data for fast access and analysis, as well as information integrity verification. Hashing techniques have also evolved from simple randomization approaches to advanced adaptive methods considering locality, structure, label information, and data security, for effective hashing. This survey reviews and categorizes existing hashing techniques as a taxonomy, in order to provide a comprehensive view of mainstream hashing techniques for different types of data and applications. The taxonomy also studies the uniqueness of each method and therefore can serve as technique references in understanding the niche of different hashing mechanisms for future development.

Graph hashing and factorization for fast graph stream classification

Graph stream classification concerns building learning models from continuously growing graph data, in which an essential step is to explore subgraph features to represent graphs for effective learning and classification. When representing a graph using subgraph features, all existing methods employ coarse-grained feature representation, which only considers whether or not a subgraph feature appears in the graph. In this paper, we propose a fine-grained graph factorization approach for Fast Graph Stream Classification (FGSC). Our main idea is to find a set of cliques as feature base to represent each graph as a linear combination of the base cliques. To achieve this goal, we decompose each graph into a number of cliques and select discriminative cliques to generate a transfer matrix called Clique Set Matrix (M). By using M as the base for formulating graph factorization, each graph is represented in a vector space with each element denoting the degree of the corresponding subgraph feature related to the graph, so existing supervised learning algorithms can be applied to derive learning models for graph classification.

A Cloud Model-based Approach for Facial Expression Synthesis

The process to synthesize feature for human facial expression often implies both fuzziness, randomness and their certain relevance in image data. By using the advantage of cloud model, this paper presents a new approaches and applications for comprehensive analysis of human facial expression synthesis using cloud model, in order to realize the rapid and effective facial expression processing in analysis and application. It gives the comprehensive analysis for the fuzziness and randomness of facial expression feature and the relationship between them based on cloud model, including the new method of facial expression synthesis with the uncertainty. It proposes the method of facial expression feature synthesis by cloud model, using the three numerical characteristics (Expectation, Entropy and Hyper Entropy) as the features and concepts of facial expression with its fuzziness, randomness and certain relevance in them. Through such three numerical characteristics, it introduces the framework of facial expression synthesis and the detail procedures based on cloud model. It puts forward the synthesis method of facial expression and gives the concrete realization and the implementation process. The facial expressions after synthesis can express the different expressions for one person, and it can meet a variety of demands for facial expression. The experimental results show that the proposed method is feasible and effective in facial expression synthesis.

Comprehensive and efficient discovery of time series motifs

Time series motifs are previously unknown, frequently occurring patterns in time series or approximately repeated subsequences that are very similar to each other. There are two issues in time series motifs discovery, the deficiency of the definition of K-motifs given by Lin et al. (2002) and the large computation time for extracting motifs. In this paper, we propose a relatively comprehensive definition of K-motifs to obtain more valuable motifs. To minimize the computation time as much as possible, we extend the triangular inequality pruning method to avoid unnecessary operations and calculations, and propose an optimized matrix structure to produce the candidate motifs almost immediately. Results of two experiments on three time series datasets show that our motifs discovery algorithm is feasible and efficient.

Efficient authorisation update on cloud data

To broaden the adoption of cloud computing, it is necessary to provide efficient security mechanisms for authorisation update, which is a core component of cloud security. In this paper, we propose an efficient and secure authorisation update mechanism, which is achieved using a double-layer encryption: inner-layer encryption and outer-layer encryption. The inner-layer encryption is performed on an original plaintext to generate ciphertext, while the outer-layer encryption is performed on a part of the inner-layer ciphertext taking a ciphertext as output. The inner-layer encryption enforces the initial authorisation policy, while the outer-layer encryption reflects the updated authorisation policy. Based on the double-layer encryption, we deal with the operations related to authorisation update including user update and data update. In addition, we implement the proposed mechanism and conduct extensive experiments. The experimental results demonstrate the efficiency and practicality of the proposed mechanism.

Lightweight Management of Authorization Update on Cloud Data

While outsourcing data to cloud, security and efficiency issues should be taken into account. However, it is very challenging to design a secure and efficient mechanism supporting authorization updates. In this paper, we aim to provide a mechanism supporting authorization updates which only incurs a lightweight cost of authorization updates and meanwhile supports a high level of security. This mechanism is consisted of two encryption schemes performed in different layers. The inner-layer encryption scheme is performed on the original plaintext and the generated cipher text is called inner-layer cipher text, while a part of the inner-layer cipher text is encrypted by the outer-layer encryption scheme to generate cipher text, called out-layer cipher text. These two encryption schemes are both performed by data owner. The inner-layer encryption realizes the initial authorization policy, while the outer-layer encryption reflects the updated authorization policy. We implement the proposed mechanism and conduct extensive experiments. The experimental results demonstrate that the proposed mechanism outperforms previous existing approaches, e.g. single-layer encryption and double-layer encryption.

Hashing for Adaptive Real-Time Graph Stream Classification With Concept Drifts

Many applications involve processing networked streaming data in a timely manner. Graph stream classification aims to learn a classification model from a stream of graphs with only one-pass of data, requiring real-time processing in training and prediction. This is a nontrivial task, as many existing methods require multipass of the graph stream to extract subgraph structures as features for graph classification which does not simultaneously satisfy “one-pass” and “real-time” requirements. In this paper, we propose an adaptive real-time graph stream classification method to address this challenge. We partition the unbounded graph stream data into consecutive graph chunks, each consisting of a fixed number of graphs and delivering a corresponding chunk-level classifier. We employ a random hashing function to compress the original node set of graphs in each chunk for fast feature detection when training chunk-level classifiers. Furthermore, a differential hashing strategy is applied to map unlimited increasing features (i.e., cliques) into a fixed-size feature space which is then used as a feature vector for stochastic learning. Finally, the chunk-level classifiers are weighted in an ensemble learning model for graph classification. The proposed method substantially speeds up the graph feature extraction and avoids unbounded graph feature growth. Moreover, it effectively offsets concept drifts in graph stream classification. Experiments on real-world and synthetic graph streams demonstrate that our method significantly outperforms existing methods in both classification accuracy and learning efficiency.