Smart contracts emerged as programs running on the blockchain. Security is one of the major concerns against smart contracts which also exist various vulnerabilities as for any other traditional programs. What was worse, security vulnerabilities in smart contracts may lead to irreversible economic losses. Hence, there is an apparent demand for security audits of contracts before deployment. In recent years, a large number of smart contract vulnerability detection tools have emerged. The methods used by these tools include formal verification, symbolic execution, machine learning, and fuzz testing. These methods can well analyze vulnerabilities, but there are still limitations. In this paper, we optimized and extended the Mythril symbolic execution tool. The optimized pruning algorithm improves the speed of symbolic execution, while the proposed detection algorithm for Transaction Order Dependence vulnerability expands the range of detecting vulnerability. In addition, a machine learning vulnerability detection model is introduced as an auxiliary detection method, which is used to build the complete smart contract vulnerability detection system. The experimental results show that the proposed system reduces the execution time, and improves the accuracy as well as the recall of vulnerability detection compared with the original Mythril tool.

In distributed storage systems, the replication mechanisms are usually used to ensure system reliability and data availability. Random replication is widely used in cloud storage systems to prevent data loss. Copyset Replication (CR) as a replication strategy, makes a nearly optimal trade-off between the number of scattered nodes and the probability of data loss. Compared with random replication, CR greatly reduces the probability of data loss caused by node failure. However, CR's random selection strategy makes it difficult to select the optimal copyset based on data characteristics such as calculation and storage. In response to this problem of CR, the Optimal Copyset Replication (OCR) proposed in this paper can select the optimal copyset according to the specified data characteristics and its corresponding node conditions. Finally, combined with Cyberspace Mimicry Defense (CMD) , we implemented OCR in a distributed object storage system and conducted related experiments. When the calculation type data reaches 300,000, the experimental results prove that compared with CR randomly selecting copyset, OCR reduces the data processing time by nearly 10% through selecting the optimal copyset. By setting relevant parameters, OCR can also ensure that the data distribution of each node is relatively uniform, and avoid data skew.

As the Internet and the big data system evolve rapidly, the deployment of distributed applications becomes widespread, promoting the development of Distributed File System (DFS). The existing defense technologies for DFS, such as detection or patching, mainly aim to protect the system from known attacks and vulnerabilities. However, it is difficult for those systems to solve the growing security issues from the unknown threats due to their passiveness and hysteresis. In this paper, we propose MDFS, a mimic defense theory based architecture for DFS with the capability to improve the data security. Mimic Defense (MD), a proactive defense embedded in MDFS, emphasizes dynamism, heterogeneity and redundancy. The key benefits of MD are transferring the attack surface as well as increasing the cost of modification.