Secure, Efficient and Searchable File System on Distributed Clouds

Abstract

Many enterprises and personals are inclining to outsource their data to public clouds, but security and privacy are two critical problems cannot be ignored. The door of cloud provider may be broken, and the data may also be dug into by providers to find valuable information. In this paper, a secure and efficient storage file (SES_FS) system is proposed to distribute files in several clouds and allows users to search the files securely and efficiently. In the proposed system, keywords were transformed into integers and secretly shared in a defined finite field, then the shares were mapped to random numbers in specified random domain in each cloud. Files were encrypted with distinct secret key and scattered within different clouds. Information about keyword/file was secretly shared among cloud providers. Legal users can search in the clouds to find correct encrypted files and reconstruct corresponding secret key. No adversary can find or detect the real file information even they can collude all the servers. Manipulation on shares by one or more clouds can be detected with high probability. The system can also detect malicious servers through introduced virtual points. One interesting property for the scheme is that new keywords can be added easily, which is difficult and usually not efficient for many searchable symmetric encryption systems. Detailed experimental result shows, with tolerable uploading delay, the scheme exhibits excellent performance on data retrieving aspect.

This work is supported by Demonstration on the Construction of Guangdong Survey and Geomatics Industry Technology Innovation Alliance (2017B090907030), The Demonstration of Big Data Application for Land Resource Management and Service (2015B010110006). Qiong Huang is supported by Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2014A030306021), Guangdong Program for Special Support of Top-notch Young Professionals (No. 2015TQ01X796), Pearl River Nova Program of Guangzhou (No. 201610010037), and the National Natural Science Foundation of China (Nos. 61472146, 61672242).

A Algorithm of Transforming String to Integer

We here give a function $HF_{SToI}$ to convert a keyword to a distinct number in $\mathbb {Z}_p$ (where p is a prime number). Below, we give the detailed process.

• (1) Convert each character of the keyword kw into a binary data bkw.

• (2) For each binary data bkw, if its length is not an exact multiple, pad digit 1 in the front of bkw until its length is a multiple which is named fbkw.

• (3) For each l bit block of fbkw, convert it to a decimal digit.

• (4) Compute the product of all decimal blocks fbkw and get an integer number Ikw.

• (5) Compute $Ikw \mod p$.

In the function, there is a small probability that two or more strings are converted to the same integer in $\mathbb {Z}_p$. Figure 10 gives an example that converts string kw to a numeric value where p = 2017. In the example the string kw is converted to bitstring 0110101101110111, and then padded with 111111. Finally, the number of kw is 559 in finite field $\mathbb {Z}_{2017}$.

Fig. 10.

Sample data type conversion process