Since Dimakis et al. [5] first introduced the idea of network coding [6] into DSS and proposed regenerating code to solve the problem of high bandwidth consumption within erasure correcting code [7], the security of cloud storage system based on regenerating code has begun to receive extensive attention from researchers. Oliveira et al. [8] used a Vandermonde matrix to design codes to increase the security capacity of a system, which indicates that regenerating codes can improve the security of a storage system. The data security strategy based on regenerating codes belongs to information-theory security. A number of simple encoding and decoding methods has been developed, though they have not limited the computing power of eavesdroppers. Hence, regenerating code is suitable for promotion and application in DSSs [9].

In recent years, research on using regenerating codes to ensure the security of DSS can be roughly divided into two categories. The first category focuses on deriving the corresponding upper bound of security capacity for different system models and eavesdropping models, and providing a regenerating code scheme that can achieve this upper bound, enabling the DSS to enhance its security. For example, Pawar et al. [10] used the max-flow min-cut theorem of graph theory [11] to solve the problem of node eavesdropping in homogeneous storage systems, and further constructed an encoding structure that can achieve the upper bound of the security capacity at the minimum bandwidth regeneration (MBR) point, enabling the system to achieve strong security. Rashmi et al. [12] used product matrix (PM) theory [13] and FR codes [14] to develop encoding schemes that meet strong security at MBR and minimum storage regeneration (MSR) points, respectively. Rawat et al. [15] and Goparaju et al. [16] used the linear subspace analysis method to give a new upper bound estimate of security capacity at MSR points, and constructed a secure storage code based on the maximum rank distance (MRD) codes [17,18]. Tandon et al. [19] studied the tradeoff between secure storage capacity and repair bandwidth, and obtained an improved upper bound on the security capacity that general regenerating codes can achieve. A team from Shanghai Jiao Tong University [20] considered the problem of multidimensional and multi-level secure regenerating codes, providing security constraints that can obtain MBR points. A team from Shandong University [21] gave a strong security coding design under the generalized cloud storage model. A team from the National University of Defense Science and Technology [22] proposed the concept of stationary MSR codes and obtained a fixed upper bound of the security capacity in linear MSR code scenarios.

The main objective of the above studies is to meet the strong security of the system. Although strong security does not reveal any information about the original file, it requires the introduction of a large number of random keys in the data symbols. As a result, the storage capacity is sacrificed, which is costly for cloud storage providers. Based on this, the second research direction on using regenerating codes to ensure data security is weak secure regenerating codes. The weak security nature of the system allows eavesdroppers to obtain partial information from the original file. However, they cannot decode any meaningful information of a single symbol in the original file [23]. Essentially, weak security does not introduce random keys and does not cause loss of storage capacity. Regarding the research on weakly secure regenerating codes, Kadhe et al. [24,25] proposed two external encoding structures using nested codes that can weakly protect PM-MBR codes and PM-MSR codes against eavesdropping. For MSR codes, Kadhe et al. [26] further proposed a generalized weakly secure encoding to meet practical application scenarios, where external encoding can be designed independently of internal encoding. Liu et al. [27] designed two types of weakly secure regenerating code schemes against eavesdropping attacks by combining the all-or-nothing transformation and a precise repair regenerating code strategy. Xu et al. [28] designed a heterogeneous encoding scheme, that satisfies weak security constraints, to address the issue of anti-link-eavesdropping in heterogeneous DSS. The team from the Beijing University of Posts and Telecommunications [29] analyzed the block security of PM-MSR codes, based on the Cauchy matrix, and proposed an improved MSR coding scheme to achieve optimal weak security.

It can be seen that research on strong or weak secure regenerating codes has achieved a series of results in recent years. However, strong security and weak security are only two properties of DSS. The former is too strict and the latter is too lenient. In practice, different applications may have different requirements for security levels [30]. Based on this consideration, this paper put forward a novel regenerating code structure with different security levels. The application of such a coding structure in cloud storage systems can not only ensure the availability and repairability of data, but also meet the personalized requirements of security level for cloud storage customers.