# A New Blockchain-Based Multi-Level Location Secure Sharing Scheme
## 1 Introduction  
With the development of technologies such as smart phones [1], smart wearable devices [2], and the Internet of Things [3], the location of target objects can be collected for enterprises or governments to meet business needs, such as vehicle navigation [4], epidemic prevention [5] and social software. For example, since December 2019, the COVID-19 virus has spread all over the world. Scientific epidemic prevention and control measures can effectively curb the spread of the epidemic and reduce the population infection rate. For this reason, it is of great significance to grasp the location information of cases, suspected cases, and the close contacts of cases. For this, location-based services (LBS) [6; 7; 8] have been developed and popularized. Location information contains a large amount of private user information, which is a valuable information resource that should be properly managed. In the process of location sharing, if the location data are leaked, tampered with or forged, this will not only reveal the private information but also negatively affect the business of location acquisition services. Especially in a large-scale location-sharing system, containing a large number of location providers and location demanders, how to ensure information security and privacy protection has become a serious problem that cannot be ignored. Undifferentiated information sharing [9] can no longer meet the needs of user privacy protection in location information sharing; hence, a secure, efficient, multi-level location information-sharing scheme is required. Location data demanders should be allowed to obtain location data with limited accuracy. For example, a social software server should be allowed to obtain a larger location area to determine the city in which the user is located. However, a healthcare provider should be allowed to obtain a street-level area or even the exact user's location coordinates. This requires multi-level access control for location data. Traditional data-sharing schemes [10; 11; 12] use centralized servers to process and save data, and implement certain access control rules. For the centralized servers, users' privacy cannot be fully protected, even with the help of some privacy protection technologies, such as k-anonymity [13; 14], which will cause data loss. If the centralized server is attacked, large-scale data security problems will arise. Moreover, even if many users in the network already have relevant data, the centralized server must also participate in all data-sharing processes, making the burden of the centralized server very high. There is no centralized node or authoritative manager in the blockchain network. Blockchain is a distributed ledger and, based on the agreed specifications and consensus protocols [15; 16; 17], it is jointly maintained by the nodes of the blockchain network in a deut trusted environment. Smart contracts [18; 19] support various data-access strategies, such as attribute-based access control (ABAC [20; 21; 22], and role-based access control (RBAC) [23; 24; 25]. Data can be circulated in the blockchain network according to the custom rules. In addition, the data structure of the blockchain has a strong immutability. All the above properties endow the blockchain technology with an inherent advantage in the field of data sharing. The following problem needs to be solved: while effectively sharing location information, the user's location privacy must be protected, and the received data must be true and credible. To solve this problem, in this paper, we propose a new blockchain-based, multi-level secure location sharing scheme. In our proposed scheme, location information is defined as multiple different accuracies. Location information demands (\(LD\)s) are assigned different access levels, so they can learn the area where location information providers (\(LPS\)s) are located with different accuracies. Moreover, we ensure that the received data can be verified. Our main contributions are summarized as follows:  
1.