# A New Blockchain-Based Multi-Level Location Secure Sharing Scheme
## 2 Related Works  
Using a centralized server to share location [9] is a common method. Dodgeball is one of the earliest location-based online social network (LSN) services; founded by [PERSON] and [PERSON] in 2000, it relied heavily on short message services (SMS). Users texted their location to the service, which then notified other users. Google launched the \"Latitude\" service in 2009. Google latitude allows users to update their locations by manually typing an address, so as to disable real-time tracking and hide their locationsfrom others. In WhatsApp [26], users are allowed to share their current coordinates to others over the system. However, if users share their location in a group consisting of individuals they have blocked, the blocked individuals will still be able to see their location. During the COVID-19 pandemic, Singapore and Australia used centralized apps [27], and sent data collected by a user's phone to a central database controlled by a national health agency or another governmental authority. In April 2020, Apple [28] and Google revealed that they were busy developing privacy-friendly contact-tracing technology, which is inspired by the ultra-privacy-sensitive DP-3T protocol. The protocol includes: (1) The use of Bluetooth. (2) The generation of random identification numbers by phones that change every 10-20 min. (3) An opt-in system. (4) The data being stored and processed on users' devices. In 2008, [PERSON] [29] proposed blockchain technology to prevent the double-spending problem using a peer-to-peer network. Although the blockchain was originally developed as a trading platform for virtual cryptocurrency, the application of this technology in different fields [30; 31] in recent years has been surprising. Among them, blockchain is expected to change the form of data management and sharing in many scenarios. [PERSON] et al. [32] proposed a blockchain-based personal data storage scheme UB-PPD. In UB-PPD, two types of transactions, namely, \(T_{access}\) for access control management and \(T_{data}\) for data storage and retrieval, are recorded in the blockchain as an automatic access controller. The user's personal data, including location data, are stored and accessed off-chain through Distributed Hash Table (DHT). [PERSON] et al. [33] proposed a blockchain-based, decentralized record-management system (MedRec) to process Electronic Medical Record (EMRs). Medical data are stored in care-providers' local database. Three types of smart contract, Registrar Contract (RC), Summary Contract (SC), and Patient-Provider Relationship Contract (PPRC), are used to manage the process of access control and data sharing. In the above two works, the requester cannot verify the received data, and the privacy protection of the data recorded in the blockchain is not considered. In UB-PPD, the nodes in the blockchain network can even freely access the user's personal data. [PERSON] et al. [34] proposed a blockchain-based medical-information-sharing scheme, Medblock. EMRs are stored in external databases, and the ciphertext and the summary information containing the storage address of the EMR are recorded in the blockchain. Any user can access the hash value in the blockchain to verify the data, while the access control protocol recorded in the block header is executed to allow legitimate users to obtain the ciphertext of the summary. Unfortunately, the access control in Medblock is inefficient. Blockchain, as a distributed database, is not suitable for recording heavyweight data. For location data, [PERSON] et al. [35] proposed a blockchain-based, multi-level, location-data-sharing scheme (BMPLS). Location requesters with different access levels can obtain location data with different accuracy, then use Order-Preserving Encryption (OPE) [36] and Merkel tree to verify them. However, BMPLS has the following serious problems: (1) The authenticity of OPE ciphertext, which is used to verify the authenticity of the location data, cannot be verified.