# Multi-Blockchain Structure for a Crowdsensing-Based Smart Parking System
## 3 Proposed System
### Public Blockchain-Based Data Provision Session and Incentive Management  
Figure 3 is a structural diagram of the proposed system, showing the functional modules and data flow of each entity with which the contributors, service providers, and users interact, and the bridge nodes through which the two blockchains communicate. The functions in the module of each blockchain can be implemented as smart contracts, and the functions of the bridge node can be implemented as a process. The overall processes of the proposed system consist of the following: the data provision process (1.1.1-1.3.2), data acquisition process (2.1.1-2.2.2), and reward payment process (3.1.1-3.2.2). The public blockchain consists of a data provision session module and an incentive management module. The data provision session module requests a session through a bridge node to a service provider operating in the private blockchain network, according to the request from the contributor. The data provision session module then provides the received session information to the contributor. The incentive management module discloses the incentive information on the block of public blockchains, for the contributors to obtain when claiming the incentive. The bridge node consists of a session management module and an incentive management module. The former exchanges the request of session information and the created session information between the public blockchain and the private blockchain, and the latter delivers the reward information. The private blockchain consists of a data gathering session module and a data processing module. The data gathering session module manages the session information created by the service provider, based on the session request transmitted through the bridge node. The data processing module  
Figure 3: Functional modules and data flow of the proposed system. calculates the effectiveness and reward amount of the contributor's data and provides the calculated incentive information. Based on the data request of a user, the data processing module searches for the service provider that can provide the requested data, and then forwards the request to that service provider. The following describes each blockchain in more detail. As shown in Figure 4, in the public blockchain, a contributor participates in crowdsensing, and a bridge node is involved. In this blockchain, smart contracts are used to obtain session information, to deliver data from contributors (i.e., named Request Session and Disclose Session Info. ), and to claim and pay incentives (i.e., named Disclose Incentive Info. and Claim Incentive). The purpose of the public blockchain is to incentivize and encourage many contributors, through mobile crowdsensing, to collect the latest data. The bridge node plays a role in providing data from the public blockchain tailored to the purpose of the private blockchain. Incentive storage (i.e., blockchain) has the information about the provided data and corresponding incentives. The information consists of the contributor (i.e., ID and public key), service provider (SP) in the private blockchain to which the contributor transmits the data, transaction ID used in the session request, session information encrypted with the public key of the contributor, use of contributed data, and disclosed incentive information. The contributor can decrypt the session information with its own private key to establish the data session with the service provider. During the performance of the data provision process, contribution information, SP ID, and transaction ID for session request are stored. Moreover, the use of contributing data and incentive information are stored for the reward payment process, and these pieces of information are mapped with the transaction ID of the session request. Contributors provide data (e.g., a picture of a parking lot) for mobile crowdsensing on the public blockchain. The contributors provide the data to the service provider, and in exchange, receive compensation.