# Multi-Blockchain Structure for a Crowdsensing-Based Smart Parking System
## 2 Literature Review
### Multi-Blockchain and Privacy-Preservation  
The popular Bitcoin is based on a public blockchain structure for the whole world. It can be used around the world in a system that is not specific to a particular region, but it can take a long time for a deal to be made. Although several blockchains, such as Ethereum, have tried various improvements, such as consensus algorithm optimization, to provide transaction speeds suitable for sensor networks with smart contracts, the transaction speed improvement of public blockchains is currently insufficient [13].  
To overcome the limitations of blockchain services targeting the world, studies to develop blockchains with locality and to exchange information among various blockchains have been conducted [14]. In other words, to utilize the multi-blockchain, a communication scheme between two blockchains has been created, demonstrating the possibility of connecting several blockchains using protocol design [15]. A system that connects several blockchains with the routers of blockchain has also been proposed [16].  
The hottest research topics in MCS are the privacy preservation of a contributor to participate in MCS without worrying about privacy exposure and the incentive mechanism encouraging a lot of participation. In general, sharing the sensing data (e.g., identity, location, and life pattern) causes a privacy exposure problem, and thus the tradeoff problem between incentive payment and privacy preservation exists. Various mechanisms [a1-a4] to resolve the tradeoff problem have been proposed. The problem of location protection issue for task allocation (i.e., a process to recruit workers and assign them with sensing tasks) under untrusted server has been dealt with [17]. The personalized location protection has been designed, considering different protection demands of workers. A location aggregation method clustered users into groups for k-anonymity preserving, mitigating the incurred information loss [18]. An incentivizing data aggregation mechanism with privacy preservation has been proposed, incorporating workers' reliability and privacy costs and integrating data aggregation and data perturbation mechanisms [19; 20]. However, collaborating \(K\)-nodes for anonymity are required for data aggregation and perturbation, and may cause privacy problems if adversarial cooperative nodes exist.  
In this paper, we design a bridge node for 1:1 connection and information exchange between two blockchains. In a multi-blockchain, we utilize the advantage of connecting public blockchain and private blockchain; i.e., automating the incentive model through a public blockchain suitable for mobile crowdsensing, and delivering the sensitive information or difficult-to-contain information in a block on a private blockchain as a medium. Moreover, the private blockchain provides an environment for sharing the collected information among service providers in real time and for providing parking information to users. That is, the proposed structure includes a process for the secure delivery of sensing data, a transparent incentive payment process, and a process for sufficient information provision through sharing information by service providers. When data, such as photographs, are collected through public-blockchain-based mobile crowdsensing, the vehicle number or the face of a person may be directly exposed. To protect the privacy of the sensing information, the contributor receives the secure session information through the blockchain and directly delivers it to the service provider. Moreover, because the blockchain is unsuitable for delivering unstructured data, such as photos, we design for the contributor to deliver the data to the service provider. If the mobile device of contributor can preprocess the data, i.e., removal of sensitive information and transformation to structured data, a method that has been previously proposed [21; 22] can be applied to resist the passive attack from honest-but-curious service providers and to enhance the efficiency of data transmission.