# Using Blockchain to Protect 3D Printing from Unauthorized Model Tampering
## 2 Design and Analysis  
### Choosing an Underlying Blockchain  
As a new technology followed with great interest, blockchain was initially used as the underlying network of digital currency (a famous example is bitcoin, which is the flat currency of the Republic of El Salvador). Now, blockchain is also involved in many industries including copyright protection, traceability, medical treatment records, etc. People create trust and credible networks for supply chains based on blockchain technology, which can ensure the records are accurate and reliable. Blockchain provides a way for each member (or node) of the entire network to obtain all the records of transactions. In this  
Figure 1: Types of 3D printing anti-counterfeiting methods.  
network, each member maintains its ledger and all the actions will be recorded. When new data are produced and stored, a block will be added to the previous block in a strict order. These blocks connect end to end and form a chain. Data on the chain can be transactions, assets, identity, or anything that can be described in digital form, and these records are permanent, transparent, and searchable. There are some reported blockchain-based 3D printing anti-counterfeiting works. [PERSON] et al. concluded that blockchain would improve additive manufacturing supply chains and act as a secure inter-organizational intellectual property management layer [20]. [PERSON] et al. developed a blockchain-based 3D printing solution to track data from the supply process of 3D-printed products [21]. The above two works focus on the data service of the 3D-printed product supply process. We pay more attention to protecting the security of the model during internet transmission which is a fragile link prone to infringement.  
When considering the blockchain industry, the first question is which blockchain to choose. In the manufacturing world, a very-high-throughput (characterized by transaction processing per second, TPS) blockchain is needed to meet the requirement of recording and processing a large amount of manufacturing data. Therefore, it is completely infeasible to use a bitcoin network to process manufacturing data since the TPS of a bitcoin is only 7 [22]. Ethereum was first proposed by [PERSON] in 2013, aiming to become a global open source blockchain platform able to support diverse applications. However, the throughput of Ethereum is also very low, around 15 TPS [23]. To improve the throughput capacity of a blockchain, some potential strategies, such as increasing the block size [24], sharding [25], and developing a second layer (also called two-layer strategies) [26], have been discussed and developed. Among them, sharding technology is very suitable for manufacturing scenarios because it considers both the security and the high throughput [27]. After examining the major characteristics of existing mainstream sharded blockchains (see Table 1), it is believed that the QuarkChain [28] network is one of the most effective platforms to implement the protection of the STL model. Because QuarkChain consists of two layers, the first layer consists of many sharding chains (or sub-chains) for recording detailed transactions. The second layer is a master chain used to confirm transactions and ensure the safety of the sub-chains. The number of sub-chains can be dynamically increased, so the overall throughput of the network will improve according to the needs of manufacturing. In a testing environment, a sharding blockchain can reach tens of thousands or even hundreds of thousands of TPS [28]. Moreover, it supports Turing complete smart contracts and can also satisfy the automatic execution of our various instructions [28]. A structure diagram of the QuarkChain system is shown in Figure 2.  
Figure 2: A high-throughput blockchain with a sharding framework.