# Using Blockchain to Protect 3D Printing from Unauthorized Model Tampering
## 3 Demonstration and Discussion  
To study the feasibility of our design, we designed an attack-defense demo referring to the previously mentioned [PERSON]'s experiment. Firstly, we bought a small UAV from 4 DRC (a UAV manufacturer) which was capable to fly for 30 min under normal takeoff conditions, and the blades of this UAV were replicable. Designer [PERSON] drew out the model STL file according to the size of the original blade and stored it on the computer connected to a 3D printer. The 3D printer used here was the photon mono SE light-curing printer from ANYCUBIC. The wavelength of the matrix light source was 405 nm, the Z-axis accuracy was 0.01 mm, the thickness of one layer was 0.01-0.15 mm, and maximum printing speed was 88 mm/h. Later, hacker [PERSON] (pretended by our lab companion) modified the mentioned drawing. [PERSON] implanted defects into the blade which could lead the fatigue development faster than expected during high-speed rotation. These changes were invisible to the naked eye from only checking the appearance of blades. See Figure 4c, where it is difficult to find out if one of them has a defect. [PERSON] replaced the defective STL file with the genuine one (created by [PERSON]) and all the access and change records were deleted. The original model (Figure 4a), invaded the blade model (Figure 4b), and their printed objects are shown in Figure 4c. To let readers see clearly, we created an obvious defect in Figure 4b, but these modifications can be hidden in a real illegal network intrusion. Figure 3: The process of hashing 3D model (obtaining the “fingerprints”) and storing “fingerprints” on a blockchain. Then, operator [PERSON] printed out the invaded blade model (she did not know this file was invaded) and installed it on the UAV. After normal takeoff, the UAV quickly fell from a 5 m height after the rotor broke, the overall flying time was less than 1 min, and this UAV was damaged (Figure 4d-f). The hacker's tampering affected the flight of the UVA and caused serious damages. As a comparison, if operator [PERSON] printed the right blade model, see Figure 4a, the printed blade can supported flying normally. Under the same meteorological conditions, the UAV can fly at a height of 5 m and safely land after completing various actions instructed by [PERSON] via remote control. To intercept the illegal blade model invasion, a further defense demonstration was carried out. The key idea is shown in Figure 5. After [PERSON]'s attack was complete, operator [PERSON] sent the dummy blade model MODEL-A' to the authorized printer which connected to a blockchain network. This authorized printer obtained the hashes of MODEL-A' firstly and compared them to the 4 DRC-UAV parts whitelist, which already included the hashes of approved MODEL-A. This authorized printer only printed out models once their hashes were on the whitelist, and MODEL-A' was not, and the process was terminated. Through this demonstration, people can believe that the printed products are qualified and from reliable models. Since UAVs may be used to perform important civilian or military missions, it is likely that the design of the UAVs or the credibility of the model may be important. Therefore, it is critical to automatically filter out inappropriate models by the network. In addition, the combination of AM and blockchain can not only reduce labor and time costs but also prevent material waste caused by printing out non-compliant parts. One must avoid misuse due to omission. Verifying the authenticity of drawings can prevent the waste of resources and reduce labor before the inappropriate product is printed. From the above demonstration, we showed that the STL model without blockchain protection was tampered with and manufactured easily. Problematic products installed on UAVs without verifying will cause UAVs to crash in minutes (as shown in Figure 4d). If the manufacturing scene is oriented to the key components of automobiles and planes, or if problematic products are directly mixed into the supply chain, grave consequences will happen.