# Research on Secure Debugging Interaction of Sensor Nodes Based on Visible Light Communication
## 1 Introduction  
The wireless sensor network is composed of decentralized and self-organized sensor nodes. Once the network deployment, it is difficult to maintain the nodes, especially the nodes are deployed in an inaccessible area. The reparation and debugging of sensor nodes have to rely on the built wireless network infrastructure. Actually, data transmission during debugging interaction between sensor nodes and the cloud is mainly completed by radio wave communication (RWC) whose frequency is from 10 kHz to 3000 GHz, such as 2.4 GHz (Zigbee and Wi-Fi) or 800 MHz (GPRS), etc. Due to the broadcast nature of RWC, the transmitted data are vulnerable to eavesdropping. Additionally, by RWC way is difficult to debug the lost sensor nodes which cannot contact with other sensor nodes or cloud. In these lost sensor node cases, some failures in wireless communication of sensor nodes are related to malicious intrusion. That means it is no longer safe to transmitting any sensitive data (such as debugging information or codes) by RWC way. Even once the intruders obtain the debugging codes, they can further inject the malicious codes into the sensor node and continuously intercept more sensitive data. The previous studies rarely consider in improving the security of debugging interaction from the physical level of communication. In practice, most developers have to abandon these lost or abnormal nodes and replace them with backup nodes [1; 2; 3]. Meanwhile, visible light communication (VLC) adopts unidirectional propagation different from RWC with broadcast way. This unidirectional propagation guarantees intruders difficult to eavesdrop on and helps to reduce security risk. Therefore, VLC can provide more secure data transmission at the physical level. In addition, VLC can be taken as supplemental communication means of RWC. Once the sensor node is not able to recover wireless communication, some debugging information can be back to the cloud by VLC. This debugging information helps developers to analyze the lost reason and possibility of malicious intrusion and further improve the overall security of the current network. Current VLC studies generally require dedicated visible light sensing components or modules. For example, [PERSON] et al. [4] applied VLC to the access control system by introducing a special VLC circuit. The circuit composes of the photodiode, amplifier, comparator, and MCU. In order to increase transmission rate and solve the problem of multi-channel transmission, [PERSON] et al. [5] designed a duplexing indoor communication system that needs dedicated RGB light emitting diodes (LED), low pass filters, electronic amplifiers. Because VLC components are sensitive to environmental optical noise, [PERSON] et al. [6] proposed a solution to reduce the influence of optical noise. This solution requires an analog filter as the front-end receiver to preprocess the received light signal. Such extra hardware modifications for VLC are hard to be afforded by low-cost sensor nodes. That also becomes a bottleneck of the VLC applying in the sensor node device. Therefore, we need to explore how to apply VLC in low-cost or Commercial Of-The-Shelf (COTS) devices. In recent years, with the development of visible light technology, the sensor node (such as TelosB, Micaz) and smartphone equip various visible light components, such as LED, camera, and ambient light sensors. These components provide a potential capacity for VLC. In this paper, we implemented a VLC-based hybrid communication debugging system (HCDS) applied in smartphones and sensor nodes. In the system, the smartphone is taken as a VLC-based gateway that connects the sensor node with the cloud and completes debugging interaction. In the system uplink, debugging information from the sensor node is transmitted by the node's LED and received by smartphone's optical camera. In the system downlink, debugging codes are transmitted by the smartphone's flashlight and received by the node's ambient light sensor.