# Trusted GNSS-Based Time Synchronization for Industry 4.0 Applications
## 1 Introduction
Among the others, the Precision Time Protocol (PTP) represents a well-known and widely adopted solution for accurate time distribution in a network of clocks organized in a master-slave hierarchy [16; 17]. Such a synchronization protocol, also known as IEEE-1588 standard [18; 19], allows for absolute time synchronization in the range of hundreds of nanoseconds through hardware assistance (SyncE) and, potentially, sub-nanosecond accuracy with the White Rabbit extension of PTP (WR-PTP) [20; 21]. Nevertheless, a conscious adoption of the Industry 4.0 paradigm also requires the introduction of appropriate cybersecurity solutions. In fact, cyberattacks and hacking of factory industrial control systems are dramatically increasing in recent years. Proper mitigation measures against these attacks are needed to avoid the emergence of an internet of insecure industrial things [22]. The same statement is also applicable to all the grids and telecommunications networks recognized as critical infrastructures [23; 24], especially the upcoming 5G [16; 17; 25] that is considered as a key enabl  
Figure 1: Simplified scheme of a Global Navigation Satellite System (GNSS)-based synchronization distribution network. 4.0 applications. In all these cases, possible synchronization inaccuracies can directly result in a degradation of the quality of service provided by the communication network (e.g., reduced throughput, increased latency and jitter) and, in extreme cases, in a complete disruption of the considered system or application. Two areas in Figure 1 represent likely targets for potential attacks (i.e., viable attack vectors) and, thus, need appropriate protection:  
1. _GNSS RF spectrum_, potentially targeted by attacks against the GNSS signals;
2. _Time Distribution Network_, potentially targeted by attacks to the synchronization information. We can further divide this category into: 1. [label=()] 2. attacks targeting the Precision Time Protocol over the network, and 2. attacks against the integrity of PTP software (SW) running at each node and the related configuration files. It is worth noting that recent scientific literature has extensively investigated attacks to the GNSS RF spectrum (i.e., attack vector 1) and to the PTP protocol and packets over the network (i.e., 2.a). Proper countermeasures against them are already available (e.g., refer to [26; 27; 28] and references therein). For this reason, the following sections will summarize these specific attack vectors, while their experimental assessment is out of the scope of this paper. On the other hand, to the best of the authors' knowledge, previous works have not widely covered the attacks to the PTP SW integrity within nodes (i.e., 2.b). This category of attacks is gaining special attention in both scientific and industrial communities, where the investigation of possible cybersecurity solutions is an active research topic, stressing the need for software integrity control at network nodes. For these reasons, we recognize this category of attacks and related solutions as especially relevant and challenging for Industry 4.0 applications and, thus, worth of further investigation. Motivated by this background, the paper has the following objectives:  
* An analysis of attack vectors for GNSS-based synchronization distribution networks, with a special attention to attacks not widely covered in prior work (i.e., 2.b);
* The proposition of a novel solution for PTP nodes augmented with Trusted Computing technologies to counteract attacks to their integrity, complementing existing security solutions that focus on GNSS RF Spectrum and PTP protocol only;
* A reference implementation on a testbed, capable to demonstrate the proposed solution by means of quantitative results. Our proposed approach differs from other authentication and authorization-based solutions already available in prior work and suitable to ensure the integrity of the network protocols and all type of exchanged packets.