# Protecting Private Communications in Cyber-Physical Systems through Physical Unclonable Functions
## 1 Introduction  
Cyber-physical systems (CPS) [1] are defined as unions between physical and computational processes. This new approach has opened a new era in industry (Industry 4.0) [2], education, and engineering. In CPS, feedback control loops [3] are employed to make physical and cybernetic processes evolve together. Several heterogenous components are interconnected to create pervasive systems supporting these mechanisms. This new paradigm is particularly interesting to support real-time control systems, fed by an information stream (biological signals, data from sensors, etc.) [4]. However, these new systems are also vulnerable to new, innovative, and more aggressive attacks, known as 'cyber-physical attacks' [5]. In fact, as CPS are made of many interconnected components, attacks may affect a critical component when non-relevant (and often less secure) elements fail and the failure propagates due to a cascade effect. Besides, CPS are vulnerable not only to cybercrimes, but also to physical attacks. Thus, in these new systems, not only must attackers accessing to thesystem through the communication networks be considered, but also people physically accessing and manipulating any device [6]. In this context, security for CPS is a critical issue. Many different schemes and solutions, then, have been proposed. Symmetric and asymmetric encryption schemes [7] or certificateless public key infrastructures [8] are probably some of the most common proposals. Nevertheless, those solutions and any other previously reported, finally require physical devices to store a secret key. Software components can protect secret keys through cyber-protection technologies, as they have no physical existence, only logical. However, hardware devices must include memory (ROM memory) to store permanent information such as secret keys [9]. This configuration is vulnerable to physical attacks (i.e., people physically manipulating the device), as the key could be read from the memory by unauthorized people. This risk, moreover, is especially relevant in CPS, as many deployments are isolated, unmanaged, and unattended [10]. New mechanisms to create or protect keys against any cyber or physical attacker are, then, required. Therefore, in this paper we propose a novel solution to protect communication links in CPS. The proposed encryption scheme is based on symmetric keys generated through physical unclonable functions (PUF) [11]. PUFs are systems whose response is unclonable, even if the manufacturing method of the system is known. They take advantage of certain naturally occurring physical properties (such as imperfections in dielectric materials) to create systems which are totally unrepeatable even by the original creator. In this paper, we are defining a new PUF based on magnetic materials which can suffer a spontaneous and unclonable modification of their properties thanks to the exchange-bias effect. This effect only needs an activation process that is done immediately after manufacturing the material. Modifications will depend on the room temperature and, probably (there is no conclusive evidence) on other environmental factors. Besides the activation process, the atomic structure of the material greatly affects the final behavior of this PUF. A unique sample of an activated material should be, then, divided into two elements to create the cipher and the receiver. The objective of this paper is to provide a mathematical and engineering framework for this novel encryption scheme and key generator system. It is described the proposed architecture and the physical foundations of the electronic device supporting the designed PUF. Moreover, using simulation techniques, the performance of the proposed solution is evaluated.