# Protecting Private Communications in Cyber-Physical Systems through Physical Unclonable Functions
## 2 State of the Art: Physical Unclonable Functions and Encryption in CPS  
Although security and CPS is one of the most interesting and popular research topics nowadays, encryption and security schemes for CPS are pretty standard. In particular, as in most application scenarios, these solutions can be classified into two groups: symmetric and asymmetric key schemes. Cyber-physical systems, in particular, are usually solutions exchanging information packets, not information streams. Thus, symmetric key schemes in CPS are usually focused on block ciphers [12] of cyclic redundancy checks (CRC) [13] which can detect and/or correct transmission errors caused by natural or intentional causes. This solution, nevertheless, is very inefficient as the amount of information to be exchanged grows. Nowadays, very large amounts of information need to be shared. In order to improve efficiency in these symmetric key solutions, hardware-supported schemes have been reported [14]. Different sequential circuits focused on encryption may be also found. Nevertheless, in real-time applications, even hardware-based block ciphers are not enough, and stream ciphers are required. Different proposals may be found, although most authors agree the encryption system must be as simple as possible, and the engineering cost must be put on key generation. In this sense, different techniques and pseudo-random number generators (PRNG) [15] for key creation have been described. These symmetric solutions, however, present a very well-known problem: key distribution. Although this is a problem in any system, in CPS where sparse and resource constrained devices must communication through very unsecure links and networks it is more critical if possible. To address this problem, asymmetric key solutions have been investigated. Different proposals to adapt cryptography based on elliptic curves to cyber-physical systems may be found [16]. In some occasions, even these mathematical procedures are mixed with nondeterministic effects such as temperature measures to improve the encryption entropy [17]. Employed algorithms are standard solutions such as ElGamal encryption [18; 19], although innovative public key infrastructure has also been reported to enable, for example, the transmission of signed information (using high-level data format as JSON) [20]. As a general problem of any of the previously described solutions, all of them require from devices to store a secret key in a ROM or non-volatile memory. A very risky situation in CPS. Other works have considered a totally new approach based on innovative characteristics of CPS. In particular, taxonomies to classify and analyze the attacks to be suffered by CPS [21] have been proposed. Based on these taxonomies, some intelligent solutions to detect and react to cyber-physical attacks in the most appropriate manner have been studied [5]. Besides, some domain-specific solutions may be also found, especially in the area of Smart Grids [22], industrial control systems [23], or feedback control loops [24]. Nevertheless, these approaches are totally reactive, and only valid to detect and react to attacks that are already running. Although these solutions are needed, schemes to prevent those attacks and keep the private information as a secret are even more important. In this work, we combine both approaches and define a preventive security scheme (a symmetric key encryption scheme), but which does not require to store a secret key in memory. This objective is reached thanks to physical unclonable functions (PUF). PUFs [25] formalize the idea of one-way functions, later named 'physical random functions', which consist of the use of systems' random nature properties to identify them [26; 27]. In particular, PUFs benefit from all these effects that are non-controllable or non-repeatable to create unique responses from systems to common excitations or challenges. Up to three different types of PUF have been reported: non-electric, electric, and intrinsic.