## 2 Fundamental Concepts
### Wireless Adversary Models  
Due to the inherent characteristics of wireless communication, such as broadcast nature, openness and the superposition of the transmission medium, wireless networks are extremely vulnerable to security attacks. Within the field of security, an adversary refers to a wireless attacker who aims to disrupt or prevent transmitted signals from reaching the intended receiver. Therefore, it is important to consider adversaries when designing a secure strategy in order to measure the effectiveness of the proposed security system. Security attacks on wireless networks can be mainly classified into two types: active attacks and passive attacks [38]. The concept of passive or active attacks is typically to do with whether the adversary is actively injecting into the communication system or is merely listening to the ongoing transmissions [39]. Due to the various types of adversaries and the vast nature of attacks, there is a need to identify the assumptions, goals and capabilities on which these different types of PLS are designed and the potential challenges that could prevent PLS from succeeding.  
A relevant set of goals is important for the modeling of a rigorous and strong adversary model. Naturally, a stronger adversary, i.e., with prior knowledge or more resources, could be able to attack the wireless network more successfully. With the adversary model, the assumptions of the adversary include its environment and resources, such as competency, knowledge, equipment, devices, etc. The adversary aims to compromise and obtain secret data content from within the communication system. These sets of data can be the legitimate transmission data or the property of the authorized user, e.g., energy consumption [40; 41]. Due to their exposure to various types of attacks, communication channels are required to have determined capacities that enable them to resist and alleviate wireless attacks.  
Figure 1: Generic system model of an Alice–Bob–Eve channel.  
These capabilities provide the adversary with reliable interactions that are based on the context of secure transmission systems. In fact, many of the PLS approaches assume that [PERSON] has no knowledge of the eavesdropper's CSI as the adversary is passive (i.e., not actively modifying the data, just silently reading and observing the communication system). On the other hand, some studies have found that [PERSON] is sometimes assumed to know about the CSI of the eavesdropper [42; 43; 44]. Furthermore, active attacks have sometimes been observed, such as the denial of service, replay and node malfunction attacks that are employed against PHY security approaches. As a security community, we need to adopt a strong adversary model in which the adversary is cleverer and more active. Under the implications of an adversary being an active attack, intruders use more intrusive and aggressive methods that aim to deteriorate the received signal quality for the intended user.  
The essential characteristics of a secure transmission system include authentication, availability, integrity, access control, and secret dissemination [38]. These can be established through appropriately designed signal processing strategies and channel coding techniques. The adversary model that is used in most PLS approaches is different from that used by the traditional cryptography and security community. Therefore, for the purpose of addressing the challenges that face PLS, it is important to bridge the gaps between the various adversary models that are used by the different communities. In the current review paper, the PLS mainly focuses on the premise that the eavesdropper is passive, i.e., it does not communicate with the other nodes in order to conceal its presence.