# Physical Layer Authentication in Wireless Networks-Based Machine Learning Approaches
## 3 Background
### Wireless Network
#### 3.1.3 Wireless Networks Physical Layer Threats  
The openness of wireless networks produces communication more vulnerable to attacks, which poses severe challenges for network security. Wireless networks have security vulnerabilities, such as [4; 6; 27; 32]:  
Eavesdropping: Unauthorized and unannounced interception of communications between devices. Through eavesdropping, the intercepted messages can be exploited for future illegal purposes. Eavesdropping attackers can be categorized as active eavesdroppers and silent eavesdroppers. The distinction is that active eavesdroppers acting as communication parties unintentionally send signals to transmitters, which CSIs can extract through estimation. On the other hand, silent eavesdroppers snoop on messages while being silent, where their CSIs are not available for transmitters. Therefore, this kind of threat can be divided into two types based on the manner of the attacker: interception and traffic analysis.  
* Interception: Eavesdropping is the most common attack on wireless devices' privacy. The attacker could find legitimate communication by snooping in the nearby wireless environment when the traffic transmits control information about the sensor network configuration.
* Traffic Analysis: The ability to track communication patterns to facilitate various types of attacks.  
Jamming: Blocks legitimate communications between devices by saturating a channel with noise, which can direct denial-of-service (DOS) attacks at the physical layer. In general, jamming attacks can be divided into proactive and reactive jamming.
* Proactive Jamming: Proactive jamming attackers spread interfering signals whether the legitimate signal communication is there or not. To save energy and toggle between the sleep and jamming phases, attackers sporadically spread random bits or normal packets into networks. Attackers sporadically broadcast either random bits or conventional packets into networks to preserve energy and rotation between the sleep and jamming phases.
* Reactive Jamming: Attackers that use reactive jamming can monitor the legitimate channel's activity. If there is an activity, the attacker transmits a random signal to interfere with the existing signal on the channel.  
Contaminating: Attackers seek to contaminate the channel estimate phase to gain unfair advantages in the communication phase that follows. In the same context, a feedback contamination attack means that the attacker can use falsified feedback to force the transmitter to command their beams to attackers different than the intended users.  
Spoofing: Attackers try to enter or corrupt legitimate communications by transmitting a deceiving signal with a higher power in the transmission phase between transceivers or monitoring the legitimate transmitter for sending a falsified signal between two legitimate  
Figure 1: A generic wireless OSI Model information consisting of the layers, main protocols, main attacks, security techniques.  
signals. This kind of attack has different implications, such as the intrusion of an adversary into the local network or injecting some falsified identity information. There are two types of spoofing attacks: identity spoofing attacks and Sybil attacks.