# An Overview of Key Technologies in Physical Layer Security
## 1 Introduction
### Overview of Cryptography and PLS  
Cryptography is the method of transforming data into an unreadable format so that only the authorized recipient can understand and be able to decode it [22]. The main process of cryptography is shown in Figure 1. Encryption uses coding to transform plain text into an unreadable format, whereas decryption uses a decoding process to convert the unreadable text to a piece of readable information using some special keys. Cryptography can be divided into three main types: hash functions, public key cryptography, and secret key cryptography. The latter uses only one digital key to encrypt and decrypt data for both the sender and the receiver. Meanwhile, the first mentioned type utilizes a pair of public digital keys. In this case, one key is a public key used by the sender to encrypt the message, and the other key is a secret key used by the receiver to decrypt the information. Lastly, hash functions are a type of cryptography method that uses a hash value of a fixed length encrypted into the plain text. Hash functions use algorithms to generate a digital fingerprint to create one-way encryption [22]. In most cases, cryptography is the main technology utilized to address security issues for conventional and some modern-day electronic communication systems. From another perspective, some novel technologies, mainly quantum computing, are a threat to systems that are based on cryptography. Quantum computers have close to unbounded computing capabilities and can easily break encryption and decryption keys. The ability to guess the secret keys or perform a quick reverse calculation using a quantum computer enables breakage of such keys, and this gives unauthorized or disguised network users the ability to intercept ongoing data transmissions or access the data [4]. With that being said, it is worth noting that quantum computing is limited to some extent as it cannot break all kinds of cryptographic algorithms. This means that it is one of the technologies which jeopardizes some of existing systems based on cryptography. Furthermore, the processes involved in cryptography can impose delays which can be unwelcome in some applications, such as fifth-generation (5G) ultra-reliable low-latency communication (URLLC) [24]. Moreover, cryptographic methods are inefficient in terms of energy consumption as they require extra resources for performing computations. Therefore, this calls for the need to implement new measures with an effort to augment  
Figure 1: The process of cryptography [23]. cryptography. One of the technologies directed towards augmenting cryptography is physical layer security (PLS) [6; 7; 25]. PLS is different from cryptography technology because it is based on the concept of information-theoretic security proposed by [PERSON] [26]. The concept of PLS describes communication between two authorized users in the presence of an unintended user by modeling a discrete memory-less wiretap channel [27]. Figure 2 shows the general case of the wiretap channel where two authorized users communicate over the main channel and are observed by an eavesdropper through a wiretap channel. Figure 3 illustrates the fundamental differences between cryptography and PLS [28]. In contrast to cryptography, PLS schemes can seamlessly prevent unintended users from intercepting data signals. PLS is able to facilitate security without any form of encryption in the upper layers. The facilitation of key-free encryption is made possible by the exploitation of some wireless channel characteristics through the application of suitable signaling and channel coding [29]. PLS techniques have proven capable of realizing verifiable security even when the network intruders have almost limitless computational resources. Despite the unparalleled benefits of PLS, it is worth noting that some shortfalls exist. It was shown in [30] that it is almost impossible to warrant maximal security with a probability of one since PLS relies mainly on the average information.