# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 3 The Importance of Quantum Key Distribution (QKD)
### The Challenges of QKD in 5G Implementation  
In general, 5G networking does not ensure encryption of data traffic. Rather it relies upon the top encryption sessions count on the end-user to ensure security maintenance and updates. Knowing that, end to end encryption will still be applied. A crucial prerequisite for unbreakable encryption is to secure the key exchange between different parties. However, nowadays, the well-known key exchange algorithms are believed to be vulnerable by large-scale quantum computers. There are two feasible routes to overcome this future risk namely quantum-resistant algorithms (QRAs) and quantum key dispersion (QKD). Each of the different approaches for QRA is based on robust mathematical proofs that reduce the power of the large-scale quantum computer. However, QKD is based on fundamental laws of quantum physics that provide strong communication security for any potential threats.  
Currently, 5G takes advantage of public-key infrastructure (PKI). It affects both user equipment (UE) authentication to the network and how control and management plane services are securely interacting with each other. This embrace of PKI is happening just as advances in quantum computing begin to make cryptographers nervous about the longevity of workhorse ciphers such as Rivest-Shamir-Adleman (RSA) and Elliptic Curve Cryptography (ECC). To date, the density of qubits in quantum computers has matched a Moore's Law curve of doubling every 18 months. If these trends continue, ciphers such as RSA will be broken and unusable before 2030. QKD provide a promising encryption strategy that ensures security by using the concept of quantum physics. For that, QKD is a promising measure to provide secure communication for a large capacity 5G network. However, improving the transmission capacity has led to many problems such as the limited research on the impact of strong carrier and carrier-free signal in the secure key rate in 5G networks. Moreover, there is limited research on the uncertainty of secret key rate effect on the QKD system in different bandwidths, cost, and power levels [64].  
QKD is a sample of quantum cryptography that depends on a distributed classical key between legitimate users to achieving unconditional security. This concept was proposed using the form of the QKD protocol (BB84). However, BB84 can be hacked if the attacker uses a system of higher dimensions without the consent of legitimate parties. This limitation can be countermeasure using devices dependence DI which allow the QKD protocol to perform in a separate dependent device model that limits the analysis in the statistical form and the security based on those statistical experiments results [65]. A long-distance QKD implementation is considered one of the vital security challenges in quantum. Quantum cryptography is based on the strength of the photon length before it fades away in optical fiber. The problem was solved by place a repeater in the fiber optic to work as an amplifier that strengthens the photons before they are lost. This process depends on reboots the optical signal as if taking a copy of the signal to secure the transmission losses. However, QKD cannot trust the repeater to transmit the quantum data in the long-distance which means that the range of QKD is limited presents a contradictory dilemma in quantum cryptography between distance and efficient communication rate that explains basically as the longer the distance the weaker the photon signals becoming the fewer photons reaches the destination. In the past 10 years, many successful pieces of research contributions strengthen the signals to travel longer distances [66]. On the other hand, each repeater in the communication channel secures the key can be revealed thus the confidentiality can be easily exposed. Rebooting the signal will indeed reduce the number of repeaters along the communication channel, but it cannot eliminate it.