# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 2 Security Challenges in 5G Network
### Communication Security  
The goals of 5G communications are to offer connectivity, low latency communication, high data bandwidth, and extensive signal coverage to any type of device in the 5G ecosystem. Consequently, the integration of 5G technologies and architectural changes will be updated in the 5G communication. However, some threats and challenges need to be mitigated in 5G mobile networks [49]. An efficient and reliable mobile network requires secure communication among various control entities. Enabling secure 5G communication will require integrating 5G core network elements and efficient key management entities. Furthermore, security parameters and key exchanges need to be frequently updated along with 5G core security network elements. In a 5G network, the overhead cost of security maintenance such as communication cost, battery life, bandwidth and processing power significantly increase because of the additional control entities such as core network elements, number of base stations, and subscribers. Therefore, a proper solution needs to be proposed to solve these challenges for future secure communication systems in the 5G network. The adversaries use various attack segments such as access networks, user equipment, and mobile operators core networks to launch an attack on 5G communication [50]. To help comprehend the challenges and security issues affecting on 5G network, Table 1 shows the security issues related to the 5G communication channel and the point of impact for each security issue. There are two categories in 5G core networks: user data traffic and control traffic. Both types are vulnerable to various security threats. However, the main security issue of control traffic is the inadequate IP level security. The security protocol in the application layer, such as Secure Sockets Layer (SSL) or Transport Layer Security (TLS), is used to secure the communication channel in the SDN-based 5G core network. The network has known TCP/IP security threats such as TCP sequence number attack, TCP session hijacking, SYN flooding, IP spoofing, eavesdropping attacks, TCP reset attacks [51]. Consequently, the use of an IP security mechanism that combines with a multilayer security mechanism is necessary. Securing the control channel using IPSec based security framework has been proposed by authors in [2; 53; 52]. Different SDN controllers are used to manage different network segments in SDN mobile networks [54]. The east/west traffic interface is utilized to initiate inter-controller communication (ICC) channel connecting various SDN controllers. The interface supports performing multiple network functions such as traffic and mobility management, network monitoring, and security policy synchronization by sharing control information. Therefore, securing the ICCchannel is important in guaranteeing the appropriate operation of the network functions. The entire system will be compromised if the core ICC channel is penetrated despite the condition of the rest of the network. The current SDN inter-controller channel is vulnerable to various web-based and IP-based attacks such as DNS hijacking, replay attack, DDoS, and IP port scan [55; 56]. 5G ICC will be imminently vulnerable to cyber-attacks and extensive physical threats such as human errors, technical failures, and disaster failures. Additionally, the only consideration for the current SDN communication system is the cyber-attacks impact. Authors in [56] introduced identity-based cryptography (IBC) based secure key exchange scheme in a multi-controller SDN network to secure the east/west-bound data transmission. However, the authors did not emphasize current physical threats and cyber-attacks. Recent advances in adaptive security provide a more resilient and flexible approach than industrial 5G security. Furthermore, the security solutions are mostly developed by different vendor proprietary solutions. Consequently, real-time synchronization and mix-and-match security systems are nearly impossible and difficult in current existing networks [57]. Therefore,