# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 3 The Importance of Quantum Key Distribution (QKD)  
The importance of quantum is derived from its high security and reliability compared to the classical key distribution. The classical key distribution is deployed and tested using cryptography techniques that are widely used. A classical key distribution is based on mathematical equations and computational complexity, which used two different fundamental distribution techniques namely symmetric techniques and asymmetric techniques. The symmetric techniques are applied when both sender and receiver share the same secure, and access protected key. On the other hand, the asymmetric technique provides both public and private key that is preserved and protected. The private key is used to keep the confidentiality and authenticity of each party in the communication. Those security methods were acceptable and widely used until [PERSON]'s algorithm was introduced which is the quantum cryptography technique that are used to enhance the security of any communication in the network. For that matter, quantum was proposed as an advanced and efficient mathematical operation that undermine the security of RSA. Furthermore, quantum can defeat the discrete logarithm problem consequently undermined the Elliptic Curve Cryptography method that is based on providing small key size with the same RSA cryptographic strength [61]. The quantum key distribution which based on quantum key distribution proves its reliability, privacy of security and high level of confidentiality by avoiding eavesdropping and providing holistic, different, and new concept on encryption mechanism that is based on the law of quantum mechanics and logic of physics. It can be considered as an innovative technology that proposing a new idea of transmitting photon lights between sender and receiver to establishing a secure connection [62]. There are two main principles of quantum mechanics which quantum cryptography are based on namely the Heisenberg Uncertainty Principle and quantum no-cloning theorem. Some of the advantages of using QKD is related to the security that established secure communication that can replace the eventually breakable mathematical algorithm. Furthermore, QKD is virtually unbreakable. QKD prove its simplicity of use, regarding resource consumption, consume fewer resources compare to the classical key encryption. Finally, QKD can firmly detect and eliminate eavesdropping in any communication process. The reason is that quantum mechanics prevents and will not replicate any alteration in a photon. This is due to the quantum properties are strengthened by the unconditional security as proved by the no-cloning theorem and Heisenberg Uncertainty theory. The polarization mode of the photon will change when the eavesdropper measures a photon, and eavesdropping will be observed according to quantum physics. The sending key is ignored when a photon is intercepted, and the rest of the key may be safe to send [63]. When it comes to a key agreement and asymmetric encryption methods, other options other than using QKD are using quantum-safe techniques if long-term security is required. Furthermore, the other option is to merge the existing mechanisms with the quantum-safe processes to prevent any security degradation caused by technical designs. In asymmetric signature techniques, replacing present algorithms is no longer necessary. Signatures, in contrast with encrypted messages, cannot be hacked after they have been sent. Moreover, signature systems based on known primitives currently exist that are unaffected by quantum computers. These schemes are suited in some scenarios even though it is not replacing the current methods completely. A feature equal to QKD might be developed using purely symmetric methods unaffected by quantum computers in a theoretical world free from asymmetric key negotiation.