# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 4 Quantum Key Distribution Categories
### QKD Based on Entanglement
The key point of using entanglement photons as the source for QKD is that the source can be untrusted. Which means, the security of QKD can be guaranteed even if the source is provided or controlled by opponents. Both [PERSON] and [PERSON] will acquire a particle from pair of entangled photons released by the entanglement photon source. The similarity of the E91 protocol and BB84 protocol is in the procedure of choosing the random basis for measuring purposes and reviewed it in the classical channel. Based on the principle of quantum, [PERSON] and [PERSON] will receive an opposite or identical result depend on the specific state of entanglement. Moreover, in the E91 protocol, detection of an eavesdropper can be performed using [PERSON]'s Inequality experiment. * BBM92 protocol is proposed in 1992 by [PERSON] et al. shortly after the E91 protocol is proposed by [PERSON] [100]. This QKD protocol is also categorized as entanglement-based protocol. The similarity between BBM92 protocol and BB84 protocol is privacy amplification, key sifting, and raw key exchange procedure. Some other DV Protocols  
The proposed protocol can be divided into two categories: One-way protocols and Two-way protocols. Some of the proposed protocols that fall into the category are as follows:  
One-way protocol:  
* Differential Phase Shift (DPS) protocol is proposed in 2002 by [PERSON] et al. [101]. This protocol is categorized as an entanglement-based protocol. The principle of quantum entanglement is used in designing this protocol. The advantages of this protocol compare to other protocol is due to its simplicity of the configuration, efficient domain time usage, and the robustness against an attack such as photon number splitting (PNS) [102]. * Round-Robin Differential Phase Shift (RRDPS) protocol is proposed in 2014 by [PERSON] et al. [103]. This protocol is attracting the researchers because of the protocol security characteristics; for example, the information leakage can be confined within the boundary without the knowledge of key bit error rate. However, there are still major doubts about the practicality of the implementation due to the difficulty in terms of the measurement device. Furthermore, from the theoretical aspect of security, the view on the optical attack remains unclear. * The Coherent One-Way (COW) protocol is proposed in 2005 by [PERSON] et al. [104]. This protocol was developed by utilizing the principle of photon entanglement. This protocol is categorized as entanglement-based QKD protocol. Furthermore, this protocol has advantages where the protocol is robust against photon number splitting (PNS) attack, resistant to low interference visibility, and efficient with distilled secret bits per qubit. In this protocol, the time function is used to encode the information. * S13 protocol is proposed by [PERSON] [105]. The similarity of the S13 protocol and BB84 protocol is in terms of the mechanism of quantum. The only difference between each protocol is the usage of private reconciliation using asymmetric cryptography and random seed. Two-way protocol:
* Ping-Pong QKD protocol [106] works by [PERSON], making sure that the Bell state is in specific pair of photons entangled inside freedom polarization degree where [PERSON] will transmit one photon, which is supposed to be lossless and noiseless, to [PERSON] by using the quantum channel. Next, [PERSON] will perform the encoding of the travelling qubit and return the qubit to [PERSON]. [PERSON] received the qubit again from [PERSON]. The remaining qubit that he will be accepted is either one of two encoded Bell states correlating with bit 0 or 1. [PERSON] will differentiate using Bell measurement. In the default ping pong protocol, the security demands the control mode and the message mode to be rotated, in which the qubit is measured by [PERSON] to check for error but will not return it. The ideal case for the Ping-Pong protocol is that [PERSON] can only obtain the onward qubit and restore the photon to a maximum mixed state [107].