# Multi-Stage Quantum Secure Direct Communication Using Secure Shared Authentication Key
## 5 Example of the Proposed Protocol
### Security Check Procedure
[PERSON]'s ability to obtain useful information is limited because the quantum states are transmitted in the quantum mechanical property, which means that the states are the non-orthogonal. The authentication keys and polarization states of photons are used to generate the non-orthogonal quantum states. It shows that the security relies on the no-cloning theorem, which means no one can make a copy of any unknown no-orthogonal states. Besides, when [PERSON] tries to measure the superposition states during the transformation phase, the result could be any non-orthogonal state. Thus, no information regarding the polarization angle is detected. Assuming that [PERSON] managed to intercept the transmitted photons from [PERSON], [PERSON] will resend the photons to [PERSON] after doing a measurement. However, [PERSON] cannot break the protocol because she does not know the polarization angles and authentication keys set by [PERSON] and [PERSON]. Let us assume that [PERSON] transmits a quantum state of \(\left|\varphi\right\rangle\), i.e., \(\left|1\right\rangle\) in \(45^{\circ}\) polarization angle and encrypts it using the authentication key. Then, [PERSON] tries to intercept them. [PERSON] will fail to get the \(\left|\varphi\right\rangle\) since she does not possess the secret polarization angle and the authentication key. In the proposed protocol, [PERSON] needs to guess two secret angles in high precision. [PERSON] can be detected if her polarization angle is not the same as [PERSON] and [PERSON] keys' possession. Since this protocol applies bit-by-bit authentication, [PERSON] cannot spend a long time analyzing the statistics of a large number of photons that she gets during her attack, or she will be exposed. Consequently, [PERSON]'s attack can be exposed as [PERSON]'s and [PERSON]'s measurements on the fake bit are not identical to the original one. Moreover, [PERSON] cannot gain any profit from this attack. #### 6.3.3 Beam Splitting Attack

In this attack, [PERSON] will locate the beam splitter between [PERSON] and [PERSON] to collect the photons illegally. Without being detected at [PERSON], [PERSON] will get the probability of 50% of the correct photons. However, this attack is not possible against the proposed protocol. Even if [PERSON] is able to collect the fraction of the transmitted photons without being detected by [PERSON], it is very difficult to find the secret polarization angles since the angle will never be disclosed publicly. In addition, the angles of polarization will be updated after a number of photons using the mutually agreed secret algorithm to maintain the level of secrecy and to obtain unconditional security [7; 28]. Apart from that, the newly updated keys will prevent information related to the keys and messages from being sniffed by an eavesdropper. ## 7 Implementation Issues of the Proposed Protocol

The security of the protocol proposed is based on how hard it is for an eavesdropper to construct the authentication key and secret angle that [PERSON] and [PERSON] exchange. [PERSON] has to find [PERSON]'s secret information using a probability of up to \(\frac{1}{2025}\) by intercepting their communications. The ease of implementation is one of the key factors in the design of QSDC. It can be observed that there are no entangled states, which are difficult to implement in the proposed protocol. The performance analysis showed that the proposed protocol is low-cost, and it can be implemented with today's technology using optical devices, as discussed in Section 4. As discussed in the literature, the proposed work intends to secure the information against an eavesdropper's attack. In the real implementation of the QSDC protocol, several practical issues need to be considered other than security issues. The practical issues that will impact the implementation of quantum communication are the noise and error in the quantum channel. Besides, photon loss may occur in the large communication distance due to the features of quantum mechanics [34].