# Multi-Stage Quantum Secure Direct Communication Using Secure Shared Authentication Key
## 2 The Importance of Authentication for QSDC  
The authentication procedure is significant to avoid the legitimate parties' identities being impersonated by an eavesdropper that contributes to the leak of secret messages. In order to avoid eavesdroppers' attacks, existing quantum cryptography protocols typically require a third-party authentication for [PERSON] and [PERSON] to initiate the communication. However, some drawbacks may occur when a third party is involved in the communication. Third-party authentication is typically based on the classical cryptography assumptions that utilize computational complexity. Another problem noted is [PERSON] can still attack the quantum communication channels after [PERSON] and [PERSON] are authenticated to circumvent the third party authentication, leaving such attack undetected as revealed in reference [16]. This type of attack is called the Intercept-Resend attack, which will be discussed later in this paper.  
Thus, to overcome the aforementioned problem, the authentication process should be performed during the quantum data communication process. It is important to ensure that the parties involved in the protocol are whom they are supposed to be; this is what we call identity authentication. In addition, the data origin authentication can be done on a bit-by-bit basis to eliminate the possibility that [PERSON] attacks the quantum channel after [PERSON] and [PERSON] are authenticated. This step is important to ensure that the data originate from the right sender and is received by the right receiver. The important part of the authentication is to share the authentication key between [PERSON] and [PERSON] to allow the recognition process [16]. The authentication key is important to avoid a MITM attack launched by [PERSON], who is pretending to be [PERSON] and [PERSON] at the same time. The authentication key must not be revealed to [PERSON] and needs to be shared secretly between [PERSON] and [PERSON] before the process of authentication has started. This process is done in the initial part of the authentication by sharing the sender's secret angle and the authentication key since pre-shared is not suitable to be adapted because it deals with the public channel.  
In quantum communication, it is essential to reduce the level of complexity and, at the same time, maintain the security of the system against eavesdropping attacks. However, a direct link between [PERSON] and [PERSON] will significantly be affected where an eavesdropper can overhear, jam, or spoof the communication between them [17]. Recent work by [18] showed that an eavesdropper might span into multiple smart attackers that can overhear the secure message depending on their location between the transmitter and the receiver. In addition, the investigation by reference [19] proved that an attacker might use several transmitter antennas to increase the attack rate. It can be concluded that the eavesdroppers can utilize various types of attacks to intercept secret messages. Thus, the improvement of data and transmission secrecy is an urgent issue to be solved in the communication area.