# Multi-Stage Quantum Secure Direct Communication Using Secure Shared Authentication Key
The multi-photon provides security features without the involvement of the key distribution step, and no entanglement is required. In the multi-photon protocol, photons act as the carrier to carry the quantum information by polarizing their quantum properties, i.e., \(|0\rangle\) and \(|1\rangle\). Given the advantages of multi-photon QSDC, it remains a challenge to enforce QSDC authentication on behalf of each entity. Throughout quantum communication, a good authentication component is essential to ensure the legitimacy of users. Moreover, the participants in the communication must be authenticated to secure the transmitted message from the man-in-the-middle (MITM) attack [11; 12]. The problem of today's authentication is the complex environment of networks. A network administrator, therefore, finds it very difficult to apply a consistent set of authentication, quality of service, and security schemes that are safe from security breaches and comply with regulations [13]. Unlike our previous work [14] that focused on increasing the transmission rates by reducing the transmission time in multi-photon over multi-stage protocol, here, the issue of authentication is focused. The aim of this paper is, therefore, to recommend a secure way to share the initial secret angle and authentication key between users over a multi-stage quantum channel. In this work, the symmetry key is applied to secure the user's data communications in QSDC protocol, where the same secret key is used to encrypt and decrypt the message. Therefore, the secret key must be exchanged securely over the communication channel, thus that the sender and receiver are symmetry on both sides. To solve the problem of initial communication in the authentication phase, a quantum handshake protocol is proposed to share Alice's secret angle and the authentication key [15]. In addition, an update function of the secret angle and the authentication key is proposed based on a mutually agreed algorithm. Figure 1: The framework of general Quantum Secure Direct Communication (QSDC) protocol. The proposed protocol in this paper sends multi-photon over the single-stage and implements bit-by-bit authentication to defend against a MITM attack. It should be noted that [PERSON] is a trusted party that does not mislead the information. This protocol will significantly improve the existing multi-photon works of references [7; 9] that need to share the secret angle at the onset of the transmission over the secret channel. This protocol significantly improves communication efficiency without losing security. The main contributions of this work are as follows:  
1. The secret angle and authentication key are generated by [PERSON], and no third party is involved in the process to enhance the confidentiality of the communication. Thus, no pre-shared authentication key and initial secret angle are exchanged between [PERSON] and [PERSON] at the onset of the communication, which occurs in public channels. 2. The secret angles and the authentication key are updated using a mutually agreed algorithm after a number of bits sent to increase the security level. 3. The QSDC protocol is operated in single-stage, and the authentication is applied bit-by-bit, thus reducing the transmission time, and the security check is done along with the data transmission process. The remainder of this paper is organized as follows. In Section 2, the importance of the authentication procedure in QSDC is discussed. Related works are presented in Section 3, while Section 4 presents the secure shared authentication key protocol. Section 5 shows the example of the proposed protocol. Section 6 discusses the performance analysis, while Section 7 provides the implementation issues of the proposed protocol. Finally, the conclusion of this paper is summarized in Section 8.