# Multi-Stage Quantum Secure Direct Communication Using Secure Shared Authentication Key
## 3 Related Works  
In the last few years, several authentication protocols for multiphoton QSDC have been proposed. The main objective of those approaches was to ensure that the messages are delivered securely to the legitimate parties. This section reviews several multiphoton QSDC's authentication approaches over multi-stage to validate the parties involved in quantum communication, discussing their basic concepts, and describing their advantages and drawbacks.  
In reference [20], a single-stage protocol was proposed to transmit the photon in a stage using secret unitary transformation. The security strength of this protocol relies on the secret value of the polarization angle, \(\theta\), and it must be secret along with the communication between [PERSON] and [PERSON]. Otherwise, the security of this protocol cannot be compromised anymore when the eavesdropper is able to get the value of \(\theta\). This protocol assumes that there is another protocol operated, such as a trusted certificate exchanged before the secure transmission begins. The authors in [9] presented a secure quantum communication over a single-stage, hereafter called it a Braided Single Stage protocol. The idea of Braided Single Stage protocol is to overcome the drawback of the three-stage protocol by reducing the overhead to encode the photons. Later, the IV Three-stage protocol [7] was proposed based on the adoption of an Initialization Vector (IV) to be a one-time pad key as the extra dimension of security for the three-stage protocol.  
All of the mentioned approaches utilized the three-stage protocol to handle the authentication process in the first phase by sharing the initial value of secret polarization angle \(\theta\) (e.g., 0\({}^{\circ}\), 25\({}^{\circ}\), 50\({}^{\circ}\),..., 180\({}^{\circ}\)) that is predetermined among users. The set is usually exchanged via a secret channel [21; 22] over the public channel or using face to face communication. However, the technique of exchanging the secret angle is omitted. Besides, the reviewed multi-photon approaches only assumed that the sender and receiver to be mutually trusted parties, and they are already authenticated to each other [7; 23]. The usage of the authentication key that is only known by legal parties is able to provide secrecy in the authentication of QSDC [16; 24; 25]. Hence, combining the unitary transformation with the authentication key has become an appropriate strategy to construct high-level security of the cryptographic protocol. Therefore, an important question in this research is how to share the secret angle and authentication key securely in the quantum channel? Most of the authentication schemes are based on pre-shared secret information, which is not so practical for the initial communication of trusted parties [15]. To mitigate these issues, a secure shared authentication key (SSAK) is proposed in this paper. The SSAK protocol shares the secret angle and authentication key using the quantum handshake mechanism. The main objective of the proposed SSAK protocol is to improve the efficiency of authentication and reduce the communication cost by avoiding public channel usage.