# Experimental Demonstration of Secure Relay in Quantum Secure Direct Communication Network
## 2 Experiment  
The schematic of the QSDC network is shown in Figure 1. It can support communication for local-area and large-scale networks. For users of the subnetwork, they can implement QSDC directly. The feasibility of QSDC has been demonstrated by several experiments [11; 12; 13; 18]. Here, we focus on the experimental study of the large-scale QSDC network. It requires a secure relay to build connections between the transmitter and the receiver. The architecture of the computationally secure relay is shown in Figure 2. The control systems (Rconsys and Tconsys) offer the calibration and compensation signals for the receiver and the transmitter. The ciphertexts are transmitted to the encoder after the quantum signals are detected and decoded. The system clock of the relay is controlled by the communication control system. With the help of the secure relay, our QSDC network can offer services to classical apparatus such as telephones of the user. The signals from the phone are encrypted using the post-quantum algorithm [4]. We then use the QSDC system to transmit the ciphertexts coded in the quantum state. The intermediate node receives the quantum signals and decodes them to recover the ciphertexts. Due to the lack of a private key, the intermediate nodes cannot decrypt the ciphertexts, even with quantum computers. Then, the ciphertexts are transmitted to the next node in the same manner until they reach the intended receiver. The receiver decrypts the ciphertexts with their private key and reads the message. Apparently, our QSDC network has end-to-end security. If the QSDC transmissions are replaced by classical communication, the network is just an example of a classical network as currently in use. The use of QSDC enhances the quantum security  
Figure 1: The architecture of QSDC network with computationally secure relay. The orange line denotes the classical channel. The green line denotes the quantum channel. PQC: post-quantum cryptography; Q-T: quantum transmitter; Q-R: quantum receiver. mechanically, because eavesdroppers cannot steal the ciphertexts in the transmission, and are confined to only the relay nodes. The ciphertexts are protected by PQC. To show the feasibility of the QSDC network, we make a real-time experimental test of the computationally secure relay. The experimental setup is shown in Figure 3. Since NIST has recommended CRYSTALS-KYBER as the key-establishment algorithm to be standardized [4], we choose it in our experiment. CRYSTALS-KYBER [20; 21] is an indistinguishability under chosen ciphertext attack (IND-CCA) secure key-encapsulation mechanism (KEM). The construction of KYBER follows a two-stage approach: first design an indistinguishability under chosen plaintext attack (IND-CPA) secure public-key encryption scheme, then use Fujisaki-Okamoto (FO) transform to construct the IND-CCA KEM. The security of KYBER is based on the hardness of solving the learning-with-errors problem in module lattices (MLWE), which is believed to be computationally hard to solve even with quantum computers. At the beginning of the experiment, we implement the algorithm of CRYSTALS-KYBER to generate the public key and private key. The public key is published to every participant of the network. The private key is only known to [PERSON]. In our experiment, we choose the parameter set of KYBER-512, which can offer 107 bits of core-SVP quantum hardness [20; 21]. We take advantage of the plug-and-play architecture to implement the DL04 protocol [9; 14]. The DL04 protocol contains four steps. First, the legitimate receiver prepares a  
Figure 3: Experimental setup of the QSDC network with a computationally secure relay. [PERSON] encrypts the message and sends the ciphertext to the relay through the quantum channel. [PERSON] receives the signals from the relay and decrypts the message after implementing the QSDC protocol. KYBER makes use of two hash functions H (SHA3-256) and G (SHA3-512) and a key derivation function KDF (SHAKE-256) in the realization.