# A Hybrid Encryption Scheme for Quantum Secure Video Conferencing Combined with Blockchain
## 3 Problem Formulation
### System Model  
The system solution architecture of the quantum secure video conference system combined with blockchain is shown in Figure 3. The architecture comprises users (main site and sub-site), the video conference system server (VCSS), quantum key management equipment (QKME), QKD equipment, classical channels, and quantum channels, wherein, the role of each component is as follows:  
1. Users: store video conferencing software, application platform for meeting minutes' recording, and local quantum key database. Among them, the video conferencing software includes a quantum-key-based \"one-time pad + AES\" encryption and decryption module, a classical key-based AES encryption and decryption module, and a QKD real-time monitoring module. Users of the main site can switch between quantum encryption and classical encryption, and it include necessary hardware for video conferences such as cameras, microphones, and monitors. The meeting minutes application platform uses the quantum key to encrypt the meeting minutes and obtains the key index value of the meeting minutes, together with the key ID used as a transaction, which is written into the blockchain by the miners. In order to facilitate users to query  
Figure 1: Leaf node structure. Figure 2: B+ tree structure. the minutes multiple times, the transaction information on the blockchain is written into the application platform cache. 2. VCCSS: VCCS creates multiple tables in the database to store the symmetric quantum keys generated by each QKD device. 3. QKME: QKME stores QKD real-time monitoring software and quantum key management software (QKMS). The QKD real-time monitoring software checks the key generation status of the QKD equipment and sends a timeout heartbeat response to the user when there is a problem with the key generation. QKMS is used to store and manage quantum keys generated by QKD devices. 4. QKD equipment: It adopts the quantum key distribution equipment based on time phase encoding, which is mature and widely used. The generation of quantum keys and the transmission of quantum states are based on quantum channels. 5. Blockchain: It stores the cipher text information of the video conference text minutes and has the query function. 6. Classical channel: used to transmit classical data. 7. Quantum channel: used to transmit quantum states and generate keys. In the model of the video conference system, we set up a standard star quantum communication network, which needs to deploy QKD devices to realize the secure communication with each other. The specific structure is as follows:  
(1) Quantum key storage: The QKD equipment completes key agreement through the quantum channel and stores the Bob-Alice symmetric key in the QKME in the form of a file. [PERSON]'s QKME creates multiple key files according to the number of [PERSON]'s and stores the symmetric key matching [PERSON]. [PERSON]'s QKME only stores the local key. [PERSON]'s user and [PERSON]'s server, respectively, read the key from the key file and store the key in the local database. (2) Hybrid encryption scheme: The user obtains the key from the database, and according, to the security level of the video conference data, the main site user chooses to use the key scheme of \"one-time pad + AES\" or AES to encrypt audio and video data. The encrypted data are transmitted to the server of the video conference system through the classical channel. After the server decrypts and encrypts them, they are sent to the other user. The user uses the local key to decrypt and display the audio and video data. The running process is shown in Figure 4.  
Figure 3: Scheme model. (3) The real-time monitoring of QKD is shown in Figure 5. The QKME monitors the key file in real-time to check whether the file size is updated.