## 4 Structure of SFrame Video Conferencing System
### SFrame's Key Management System
The host uploads a ciphersuite (\(ciphersuite_{A}\)), the server's signature for the verification key (\(Sig_{Server}^{A}\) (\(IVK_{A}\))), and their own signature (\(Sig_{A}\)). Subsequently, the client generates a public key (\(pk_{C}\)) and a private key (\(sk_{C}\)) using the host's ciphersuite. The client then uploads their public key (\(pk_{C}\)), the server's signature for the verification key (\(Sig_{Server}^{B}\) (\(IVK_{B}\))), and their own signature (\(Sig_{B}\)). Next, the host creates a session key (\(ss\)) using a random source extracted from a cryptographically secure RNG. The host encrypts the session key (\(ss\)) with the client's public key (\(pk_{C}\)), and uploads it. The client decrypts the ciphertext with the secret key (\(sk_{C}\)) to obtain the session key (\(ss\)). Finally, the host and client can securely encrypt/decrypt with the session key (\(ss\)) and share conference data. The next-generation video conferencing system depicted in Figure 5, which utilized the PQC KEM, addresses the integrity requirement by leveraging authentication and signing

Figure 5: Application of PQC KEM to GPKI-based video conferencing system. provided by the GPKI. Moreover, this system satisfies the confidentiality requirements through public key encryption. Hence, the PQC encryption protocol employed in the GPKI-based video conferencing system fulfills the E2 EE security feature requirements outlined in Table 1.

Figure 6 illustrates an example of applying NTRU KEM, one of the third-round candidates for the National Institute of Standards and Technology (NIST) PQC standardization contest, to a GPKI-based video conferencing system. When using a ciphersuite of NTRU KEM with a security level 1, such as 'ntruhps2048509', the size of the shared session key (ss) is 32 bytes, which is the same size as the _MK_ in Zoom [14]. First, the host (_A_) uploads the server's signature for the verification key and ciphersuite. The client (C) generates a private key (\(f\), \(f_{p}\), \(h_{q}\), s) and a public key (\(h\)) using the NTRU algorithm, and uploads the public key (\(h\)) and signature. Second, the host extracts a random value (\(r\), \(m\)) using a cryptographically secure RNG, and encrypts it with public key (\(h\)). During this procedure, the host creates a 32-byte session key (ss) with a random value (\(r\), \(m\)). Third, the client decrypts the ciphertext with their own private key (\(f\), \(f_{p}\), \(h_{q}\), s), and gets (\(r\), \(m\)). Finally, they generate a 32-byte session key (\(ss\)) with (\(r\), \(m\)) in the same way as the host. Consequently, the host and client both have the 32-byte shared symmetric session key (ss), without having exposed the key to the central server. The NTRU KEM was used as the PQC scheme shown in Figure 6, but other PQC schemes could also be applied to the proposed PQC encryption protocol. As a possible option, since CRYSTALS-KYBER has been selected as the PQC KEM standard after the third round of the NIST PQC standardization competition [15], applying this KEM to the proposed GPKI-based video conferencing system's E2 EE PQC protocol could contribute to improving the security of the next-generation video conferencing system. ### Security of GPKI-Based Video Conferencing Systems

In Sections 3 and 4, we reviewed Zoom, a commercially available video conferencing system, and an open library, SFrame. Both systems rely on environments without any PKI or CA that is admitted by external participants. However, in Korea, there exists a video conferencing system, On-Nara, based on the Korean GPKI [7]. Therefore, we aim to obtain security for next-generation video conferencing systems by applying PQC KEM to the

Figure 6: Application of NTRU KEM to GPKI-based video conferencing system. On-Nara GPKI system. As a result, the security of our GPKI-based video conferencing system could be enhanced to a quantum-safe level. We performed a comparative analysis of the GPKI-based video conferencing system. For analysis, we compared the NTRU KEM proposed in Section 5.2 with the On-Nara system.