## 4 Structure of SFrame Video Conferencing System
### SFrame's Key Management System
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Property** & **Zoom** & \begin{tabular}{c} **SFrame** \\ **(with MLS)** \\ \end{tabular} \\ \hline Key Agreement & ECDH, HKDF & \begin{tabular}{c} HPKE (ECDH-based KEM) \\ (RFC 9180) \\ \end{tabular} \\ \hline Signature & EdDSA & \
\begin{tabular}{c} EdDSA \\ ECDSA \\ \end{tabular} \\ \hline Shared-Key Exchange & \(Enc(MK)\)\(\leftarrow\) Symmetric(\(K_{IJ}\)) &
\begin{tabular}{c} \(Enc(joiner\_secret,\) \\ \(path\_secret[]\) \\ \(\leftarrow\) Asymmetric(\(pk_{I}\), \(sk_{I}\)) \\ \end{tabular} \\ \hline E2 EE & Authenticity & \(\Circle\) & \(\Circle\) \\ \cline{2-3}  & Confidentiality & \(\Circle\) & \(\Circle\) \\ \cline{2-3}  & Integrity & \(\Circle\) & \(\Circle\) \\ \hline Transparency Tree & \(\times\) & \(\Circle\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison between Zoom and SFrame. can manage the list of public keys. The GPKI is an example of such an infrastructure that provides the necessary mechanisms for managing and distributing public keys securely. The GPKI, also known as an administrative electronic signature, utilizes GPKI certificates issued by certification authorities. These certificates serve as electronic information that verifies and provides evidence of the authenticity of signatures. Electronic information is issued to administrative, auxiliary, and assistant agencies, public infrastructure, banks, and users [13]. Figure 4 depicts GPKI-based and National Public Key Infrastructure (NPKI)-based certification schemes. First, the GPKI system uses a top-level certification authority (CA1) to manage a list of public keys for public service workers (\(A\), \(B\)). The CA1 issues a certificate (\(cert_{I}\)) containing a signature for the public key (\(pk_{I}\)) that signs with its private key (\(sk_{CA}\)). Second, the NPKI system uses another top-level certification authority (CA2) that manages multiple CAs. These multiple CAs maintain public keys for civilians (\(C\), \(D\)). Also, these CAs issue a certificate (\(cert_{I}\)) to civilians. These two authentication systems mutually recognize each other and collaborate to manage a certificate trust list. As CA1, CA2, and the CAs managed by them are trusted authorities, users can trust the public keys contained in the certificates they issue. ### End-to-End PQC Encryption Protocol Applicable to GPKI-Based Video Conferencing Systems

GPKI-based video conferencing systems employ user authentication mechanisms that differ from those used in the commercial sector. Unlike Zoom's login and signature method, and authentication through SFrame's credentials, GPKI-based video conferencing systems require a login with a certificate issued by a government agency. Consequently, if all meeting participants are affiliated with a government agency and possess GPKI-based certificates, there are no obstacles to their participation. Meanwhile, ordinary people who participate in meetings do not have GPKI certificates; therefore, another authentication method is required. Access through a provided link or a one-time password authentication method may be a possible method for authenticating those in the commercial sector. We propose the application of PQC KEM to the user key exchange process of the E2 EE protocol in a GPKI-based video conferencing system. Previously, in Zoom's E2 EE security Phase 1, Client Key Management and meeting data could be encrypted or decrypted if a 32-byte \(MK\) was shared securely. In the proposed next-generation video conferencing system, the session key (\(ss\)) shared through KEM plays the same role as the 32-byte \(MK\) in Zoom. Figure 5, below, shows the application of the PQC KEM to the GPKI-based video conferencing system. We describe the progress of the Figure 5 in terms of Algorithm 3:

Figure 4: (a) GPKI-based certification scheme; (b) NPKI-based certification scheme. First, it is assumed that both the host (\(A\)), who owns the GPKI, and the general client (\(C\)) are normally authenticated by the NPKI.