## 2 Research Trends  
### E2 EE-Related Research  
Since non-face-to-face environments are becoming more prevalent, there is an increase in the usage of online video conferencing systems. As a result, the importance of security technology for protecting the privacy of entities is receiving attention. However, a problem may occur in which the private information of an entity is exposed to a central server when video conferencing information is transmitted. A concept that has emerged to resolve this problem is E2 EE, that is, an encryption technology for protecting the privacy of entities. If E2 EE technology is applied to encrypt using a key that is shared only by the entities who are communication parties, the entities' privacy is protected because messages cannot be verified except by those entities. On the other hand, there are several practical barriers to the actual application and deployment of E2 EE technology. The most significant obstacle is when the communicating party does not utilize a reliable certification authority (CA)'s Public Key Infrastructure (PKI) during the secret key generation process and instead relies on an unreliable third party as a server. This compromises the privacy of the entities, as the third party gains access to the secret key information, rendering the E2 EE security function ineffective. Apart from the security issues arising from the perspective of E2 EE, the following obstacles should also be taken into consideration [1]. * The government's national policy does not actively encourage the application of E2 EE technology. * Problems arise when an entity wants services provided by a third party with low security reliability. Despite these obstacles, E2 EE security needs to be applied for the security of the entity's video conferencing system. Furthermore, entities need to check if the system has applied E2 EE security. As a representative protocol with E2 EE security, the Signal Protocol, applied to various message applications, is a cryptographic protocol that provides E2 EE security for word, voice, and video encryption. This protocol has five features, Immediate Decryption, Long-lived Sessions, Usability, Forward Secrecy, and Post-Compromise Security [1]. The communication parties of the Signal Protocol receive messages immediately and decryption without loss or delay is possible. To this end, the Signal Protocol has a feature that maintains the session between communication parties until they reinstall an application or change the device. In addition, the protocol has usability because it does not use its own password or CA's PKI. Forward Secrecy refers to the feature that even if a user in the group is attacked and the group secret key is exposed, the attacker cannot decrypt the message before the attack. Contrary to Forward Secrecy, Post-Compromise Security refers to the feature that an attacker cannot decrypt messages after a user has been attacked. Therefore, Post-Compromise Security is also referred to as Future Secrecy. E2 EE security requirements, including Forward Secrecy and Post-Compromise Security features, are defined according to the IETF standardization document [6]. In this document, the requirements for E2 EE security are divided into necessary and optional features, as summarized in Table 1.  
E2 EE must satisfy the following three requirements: authenticity, confidentiality, and integrity. There are additional features that enhance E2 EE security and contribute to its effectiveness. There are seven conditions, including Forward Secrecy and Post-Compromise Security, provided by the Signal Protocol. The Cisco Webex video conference application provides E2 EE security. It securely exchanges the shared meeting key based on MLS and encrypts meeting content using SFrame. It claims the security model of Webex. Also, it is able to exchange video conference data securely that are encrypted via the use of specific ciphersuites. They have described their strategy in the whitepaper [8].