如果用 Matrix/Element，有metadata leak嘅問題。可以參考呢篇（ https://www.andreafortuna.org/2019/08/12/why-whatsapp-and-telegram-messages-are-not-really-private/ ）19年嘅文。
Session is an open-source encrypted messenger powered by Oxen, designed to provide both security and anonymity through metadata minimisation, fully anonymous signup, and secure encryption and networking protocols. Session uses the Oxen Service Node network to route and temporarily store messages.
The past decade has seen a massive increase in the number of people using — and relying on — instant messengers for everything from chatting with friends to mission-critical business tasks. The most widely-used messengers have amassed more than a billion users. However, these popular messengers have some serious privacy and security shortfalls that make them completely unsuitable for anyone concerned about their online privacy.
Some messengers, such as WhatsApp, have deployed end-to-end encryption in an attempt to alleviate user concerns about privacy and security. However, simply encrypting the contents of user messages doesn't fix the wider underlying issue: the companies that operate these services can still see massive amounts of metadata about their users' messaging activities. This metadata can include user IP addresses and phone numbers, the time and #️quantity of sent messages, and the relationship each account has with other accounts. Increasingly, it is the existence and analysis of this metadata that poses a significant privacy risk to journalists, human rights activists, and other groups with a real need for private communications.
Session is, in large part, a response to this growing risk. Session is built to minimise metadata creation and leakage at every step of the messaging process. Through its fully anonymous signup process, decentralised networking infrastructure, and encryption and onion routing protocols, Session ensures that users send messages — not metadata.
Session integrates directly with Oxen Service Nodes, which are a set of distributed, decentralized and Sybil resistant nodes. Service Nodes act as servers which store messages offline, and a set of nodes which allow for onion routing functionality obfuscating users IP Addresses. For a full understanding of how Session works, read the Session Whitepaper.
Oxen係Monero fork， Session係Signal fork， 應該幾正