Message Security in Matrix

In Matrix, message security is provided by Megolm. The goal of message security
is twofold. We (a) want messages to be encrypted on the sender’s device and (b)
only be able to be decrypted on devices of users who are a part of the
conversation. As a corollary, we want to prevent other parties (rogue
homeservers, man-in-the-middle attackers, etc.) from decrypting the messages.