Matrix end-to-end encryption is called Megolm.
"The Megolm ratchet is intended for encrypted messaging applications where there may
be a large number of recipients of each message, thus precluding the use of peer-to-peer
encryption systems such as Olm.
It also allows a recipient to decrypt received messages multiple times. For instance, in
client/server applications, a copy of the ciphertext can be stored on the (untrusted)
server, while the client need only store the session keys."
Encryption is implemented in Riot since November 2016, so still very new. Several
user-visible problems have occurred in real-world usage, and are being tackled down
at the moment.
