e present a protocol for secure online com-munication, called “off-the-record messaging”, which hasproperties better-suited for casual conversation than do sys-tems like PGP or S/MIME.

"Off-the-Record Communication, or, Why Not To Use PGP"

In 2013, the Signal Protocol was introduced, which is based on OTR Messaging and the Silent Circle Instant Messaging Protocol (SCIMP). It brought about support for asynchronous communication ("offline messages") as its major new feature, as well as better resilience with distorted order of messages and simpler support for conversations with multiple participants.[11] OMEMO, introduced in an Android XMPP client called Conversations in 2015, integrates the Double Ratchet Algorithm used in Signal into the instant messaging protocol XMPP ("Jabber") and also enables encryption of file transfers. In the autumn of 2015 it was submitted to the XMPP Standards Foundation for standardisation.

the OTR protocol also reveals used MAC keys as part of the next message, after they have already been used to authenticate previously received messages, and will not be re-used