Start adding info to xep

xep-xxxx-current-off-the-record-messaging-usage.md

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 ## Introduction
 
 The Off-the-Record messaging protocol (OTR) was originally introduced in the
-2004 paper Off-the-Record Communication, or, Why Not To Use PGP [1] and has
-since become the de facto standard for performing end-to-end encryption. OTR
-provides confidentiality, deniability, authentication, and forward and
-backwards secrecy.
+2004 paper [Off-the-Record Communication, or, Why Not To Use PGP](https://otr.cypherpunks.ca/otr-wpes.pdf) [1]
+and has since become an standard for performing end-to-end encryption. OTR
+provides confidentiality, deniability, authentication, forward secrecy and
+post-compromise security.
 
-The OTR protocol itself is currently described by the document: Off-the-Record
-Messaging Protocol version 4 [2] and will not be described here. Instead, this
-document aims to describe OTR's usage in its current version (version 4) and
-best practices within XMPP.
+The OTR protocol itself (in its latest version) is currently described by the
+document: Off-the-Record Messaging Protocol version 4 [2] and will not be
+described here. Instead, this document aims to describe OTR's usage in its
+current version (version 4) and best practices of using it within XMPP.
 
+## Overview
+
+Though this document will not focus on the OTR protocol itself, a brief overview
+is warranted to better understand the protocols strengths and weaknesses.
+
+OTR (in its version 4) uses several cryptographic algorithms, such as SHAKE-256
+and ChaCha20. An OTR session can be held only between two parties, meaning that
+OTR is incompatible with Multi-User Chat (XEP-0045) [3] and Mediated Information
+eXchange (MIX) (XEP-0369) [4]. It provides deniability in its online and offline
+way. It also provides forward secrecy and post-compromise security.
+
+## Discovery
+
+Clients that support the OTR protocol can advertise it in different ways.
+OTR can be implemented in different modes that impact how the advertisement
+works.
+
+// TODO: modes, policies, different advertisement mechanisms