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What Is XMPP?

XMPP is an open source and standardized federated messaging protocol that has existed in some form for more than 22 years (since 1999). In this time, many extensions have been standardized, and it has been used as the basis for many commercial (closed source) messaging products like google hangouts and facebook messenger.

Today, it functions as a good low-latency secure end-to-end encrypted messenger when used with the OMEMO encryption plugin. OMEMO encryption is based on the Open Whisper Systems protocol, and supports "many-to-many encrypted chats, offline messages queuing, forward secrecy, file transfer" (wikipedia).

To use OMEMO you need an XMPP client that supports it, unfortunately, there exists a myriad of clients but many are very old and their quality varies.

For Linux Dino is a good, modern client with most of the important features. Written in Vala using GTK. There is also Gajim which has been around forever and should be stable and reliable.

For Windows, I believe Gajim is the best option, it works quite reliably. Written in Python using GTK. You need to go into plugins settings to download and install the OMEMO plugin before using encrypted messaging.

For Android Conversations is the good, which has all the features you'd want and is probaby the best xmpp client around right now on any platform.

For iOS there is Chatsecure, which I have not personally used but hear is very usable and provides the best experience.

For MacOS there is Monal Catalyst, again I can't comment as I haven't used it.

There is a list of all OMEMO-supporting clients here.