outdated https://otr.im/clients.html
cant get it work https://play.google.com/store/apps/details?id=org.frozenbox.frozenchat&hl=en
went paid (latest technology) https://github.com/siacs/Conversations last free - https://f-droid.org/en/packages/eu.siacs.conversations/
Delta Chat
Is a project that aims to create a messaging app that is completely compatible to the existing e-mail infrastructure, implementation of Autocrypt https://delta.chat download by f-droid use mail from - https://dismail.de/register.html (mail portal https://webmail.dismail.de/)
Riot.im
https://play.google.com/store/apps/details?id=im.vector.alpha https://f-droid.org/packages/im.vector.alpha/ homepage
https://guardianproject.info/releases/
https://guardianproject.info/howto/chatsecurely/
https://github.com/guardianproject/ChatSecureAndroid
Since 2004 XMPP is the IETF-approved instant messaging and presence protocol. XMPP is based on the Jabber protocol that evolves since 1998. Basically programming Jabber software today means programming XMPP compliant software. XMPP is the new name (Google Talk used to run on this), Jabber the old.
online https://chat.frozenbox.org/inverse/ based on https://conversejs.org + https://inverse.chat/
XMPP Servers - https://xmpp.org/software/servers.html XMPP Clients - https://xmpp.org/software/clients.html
Public XMPP servers - https://list.jabber.at/ stable servers - https://jabber.hot-chilli.net/ or http://yam.im
create your own XMPP server with : -https://www.process-one.net/en/ejabberd/ tutorial -https://prosody.im/ -http://www.igniterealtime.org/projects/openfire/index.jsp
Comparison of XMPP server software
WhatApp uses a variation of XMPP for its popular chat service - https://github.com/mgp25/Chat-API/wiki/FunXMPP-Protocol
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