XMPP is a communications protocol that enables near-real-time chats between any two or more network entities. That means it’s like a skype or Facebook chat between two or more users. It was originally named Jabber, a name which sometimes still gets used for it.
Following this guide, you will be able to send end-to-end encrypted messages in real-time for free.
OMEMO
OMEMO is an extension to the Extensible Messaging and Presence Protocol (XMPP) for multi-client end-to-end encryption developed by Andreas Straub. According to Straub, OMEMO uses the Double Ratchet Algorithm “to provide multi-end to multi-end encryption, allowing messages to be synchronized securely across multiple clients, even if some of them are offline”. The name “OMEMO” is a recursive acronym for “OMEMO Multi-End Message and Object Encryption”. It is an open standard based on the Double Ratchet Algorithm and the Personal Eventing Protocol (PEP, XEP-0163). OMEMO offers future and forward secrecy and deniability with message synchronization and offline delivery.
In comparison with OTR, the OMEMO protocol offers many-to-many encrypted chat, offline messages queuing, forward secrecy, file transfer, verifiability, and deniability at the cost of slightly larger message size overhead
OTR(Off the record)
OTR and Pidgin are both currently included with the Zero Trace Pen. However, both have become very dated. It is strongly encouraged that you start using OMEMO.
Pidgin (formerly named Gaim) is a free and open-source multi-platform instant messaging client. It has support for many instant messaging protocols, allowing the user to simultaneously log into various services from one application. That means you could chat with your Facebook/google talk / AIM friends using only Pidgin and not visiting the website itself (e.g. facebook.com).
Pidgin is widely used for its Off-the-Record Messaging (OTR) plugin, which offers end-to-end encryption. For this reason, both (Pidgin and the OTR plugin) are included in the Zero Trace Pen and you just have to set it up correctly. However, your chat partners have to have the OTR plugin too (Pidgin is not necessary, they could use a similar tool) in order to be able to chat with you this way.
The OTR plugin ensures the messages cannot be recovered by a third party because it uses Perfect Forward Secrecy. However, as always your other chat partner could always keep logs of your conversation without you knowing or being compromised.
