Encrypted Instant Messengers

Federated

Federated messengers use multiple, independent servers that are able to talk to each other (email is one example of a federated service). Federation allows system administrators to control their own server and still be a part of the larger communications network.

Advantages

• Allows for greater control over your own data when running your own server.
• Allows you to choose who to trust your data with by choosing between multiple "public" servers.
• Often allows for third party clients which can provide a more native, customized, or accessible experience.
• Generally a less juicy target for governments wanting backdoor access to everything as the trust is decentralized. The server may be hosted independently from the organization developing the software.
• Server software can be verified that it matches public source code, assuming you have access to the server or you trust the person who does (e.g., a family member)
• Third-party developers can contribute code and add new features, instead of waiting for a private development team to do so.

Disadvantages

• Adding new features is more complex, because these features need to be standardized and tested to ensure they work with all servers on the network.
• Some metadata may be available (e.g., information like "who is talking to whom," but not actual message content if E2EE is used).
• Federated servers generally require trusting your server's administrator. They may be a hobbyist or otherwise not a "security professional," and may not serve standard documents like a privacy policy or terms of service detailing how your data is utilized.
• Server administrators sometimes choose to block other servers, which are a source of unmoderated abuse or break general rules of accepted behavior. This will hinder your ability to communicate with users on those servers.

Worth Mentioning

• Other Matrix clients, that may however be less feature complete than Riot.im.
• XMPP (Extensible Messaging and Presence Protocol) is an open-source communications protocol that began development in 1999. Since then, XMPP has been extended by the publishing of XEPs (XMPP Extension Protocols). OMEMO is the most popular XEP (XMPP extension) for E2EE. Unlike Matrix, clients are only developed by the community and not by the foundation itself. Inconsistent E2EE
• Gajim
• Conversations
• Siskin
• Other OMEMO capable clients for XMPP.
• Kontalk is a community-driven instant messaging network based on XMPP.

Peer to Peer (P2P)

Peer-to-Peer instant messengers connect directly to each other without requiring third-party servers. Clients (peers) usually find each other through the use of a distributed computing network. Examples of this include DHT (distributed hash table) (used with technologies like torrents and IPFS, for example), or Ethereum's Whisper protocol (used with some newer DApps). Another approach is proximity based networks, where a connection is established over WiFi or Bluetooth (for example, Briar or the Scuttlebutt social networking protocol). Once a peer has found a route to its contact via any of these methods, a direct connection between them is made.

Advantages

• Minimal information is exposed to third parties.
• Modern P2P platforms implement end-to-end encryption by default. There are no servers that could potentially intercept and decrypt your transmissions, unlike centralized and federated models.

Disadvantages

• Reduced feature set:
• Messages can only be sent when both peers are online, however, your client may store messages locally to wait for the contact to return online.
• Generally increases battery usage on mobile devices, because the client must stay connected to the distributed network to learn about who is online.
• Your IP address and that of the contacts you're communicating with may be visible if you do not use the software in conjunction with a self contained network, such as Tor or I2P. Many countries have some form of mass surveillance and/or metadata retention.