Comment by sunaookami

It's a bit more complicated:

>In order to maximize user security, we would prefer third-party providers to use the Signal Protocol. Since this has to work for everyone however, we will allow third-party providers to use a compatible protocol if they are able to demonstrate it offers the same security guarantees as Signal.

>To send messages, the third-party providers have to construct message protobuf structures which are then encrypted using the Signal Protocol and then packaged into message stanzas in eXtensible Markup Language (XML).

>Meta servers push messages to connected clients over a persistent connection. Third-party servers are responsible for hosting any media files their client applications send to Meta clients (such as image or video files). After receiving a media message, Meta clients will subsequently download the encrypted media from the third-party messaging servers using a Meta proxy service.

You also have to connect over XMPP and through a proprietary "Enlistment API", etc.

https://engineering.fb.com/2024/03/06/security/whatsapp-mess...