> This document describes the Sesame algorithm for managing message encryption sessions in an asynchronous and multi-device setting.
But if you are doing regular messages between people, those messages are certainly readable by the server and because adding a new device decrypts all previous messages, the server has decryption knowledge.
Note: At each point in their sesame algorithm the user has a non-empty set of devices. So if you want to sync another device acts as a p2p syncer.
What happens when you remove your last device and add another new one. Hence why whatsapp has a non-privacy mode. Or am I misunderstanding?
If you switch devices, other clients will use the old keys until they have received the new one (and then they will silently re-encrypt and resend undelivered messages, something WhatsApp was heavily criticized for).
They store undelivered, encrypted messages on the server.