Everything you post is encrypted in this browser before it is sent. The server still learns how many messages you have and when, each event's type, which events concern the same message, which pictures belong to which message, and — while link previews are on — every URL you paste. It never sees a word of text or a pixel of an image.
Copy this key somewhere safe now: it is shown once. To read your messages on another device, paste it there. Lose it and everything is gone — there is no reset, and no one, including the operator, can recover it.
This account already has a key. Paste it to read your messages on this device.
Paste this on any other device to read your messages there. Lose it and everything is gone — there is no reset.