I think the ephemeral keys make a big difference
Discussion
The new idea does use ephemeral keys to send. We could do ephemeral keys to receive as well, but I am not sure if it actually adds much security.
Wouldn't it solve the problem of leaking your entire message history?
Not if you need to save all these keys somewhere (locally or on the cloud) to see the same feed in another client or device. That's what SimpleX does with the local database. Keys don't leak randomly. They leak when you are inserting them in a new device/client or when somebody gets access to it. If you keep your keys and the local db together, the attackers gets both.
Couldn't you save ephemeral receiving keys for like a week, then discard them? Just long enough to sync up all the devices.
We can rotate keys, but if you discard them, you won't see the messages address to that key anymore. In all cases, you will need a collection of all your past receiving keys to move to a new phone, for instance.
Wouldn't it make more sense to just link your nostr ID to your simplex ID for DMs and then when you want to DM someone you can choose whether to do it over nostr (if they're not a SimpleX user) or over SimpleX if they are.🤷🏼♂️
Simplex doesn't want to have a link to a known identity like Nostr would have.