Earlier quoted context omitted.
The thing I like about Keybase is that keys are always generated client-side and never leave the client, and all of the functionality associated with adding/removing devices is done in a way so that there's no way for a server to tamper with it (aside from denying service). Is that true in Matrix? Several services advertise themselves as "end-to-end" encrypted, but then when you poke harder it turns out either there…
Yes, Matrix is properly end-to-end encrypted (with all keys generated clientside) and has been independently audited as such: https://www.nccgroup.trust/us/our-research/matrix-olm-crypto... . We have gone to huge efforts to prevent MITMs via device verification and cross signing - which specifically addresses both problems of a) losing chat history when you move between devices (via https://github.com/uhoreg/matrix-d…
Edit: Specifically, is key backup tied to the ability to recover account history on a new device, or can I still get that with key backup disabled as long as I have at least one other device active?
Edit 2: Can you address this paragraph:
> One point for super-paranoid users: currently the private key used to sign your own devices and the private key used to sign other users are encrypted by your recovery passphrase/key and stored on the server to allow recovery if you lose all your devices. We also allow signing keys to be shared (gossiped) between devices, but right now the implementation also stores them encrypted on the server too. This restriction will be fixed in future, but for now if you don’t trust your server with encrypted keys, you may want to hold off on using cross-signing.
If I understand correctly, sounds like security is based on the complexity of your recovery passphrase and an implicit assumption that the passphrase doesn't get transmitted to the server... is that correct?