If you want to consider this sort of thing I would suggest looking at something like Apple iMessage for inspiration (and iCloud in general) Apple is trying ver

If you want to consider this sort of thing I would suggest looking at something like Apple iMessage for inspiration (and iCloud in general) Apple is trying very had to do proper public-private key crypto, in a way where not even they have access to the keys, but that is also convenient for the user. There's a reason, for example (this is the best example) you have to approve new iCloud Keychain devices from an existing device. Because that device re-encrypts the database with the private key of the new device. Even that can cause problems as once someone forgets their password they may not be able to recover that keychain and by proxy other services. There are some trade-offs including an iCloud recovery key that is protected by HSMs that Apple Have with the complete inability to update their firmware - but that has other trade-offs such as they can't fix any bugs on those devices, etc. I won't fully detail further I would suggest doing your own reading. You may also wish to look at FIDO, in a sort of related unrelated way.