The only attack is to get the users private key, which can be encrypted on their machine behind passwords, biometrics, or whatever.
Here is the problem with your suggestion. I don't think Joe Sixpack or my aunt Monique would take proper care of their private keys.
Biometrics may help with that, but there is no universal&practical solution right now.
But, excellent idea. Mate is the way to go for LM12 (IMHO), and I'm sure this will be a very popular decision.
I thought Mate was a Gnome2 fork, this (Cinnamon) is a Gnome3 fork.
So, which is the way to go ?
"Open the pod bay doors, HAL." -- Dave Bowman, 2001