luks root partition and grub opens it using usb - how to?

[lejeczek]

hi guys

I wonder if it is possible to have a passphrase to the root 
partition on a usb/sd and tell the grub to check and use it 
and if that fails then fall back to user providing the 
passphrase, as normally by the means of keyboard.

Would you know if that works and if there is any doc/manual 
for it?

many thanks, L.