[CentOS] system encryption question

Wed Mar 1 23:35:09 UTC 2017
FHDATA <fhdata at unm.edu>


Centos 7.x

Let say a clean-install of Centos 7.x is
being performed on a box simply with two
partitions  / and /boot ... and that there is
no extra complexity like lvm , raid ,etc,etc ...

Centos 7.x process allows /  to be encrypted with pass phrase...

How can a key,  residing on a usb flash drive, 
to be added to the  encryption of / ?

So the system needs both manual input of pass phrase
as well as presence of  usb flash drive to
come up after getting  rebooted.

Thank you,