This has been discussed previously... And recently.
Search the mailing lists and you will find your answers.
On Oct 17, 2016, 23:12, at 23:12, "Tito Mari Francis EscaÃ±o"
>Is full disk encryption via auto install script feasible? Has anyone
>tried this before? Maybe somebody can share pointers on what to watch
>out for if it's already been done.
>I was wondering how the full disk encryption password can be secured
>during auto install. Maybe somebody can share their practices on this.
>Thanks so much.