To make unreadable a functional system.

[Sthu Deus <sthu.deus@gmail.com>]

Good day.

Can You please help me to direct to some documentation or project in
Debian or explain how to make the following:

after a system has been installed and configured - to make it encrypted
so that it will make impossible (or almost so) to read its files,
configuration, etc (even though the HDD be removed from the host and
connected to another running OS) - yet that it will run from turning on power on the host itself (boot).

- In short - possible to use, impossible to read w/o a password.

Thank You for Your time.