This short tutorial will show you how to password protect your Grub bootloader rendering your machine unbootable to the casual snoop.
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There is a lot more to securing your Debian system than just password protecting Grub but physical security plays a large role in the overall scheme of securing your computer. However, most people shrug this off. This, along with setting your BIOS to only boot to your “hdd-0” and disabling all other boot devices, is an important first step. Don't forget to set a password for your BIOS as well while you're in there. If you lock the gate with a good enough lock you'll keep all but the most determined trespassers out – and for those malcontents we have more security in store but that's for another tutorial.