Gentoo Linux: Unlocking a LUKS Encrypted LVM Root Partition at Boot Time using a Key File stored on an External USB Drive


Gentoo can be configured to use a key file stored on an external USB drive to unlock a LUKS encrypted LVM root partition.

We will explore in this article the general steps involved in configuring Gentoo to use an external USB drive as a key file to unlock a LUKS encrypted LVM root partition.

1. Create a key file on the USB stick and add it to the LUKS encrypted partition

Generate a key file on a mounted ext4 or vfat partition of a USB stick, which will be used by initramfs to unlock the LUKS partition:

dd if=/dev/urandom of=/PATH/TO/USBSTICK/keyfile bs=1024 count=4Code language: plaintext (plaintext)

Ensure that the partition on the USB drive has a label, as the initramfs will use this label to find where the key file is located.

Afterward, add the key file to the LUKS partition to enable decryption of the partition using that key file:

cryptsetup luksAddKey /dev/PART1 /PATH/TO/USBSTICK/keyfile

In this example, “/dev/PART1” is the partition where the LUKS encryption is enabled, and “/PATH/TO/USBSTICK/keyfile” is the location of the keyfile.

2 – Find the UUID of the encrypted partition and the label of the USB drive

Use the lsblk command to find the UUID of the encrypted partition and the label of the USB drive:

lsblk -o +UUID,LABEL

3. Configure the boot loader (such as Systemd-boot, GRUB, Syslinux…)

Add to the boot loader configuration the following initramfs kernel parameters:

  • crypt_root=UUID=A1111111-A1AA-11A1-AAAA-111AA11A1111
  • root=/dev/LVMVOLUME/root
  • root_keydev=/dev/disk/by-label/LABELNAME
  • root_key=keyfile

Here is an example for Systemd-boot:

options dolvm crypt_root=UUID=A1111111-A1AA-11A1-AAAA-111AA11A1111 root=/dev/LVMVOLUME/root root_keydev=/dev/disk/by-label/LABELNAME root_key=keyfileCode language: plaintext (plaintext)

To ensure proper setup:

  • Customize the initramfs options for LVMVOLUME, LABELNAME, and UUID=A1111111-A1AA-11A1-AAAA-111AA11A1111 to match your specific case.
  • Verify that the ext4 or vfat partition of the USB drive that is labeled “LABELNAME” contains a file named “keyfile”.
  • Make sure that the modules “dm_mod” and “usb_storage” are included in the initramfs.

This method offers a convenient way to unlock a LUKS encrypted root LVM partition. The implementation process is well-documented, making it a suitable choice for those looking to secure their Gentoo Linux systems.

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