Unlock a LUKS partition from a file

By Dan Aukes

Why would you want to do this? Sometimes, after encrypting a hard drive, you need it to auto-start. Following these instructions, you can enable a computer with an encrypted disk to start up, de-encrypt itself, and boot.

This tutorial is derived from the following:

lsblk
sudo cryptsetup luksDump /dev/sda3
sudo cp /boot/initrd.img-5.15.0-58-generic /boot/initrd.img-5.15.0-58-generic.safe

Edit /boot/grub/grub.cfg to make a copy of your top menu entry in the advanced menu:

make sure to replace initrd img with the safe copy you just created

sudo nano /boot/grub/grub.cfg

Note: sudo update-grub will probably overwrite your changes, so don’t use that.

menuentry 'Debian GNU/Linux, with Linux 4.19.0-13-amd64 (crypto safe)' --class debian --class gnu-linux --class gnu --class os {
       load_video
       insmod gzio
       insmod part_msdos
       insmod ext2
       set root='hd0,msdos1'
       search --no-floppy --fs-uuid --set=root 2a5e9b7f-2128-4a50-83b6-d1c285410145
       echo    'Loading Linux 4.19.0-13-amd64 ...'
       linux   /vmlinuz-4.19.0-13-amd64 root=/dev/mapper/dradispro-root ro  quiet
       echo    'Loading initial ramdisk ...'
       initrd  /initrd.img-4.19.0-13-amd64.safe
}

sudo dd if=/dev/urandom of=/boot/keyfile bs=1024 count=4 sudo chmod 0400 /boot/keyfile sudo cryptsetup -v luksAddKey /dev/sda3 /boot/keyfile

find the drive the mounts at /boot

get its uuid

ls -l /dev/disk/by-uuid/ lsblk -o NAME,UUID

sudo nano /etc/crypttab

. Edit /etc/crypttab

Edit the contents of file /etc/crypttab (use the UUID of /dev/sda1 from the previous step)

vi /etc/crypttab

This contents should be: sda5_crypt UUID=9b7200b5-0e0a-447a-93a8-7eb8f1f4a1ee none luks,discard (The UUID may be different) The changes we’ll be making:

  • Replace the 3rd parameter ‐ none ‐ with /dev/disk/by-uuid/<uuid>:/keyfile with the UUID for the hard drive at /boot
  • Replace the 4th parameter ‐ luks‐ with luks,keyscript=/lib/cryptsetup/scripts/passdev The final result: sda5_crypt UUID=9b7200b5-0e0a-447a-93a8-7eb8f1f4a1ee /dev/disk/by-uuid/2a5e9b7f-2128-4a50-83b6-d1c285410145:/keyfile luks,keyscript=/lib/cryptsetup/scripts/passdev In this case the UUID for our /dev/sda1 UUID was 2a5e9b7f....

sudo update-initramfs -u -k all sudo reboot now

sudo cryptsetup luksKillSlot /dev/nvme0n1p3 1 sudo cryptsetup luksDump /dev/nvme0n1p3 sudo nano /etc/default/grub sudo update-grub sudo update-initramfs -u -k all sudo reboot now

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