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Encrypted root on Debian with keyfile/keyscript

Thursday, December 15, 2016

I've recently set up another laptop with whith whole-disk encryption, including /boot. This is all fine --the debian-installer almost gets it right, you just need to edit /etc/default/grub to add GRUB_ENABLE_CRYPTODISK=y and then re-run grub-install on the target-- but once you get this working you end up having to type in the password for the LUKS disk encryption twice at boot: once for GRUB and once for the booted system. You can solve this by adding a keyfile to the encrypted disk so it can be decrypted by either having this keyfile or the original password. The idea is then to put this keyfile into the initrd image where the bootup sequence uses it instead of having the type in the passphrase again. The initrd itself is then decrypted by GRUB together with the kernel etc. when you type it the password. This sounds simple enough, but on Debian this still involves creating some manual scripts to get this all working.

I'm going to skip through the initial setup of getting GRUB to work, instead just covering the keyfile part. But I'll quickly describe my setup so you know how the starting point. I have a GPT partition table with 2 partitions: one FAT partition for EFI and the second parition is used as the LUKS encrypted block device. This LUKS-device is then used as a Physical Volume (PV) for LVM, with two LVM partitions: one for swap and the other for BTRFS on which my root filesystem lives directly at the top of the main BTRFS volume.

So once you got this booting with a password you want to create a keyfile which will also be able to unlock the LUKS volume. LUKS will only use the required number of bits out of a keyfile so you can just make a large one, or you can look how many bits you need:

# cryptsetup luksDump /dev/nvme0n1p2
...
MK bits:       512

So we need a minimum of 512 bits in our keyfile, or 64 bytes. I store the keyfile in /etc/cryptroot/ but you can store it anywhere you like, just make sure only root can read it. Finally you want to add the keyfile as a way to decrypt the LUKS volume:

# dd if=/dev/urandom of=/etc/cryptroot/keyfile.bin bs=1 count=64
# chmod go-rwx /etc/cryptroot/keyfile.bin
# cryptsetup luksAddKey /dev/nvme0n1p2 /etc/cryptroot/keyfile.bin

This is the easy part, now you want to add this keyfile to the initramfs and make sure that the initramfs will use it to decrypt the disk early at boot, rather then prompting you for the password.

Firstly start with adding the keyfile to the Debian-specific /etc/crypttab, the third field there is the path of the keyfile so it should look similar to this:

# name          device                                    keyfile                    options
nvme0n1p2_crypt UUID=e5216e6f-f89c-49c4-8aca-33cde0d4648d /etc/cryptroot/keyfile.bin luks,discard,keyscript=/etc/cryptroot/getinitramfskey.sh

The fourth field in this file are the options, you should have luks as one option and I also use discard since I use a solid state disk and want the TRIM command to be issued. If you only had these options then Debian would be able to decrypt your disk on startup, *if* it could read the keyfile from the filesystem. But since we got the partition with the keyfile on itself encrypted we need to add the keyscript option to enable it to be used in the initrd. Again the keyscript can live anywhere, but I also keep it in /etc/cryptroot.

Needing to create this keyscript is a bit weird and I'm not sure why a keyfile in the initrd is not supported directly, probably historical reasons, but the keyscript will be copied into the initramfs and is responsible for printing the contents of the keyfile to stdout. This allows you to do lots of crazy things if you so wish, but I simply use this:

#!/bin/busybox ash

# This script is called by initramfs using busybox ash.  The script is added
# to initramfs as a result of /etc/crypttab option "keysscript=/path/to/script"
# updating the initramfs image.
# This script prints the contents of a key file to sdout using cat.  The key
# file location is supplied as $1 from the third field in /etc/crypttab, or can
# be hardcoded in this script.
# If using a key embedded in initrd.img-*, a hook script in
# /etc/initramfs-tools/hooks/ is required by update-initramfs.  The hook script
# copies the keyfile into the intramfs {DESTDIR}.

KEY="${1}"
if [ -f "${KEY}" ]; then
 cat "${KEY}"
else
 PASS=/bin/plymouth ask-for-password --prompt="Key not found.  Enter LUKS Password: "
 echo "${PASS}"
fi

As the comment in the script describes, this keyscript itself will be included in the initramfs image automatically but our keyfile itself is still not included. We set the keyfile location in /etc/crypttab to /etc/cryptroot/keyfile.bin and we will get this value as $1, but this script is executed in the initramfs and the real filesystem is not yet there. So lastly we need to provide a hook for update-initramfs which will copy the keyfile to the right location in the initramfs image. We could have chosen any hardcoded location in the script instead of using the one from /etc/cypttab, but in this case I decided to use the same location in the initramfs as on the real system.

So the last thing to do is create this hook scrypt in /etc/initramfs-tools/hooks/loadinitramfskey.sh:

#!/bin/sh

# This hook script is called by update-initramfs. The script checks for the
# existence of the key file loading script getinitramfskey.sh and copies it
# to initramfs if it's missing.
# This script also copies the key file autounlock.key to the /root/ directory
# of the initramfs. This file is accessed by getinitramfskey.sh, as specified
# in /etc/crypttab.

PREREQ=""

prereqs() {
  echo "$PREREQ"
}

case "$1" in
  prereqs)
    prereqs
    exit 0
  ;;
esac


. "${CONFDIR}/initramfs.conf"
. /usr/share/initramfs-tools/hook-functions

if [ ! -f "${DESTDIR}/lib/cryptsetup/scripts/getinitramfskey.sh" ]; then
 if [ ! -d "${DESTDIR}/lib/cryptsetup/scripts" ]; then
  mkdir -p "${DESTDIR}/lib/cryptsetup/scripts"
 fi
 cp /etc/cryptroot/getinitramfskey.sh "${DESTDIR}/lib/cryptsetup/scripts/"
fi
if [ ! -d "${DESTDIR}/etc/cryptroot/" ]; then
 mkdir -p "${DESTDIR}/etc/cryptroot/"
fi
cp /etc/cryptroot/keyfile.bin "${DESTDIR}/etc/cryptroot/"

And now you have everything in place to build a new initramfs image:

update-initramfs -u

You could now pry appart your new initramfs image to check everything is in place. Or you could simply reboot and see if it all works.

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