Full disk encryption with Yubikey (Yubico key) for mkinitcpio

This enables you to automatically unlock a LUKS encrypted filesystem from a systemd-enabled initramfs.

Requirements

To compile and use Yubikey full disk encryption you need:

yubikey-personalization
iniparser
systemd
cryptsetup
keyutils and linux with CONFIG_KEYS enabled
mkinitcpio
markdown (HTML documentation)
libarchive (Update challenge on boot)

Additionally you will need to have make and pkg-config installed to successfully compile.

Build and install

Building and installing is very easy. Just run:

make

followed by:

make install-mkinitcpio

This will place the files in their desired places in the filesystem. Keep in mind that you need root privileges for installation, so switch user or prepend the last command with sudo.

Usage

config files `/etc/crypttab.initramfs` and `/etc/ykfde.conf`

Make sure systemd knows about your encrypted device by adding a line to /etc/crypttab.initramfs. It should read like:

mapping-name /dev/LUKS-device -

Usually there is already an entry for your device. If you do not already have a systemd-enabled initramfs, you will need to create this file from scratch.

Update /etc/ykfde.conf with correct settings. Add the value of mapping-name from above to device name in the general section. Then add a new section with your key's decimal serial number containing the key slot setting. The minimal file should look like this:

[general]
device name = crypt

[1234567]
luks slot = 1

Be warned: Do not remove or overwrite your interactive (regular) key! Keep that for backup and rescue - LUKS encrypted volumes have a total of 8 slots (from 0 to 7).

Key setup

ykfde will read its information from these files and understands some additional options. Run ykfde --help for details. Then prepare the key. Plug it in and make sure it is configured for HMAC-SHA1. This can be done with ykpersonalize from terminal (package yubikey-personalization) or with GUI application YubiKey Personalization Tool (package yubikey-personalization-gui). After that, run:

ykfde

This will store a challenge in /etc/ykfde.d/ and add a new slot to your LUKS device based on the /etc/ykfde.conf configuration. When ykfde asks for a passphrase it requires a valid passphrase from a previously available slot.

Alternatively, adding a key with second factor (foo in this example) is as easy:

ykfde --new-2nd-factor foo

To update the challenge run:

ykfde --2nd-factor foo

And changing second factor (from foo to bar in this example) is straight forward:

ykfde --2nd-factor foo --new-2nd-factor bar

The current and new second factor can be read from terminal, increasing security by not displaying on display and not writing to shell history. Use switches --ask-2nd-factor and --ask-new-2nd-factor for that.

Make sure to enable second factor in /etc/ykfde.conf.

cpio archive with challenges

Every time you update a challenge and/or a second factor run:

ykfde-cpio

This will write a cpio archive to /boot/ykfde-challenges.img containing your current challenges. Enable systemd service ykfde to do this automatically on every boot:

systemctl enable ykfde.service

mkinitcpio hook `ykfde`

Lastly, add ykfde to your hook list in /etc/mkinitcpio.conf. You should already have systemd and sd-encrypt there as a systemd-enabled initramfs is prerequisite. A working example config is as follows:

HOOKS="base systemd keyboard autodetect modconf block ykfde sd-encrypt sd-lvm2 filesystems fsck"

Now rebuild your initramfs with:

mkinitcpio -p linux

Boot loader

Make sure to load the cpio archive /boot/ykfde-challenges.img as an additional initramfs. It has to be listed after microcode updates (if available), but before main initramfs.

With grub you need to list ykfde-challenges.img in configuration variable GRUB_EARLY_INITRD_LINUX_CUSTOM in /etc/default/grub:

GRUB_EARLY_INITRD_LINUX_CUSTOM="ykfde-challenges.img"

Then update your grub configuration by running:

grub-mkconfig -o /boot/grub/grub.cfg

A valid configuration for systemd-boot should be placed in /boot/loader/entries/default.conf and look something like this:

title   Default
linux   /vmlinuz-linux
initrd  /intel-ucode.img
initrd  /ykfde-challenges.img
initrd  /initramfs-linux.img
options root=... rw quiet

Reboot and have fun!