Unlock LUKS volume with a YubiKey

Update: The dracut configuration has been updated and now udev consistently recognizes the YubiKey in the initramfs.

Unlocking LUKS encrypted drives with a YubiKey has been supported since systemd 248. In Debian, systemd>=250 is required, as the feature has not been enabled in prior versions. This tutorial is geared towards Yubikeys, but it should work with slight modifications with any other FIDO2 token.

YubiKey series 5 and later should support the hmac-secret extension. You can make sure your Yubikey supports the needed hmac-secret extension by querying it with ykman:

$ ykman --diagnose 2>&1 | grep hmac-secret

Backup your LUKS header

In case you mess anything up, you would need a backup of your LUKS header. Remember to save your backup to some external storage, so you can actually access it if anything goes sideways.

# cryptsetup luksHeaderBackup /dev/nvme0n1p3 --header-backup-file /media/guyru/E474-2D80/luks_backup.bin

Set FIDO2 PIN

We would like to set a FIDO2 PIN for the Yubikey, so unlocking the encrypted drive would require both the physical Yubikey and the PIN. You can set the PIN using:

$ ykman fido access change-pin

Enroll the Yubikey

Start by verifying that systemd-cryptenroll can see and can use your YubiKey:

$ systemd-cryptenroll --fido2-device=list
PATH         MANUFACTURER PRODUCT
/dev/hidraw0 Yubico       YubiKey FIDO+CCID

Now, enroll the Yubikey, replacing /dev/nvme0n1p3 with the block device of the LUKS encrypted drive.

$ sudo systemd-cryptenroll /dev/nvme0n1p3 --fido2-device=auto  --fido2-with-client-pin=yes
๐Ÿ” Please enter current passphrase for disk /dev/nvme0n1p3: (no echo)
Initializing FIDO2 credential on security token.
๐Ÿ‘† (Hint: This might require confirmation of user presence on security token.)
๐Ÿ” Please enter security token PIN: (no echo)
Generating secret key on FIDO2 security token.
๐Ÿ‘† In order to allow secret key generation, please confirm presence on security token.
New FIDO2 token enrolled as key slot 0.

Modify /etc/crypttab

We need to modify /etc/crypttab in order to tell cryptsetup to unlock the device using the YubiKey. Add fido2-device=auto in the options field of the crypttab entry for your device. For example:

nvme0n1p3_crypt UUID=307a6bef-5599-4963-8ce0-d9e999026c1a none luks,discard,fido2-device=auto

Switch to dracut

Debian’s default initramfs generator, update-initramfs of the initramfs-tools is using the old cryptsetup for mounting encrypted drives. However, cryptsetup doesn’t recognize the fido2-device option. Running update-initramfs will fail with the following error:

$ sudo update-initramfs -u
update-initramfs: Generating /boot/initrd.img-5.15.0-3-amd64
cryptsetup: WARNING: nvme0n1p3_crypt: ignoring unknown option 'fido2-device'

This is unfortunate. The simplest solution is to switch to dracut, a more modern initramfs generator, which among other things relies on systemd to activate encrypted volumes. This solves the issue of the unknown fido2-device.

Before installing dracut, I would highly recommend creating a copy of the existing initramfs in the boot partition in case something goes wrong.

$ sudo apt install dracut

Dracut includes systemd-cryptsetup by default. systemd-cryptsetup depends on libfido for unlocking devices using FIDO2 tokens. At least in Debian, systemd-cryptsetup dynamically loads libfido2.so (as opposed to being dynamically linked), which causes dracut not to have libfido2.so in the initramfs. This causes systemd-cryptsetup to issue the following error upon boot:

FIDO2 tokens not supported on this build. 

We fix it by manually adding libfido2.so to the initramfs. Of course, we also need to include libfido2’s dependencies as well. Dracut has a mechanism for automatically adding dependencies for executables, but it doesn’t work on libraries. As a workaround, instead of adding libfido2 directly, we will add an executable that depends on libfido2, which will add libfido2 and its dependencies to the initramfs. We will usefido2-token from the fido2-tools package for this trick.

$ sudo apt install fido2-tools
$ cat << EOF | sudo tee /etc/dracut.conf.d/11-fido2.conf
## Spaces in the quotes are critical.
# install_optional_items+=" /usr/lib/x86_64-linux-gnu/libfido2.so.* "

## Ugly workround because the line above doesn't fetch
## dependencies of libfido2.so
install_items+=" /usr/bin/fido2-token "

# Required detecting the fido2 key
install_items+=" /usr/lib/udev/rules.d/60-fido-id.rules /usr/lib/udev/fido_id "
EOF

Now, recreate the initramfs images:

$ sudo dracut -f

Last remarks

At this point, we are done. Reboot you’re machine and it will prompt you for your YubiKey and allow you to unlock your LUKS encrypted root patition with it. If you don’t have your YubiKey, it will give the following prompt:

Security token not present for unlocking volume root (nvme0n1p3_crypt), please plug it in.

After around 30 seconds, it would time out and display the following message:

Timed out waiting for security device, aborting security device based authentication attempt.

Afterwards, it would allow you to unlock the partition using a password (or a recovery key).

In case you run into any trouble, append rd.break=initqueue to the kernel command line, and dracut will enter a shell before attempting to mount the partitions. You can manually mount the drive using the following command:

# /usr/lib/systemd/systemd-cryptsetup attach root /dev/nvme0n1p3

Exit the emergency shell, and the system will continue its normal boot.

8 thoughts on “Unlock LUKS volume with a YubiKey”

  1. Great howto, thanks…

    With some trial and error (comparing the contents of working and non-working initrds), I determined that the only missing library that the /usr/bin/fido2-token trick adds to the initrd is libz.

    So the installation of the fido2-tools package and the fido2-token binary can be skipped and the /etc/dracut.conf.d/11-fido2.conf can be reduced to this:

    install_optional_items+=” /usr/lib/x86_64-linux-gnu/libz.so.* “

  2. I know these instructions were for Debian, but I gave it a shot in Ubuntu 22.04 and it pretty much worked with one exception: if I reboot without my Yubikey being connected, I’m not prompted to connect it and there is no timeout that prompts me for a regular LUKS passphrase. Did I miss a step, or is this expected behavior with Ubuntu 22.04?

  3. Here is output from “systemd –version” on my Ubuntu 22.04 system:

    $ systemd –version
    systemd 249 (249.11-0ubuntu3.4)
    +PAM +AUDIT +SELINUX +APPARMOR +IMA +SMACK +SECCOMP +GCRYPT +GNUTLS +OPENSSL +ACL +BLKID +CURL +ELFUTILS +FIDO2 +IDN2 -IDN +IPTC +KMOD +LIBCRYPTSETUP -LIBFDISK +PCRE2 -PWQUALITY -P11KIT -QRENCODE +BZIP2 +LZ4 +XZ +ZLIB +ZSTD -XKBCOMMON +UTMP +SYSVINIT default-hierarchy=unified

    I don’t know if Canonical has any plans to update to a newer version, or if they plan to backport any upstream changes. There have been tons of problems reported with FDIO2 and systemd-related implementations for unlocking during boot, this seems to be a workaround to get it working. Thanks for your guide, it was very helpful.

  4. I am trying on Ubuntu 22.04.01 jammy, with no success! When I reboot, with Yubikey connected, I am not prompted to connect Yubikey ….

    Any idea? Thanks

    hrouhan@hrz:~$ sudo dracut -f
    dracut: Executing: /usr/bin/dracut -f
    dracut: dracut module ‘bootchart’ will not be installed, because command ‘/sbin/bootchartd’ could not be found!
    dracut: dracut module ‘modsign’ will not be installed, because command ‘keyctl’ could not be found!
    dracut: dracut module ‘rngd’ will not be installed, because command ‘rngd’ could not be found!
    dracut: dracut module ‘plymouth’ will not be installed, because command ‘plymouth-set-default-theme’ could not be found!
    dracut: dracut module ‘btrfs’ will not be installed, because command ‘btrfs’ could not be found!
    dracut: dracut module ‘dmraid’ will not be installed, because command ‘dmraid’ could not be found!
    dracut: dracut module ‘nvmf’ will not be installed, because command ‘nvme’ could not be found!
    dracut: dracut module ‘biosdevname’ will not be installed, because command ‘biosdevname’ could not be found!
    dracut: dracut module ‘masterkey’ will not be installed, because command ‘keyctl’ could not be found!
    dracut: dracut module ‘modsign’ will not be installed, because command ‘keyctl’ could not be found!
    dracut: dracut module ‘rngd’ will not be installed, because command ‘rngd’ could not be found!
    dracut: dracut module ‘btrfs’ will not be installed, because command ‘btrfs’ could not be found!
    dracut: dracut module ‘dmraid’ will not be installed, because command ‘dmraid’ could not be found!
    dracut: dracut module ‘nvmf’ will not be installed, because command ‘nvme’ could not be found!
    dracut: dracut module ‘masterkey’ will not be installed, because command ‘keyctl’ could not be found!
    dracut: *** Including module: bash ***
    dracut: *** Including module: dash ***
    dracut: *** Including module: systemd ***
    dracut: *** Including module: systemd-initrd ***
    dracut: *** Including module: console-setup ***
    dracut: *** Including module: crypt ***
    dracut: *** Including module: dm ***
    dracut: Skipping udev rule: 10-dm.rules
    dracut: Skipping udev rule: 13-dm-disk.rules
    dracut: Skipping udev rule: 64-device-mapper.rules
    dracut: *** Including module: kernel-modules ***
    dracut: *** Including module: kernel-modules-extra ***
    dracut: *** Including module: lvm ***
    dracut: Skipping udev rule: 11-dm-lvm.rules
    dracut: Skipping udev rule: 69-dm-lvm-metad.rules
    dracut: Skipping udev rule: 64-device-mapper.rules
    dracut: *** Including module: overlay-root ***
    dracut: *** Including module: rootfs-block ***
    dracut: *** Including module: terminfo ***
    dracut: *** Including module: udev-rules ***
    dracut: Skipping udev rule: 40-redhat.rules
    dracut: Skipping udev rule: 91-permissions.rules
    dracut: Skipping udev rule: 80-drivers-modprobe.rules
    dracut: Skipping udev rule: 70-persistent-net.rules
    dracut: *** Including module: dracut-systemd ***
    dracut: *** Including module: usrmount ***
    dracut: *** Including module: base ***
    dracut: *** Including module: fs-lib ***
    dracut: *** Including module: shutdown ***
    dracut: *** Including modules done ***
    dracut: *** Installing kernel module dependencies ***
    dracut: *** Installing kernel module dependencies done ***
    dracut: *** Resolving executable dependencies ***
    dracut: *** Resolving executable dependencies done ***
    dracut: *** Hardlinking files ***
    Mode: real
    Files: 627
    Linked: 2 files
    Compared: 0 xattrs
    Compared: 25 files
    Saved: 738 B
    Duration: 0.002411 seconds
    dracut: *** Hardlinking files done ***
    dracut: *** Generating early-microcode cpio image ***
    dracut: *** Constructing GenuineIntel.bin ***
    dracut: *** Store current command line parameters ***
    dracut: Stored kernel commandline:
    dracut: rd.luks.uuid=luks-b18dacce-a670-48e6-aa0a-46da3538c4b7
    dracut: rd.lvm.lv=vgubuntu/root rd.lvm.lv=vgubuntu/swap_1
    dracut: root=/dev/mapper/vgubuntu-root rootfstype=ext4 rootflags=rw,relatime,errors=remount-ro
    dracut: *** Stripping files ***
    dracut: *** Stripping files done ***
    dracut: *** Creating image file ‘/boot/initrd.img-5.15.0-50-generic’ ***
    dracut: *** Creating initramfs image file ‘/boot/initrd.img-5.15.0-50-generic’ done ***

  5. Have you checked that the required files binaries, libraries and the udev rules are included in the initramfs image?

  6. Thanks for great howto, worked perfectly fine on Debian testing. Only thing I would add is when you use different brand than Yubikey (like mine GoTrust Idem Key), you may need to use fido2-token command instead of ykman.

    Checking hmac-secret support:
    $ fido2-token -I /dev/hidraw4 | grep hmac-secret
    extension strings: hmac-secret

    Setting PIN:
    $ fido2-token -S /dev/hidraw4

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