Yubikey and Full Disk Encryption

YubiKey NEO

Yubikey’s HMAC-SHA1 challenge-response mode can be used to unlock your encrypted hard disk at boot time.

Required Software

Yubikey for LUKS is available from package manager:

$ sudo apt install yubikey-luks

Setup and Configuration

We assume you already have full disk encryption enabled on your desktop system and you unlock the disk with a password at boot time.

Important Information

  1. The disk device and partition number where your encrypted file system resides.

  2. The LUKS key-slot to use. By default slot 7 (the last one) will be used.

On my personal computer the physical device holding the encrypted file-system is on the third partition of a a NVMe M.2 SSD known as /dev/nvme0n1.

Thus my device to un-encrypt is /dev/nvme0n1p3.

LUKS partitions have eight rewritable key-slots, each one can hold a password to un-encrypt the partition.

Display the already used slots in the LUKS header information:

$ sudo cryptsetup luksDump /dev/nvme0n1p3

If you only used one password to decrypt your disk, it is usually stored in slot 0 and slot 1 to 7 are still unused.

Warning

Beware! if a previously used slot is overwritten, that password can no longer be used to decrypt your disk!

Backup you LUKS header

Anything goes wrong, you might not be able to access the encrypted data again. Backup your LUKS header in any case before modifying it:

$ sudo cryptsetup luksHeaderBackup /dev/nvme0n1p3 \
    --header-backup-file /media/user/safe-storage/${HOSTNAME}-LUKS-header.backup-$(date -u +%Y-%m-%d_%H-%M-%S)

The backup is stored on /media/user/safe-storage/ a Safe Storage.

Initialize your Yubikey

Initialize the Yubikey for HMAC-SHA1 challenge/response mode in slot 2:

$ ykpersonalize -2 -ochal-resp -ochal-hmac -ohmac-lt64 -oserial-api-visible

Enroll your Yubikey to a LUKS slot

$ sudo yubikey-luks-enroll -d /dev/nvme0n1p3
setting disk to /dev/nvme0n1p3.
setting slot to 7.
This script will utilize slot 7 on drive /dev/nvme0n1p3.  If this is not
what you intended, exit now!
Adding yubikey to initrd
Please enter the yubikey challenge password. This is the password that will
only work while your yubikey is installed in your computer:
********
Please enter the yubikey challenge password again:
********
Please provide an existing passphrase. This is NOT the passphrase you just
entered, this is the passphrase that you currently use to unlock your LUKS
encrypted drive:
********

Enable at System Boot

Open the file /etc/crypttab and change the line:

nvme0n1p3_crypt UUID=baa9d1c2-3b57-440a-9148-52570dba9814 none luks,discard

as follows:

nvme0n1p3_crypt UUID=baa9d1c2-3b57-440a-9148-52570dba9814 none luks,keyscript=/usr/share/yubikey-luks/ykluks-keyscript,discard

This will tell the boot process, that the script /usr/share/yubikey-luks/ykluks-keyscript needs to be called, which in turn will send the password typed by the user as challenge to the Yubikey and send the response from the Yubikey to LUKS to decrypt the disk.

Save and close the file, then update the initial RAM disk:

$  sudo update-initramfs -u

Enable at Suspend/Resume

$ systemctl enable yubikey-luks-suspend.service