dracut/modules.d/91crypt-gpg

# Directions for changing a system from password-based gpg keyfile
# to smartcard-based gpg keyfile

# Be sure that you meet the following requirements:
#  1. GnuPG >= 2.1 installed with
#     * Smartcard support enabled (scdaemon must be built)
#     * Direct CCID access built into scdaemon
#  2. A password-based gpg keyfile ${KEYFILE} (e.g. "keyfile.gpg"):
#     That is, a file containing the slot key for LUKS, which
#     has been encrypted symmetrically with GnuPG using
#     a password.
#  3. Your public OpenPGP identity ${RECIPIENT} (e.g. "3A696356")
#  4. An OpenPGP smartcard holding the decryption key associated
#     with your public identity
#  5. A CCID smartcard reader

#  Notes: Requirement 4. and 5. can of course be one device, e.g.
#         a USB token with an integrated OpenPGP smartcard

# Make a backup of your keyfile (assuming it lies on the boot partition)
$ cp /boot/${KEYFILE} /safe/place/keyfile.bak.gpg

# Change your keyfile from purely password-based to both
# password-based and key-based (you can then decrypt the keyfile
# with either method). As an example aes256 is chosen, the cipher
# is not important to this guide, but do note that your kernel
# must support it at boot time (be it built into the kernel image
# or loaded as a module from the initramfs).
$ cat /safe/place/keyfile.bak.gpg | gpg -d | gpg --encrypt --recipient ${RECIPIENT} --cipher-algo aes256 --armor -c > /safe/place/keyfile_sc.gpg

# Verify that you can decrypt your new keyfile both with the password
# and your smartcard.
# (with smartcard inserted, you should be prompted for your PIN, unless
#  you already did so and have not yet timed out)
$ gpg -d /safe/place/keyfile_sc.gpg
# (with smartcard disconnected, you should be prompted for your password)
$ gpg -d /safe/place/keyfile_sc.gpg

# After verification, replace your old keyfile with your new one
$ su -c 'cp /safe/place/keyfile_sc.gpg /boot/${KEYFILE}'

# Export your public key to where crypt-gpg can find it
$ gpg --armor --export-options export-minimal --export ${RECIPIENT} > /safe/place/crypt-public-key.gpg
$ su -c 'cp /safe/place/crypt-public-key.gpg /etc/dracut.conf.d/crypt-public-key.gpg'

# Rebuild your initramfs as usual
# When booting with any of the requirements not met, crypt-gpg will default to password-based keyfile unlocking.
# If all requirements are met and smartcard support is not disabled by setting the kernel option "rd.luks.smartcard=0"
# crypt-gpg will try find and use a connected OpenPGP smartcard by prompting you for the PIN and then
# unlocking the gpg keyfile with the smartcard.