Using an OpenPGP smartcard for LUKS dm-crypt devices in Debian

The Debian cryptsetup package provides the keyscript decrypt_gnupg-sc for setups with a keyfile that is encrypted using an OpenPGP smartcard.

The following example assumes that you store the encrypted keyfile in /etc/keys/cryptkey.gpg. LUKS device is /dev/<luks_device>.

First, you’ll have to create the keyfile and encrypt it with your key 0xDEADBEEF:

dd if=/dev/random bs=1 count=256 | gpg --recipient 0xDEADBEEF \
        --output /etc/keys/cryptkey.gpg --encrypt

Next the LUKS device needs to be formated with the key. For that, the decrypt_gnupg-sc keyscript can be used:

/lib/cryptsetup/scripts/decrypt_gnupg-sc /etc/keys/cryptkey.gpg | \
        cryptsetup --key-file=- luksFormat /dev/<luks_device>

In order to unlock the encrypted LUKS device automatically during boot process, add the following to /etc/crypttab:

cdev1   /dev/<luks_device>      /etc/keys/cryptkey.gpg luks,keyscript=decrypt_gnupg-sc

In order to avoid data loss if the smartcard is damaged or lost, you may want to decrypt /etc/keys/cryptkey.gpg and store the plaintext in a safe place. Or alternatively, use another slot with your backup key:

cryptsetup luksAddKey /dev/<luks_device> /path/to/backup.key

Decrypting the keyfile at initramfs stage

If the device is to be unlocked at initramfs stage (such as for the root FS or the resume device), you need to copy the public part of the encryption key to /etc/cryptsetup-initramfs/pubring.gpg:

gpg --export 0xDEADBEEF >/etc/cryptsetup-initramfs/pubring.gpg

Then the provided initramfs hooks should do all additionally required work for you when the initramfs is created or updated.

Be warned though, that for such devices the OpenPGP encrypted key is copied to the initramfs by the initramfs cryptgnupg-sc hook. If you don’t want this, you should take a look at the initramfs cryptgnupg-sc hook, which is located at /usr/share/initramfs-tools/hooks/cryptgnupg-sc.

Moreover, note that unlocking at initramfs stage is currently not compatible with plymouth or other bootsplash, as a curses-based prompt is used for PIN entry.

– Guilhem Moulin Sun, 23 Sep 2018 03:28:31 +0200