Using GnuPG keys for LUKS dm-crypt devices in Debian

The Debian cryptsetup package provides the keyscript decrypt_gnupg for setups with a GnuPG encrypted LUKS keyfile.

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 encrypted keyfile:

dd if=/dev/random bs=1 count=256 | gpg --no-options --no-random-seed-file \
        --no-default-keyring --keyring /dev/null --secret-keyring /dev/null \
        --trustdb-name /dev/null --symmetric --output /etc/keys/cryptkey.gpg

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

/lib/cryptsetup/scripts/decrypt_gnupg /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,discard,keyscript=decrypt_gnupg

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), 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 GnuPG encrypted key is copied to the initramfs by the initramfs cryptgnupg hook. If you don’t want this, you should take a look at the initramfs cryptgnupg hook, which is located at /usr/share/initramfs-tools/hooks/cryptgnupg.

– Jonas Meurer jonas@freesources.org Thu, 04 Mar 2010 17:31:40 +0100

– Guilhem Moulin guilhem@guilhem.org Sat, 17 Sep 2016 16:14:41 +0200