Encrypted root volume with OpenPGP smartcard

Things have sure come along since I wrote gpgcryptroot.tar.gz. We can now use GnuPG properly from an initramfs in Debian jessie. Unfortunately, the supplied scripts only handle the case where GnuPG is used with symmetric (password based) encryption. I’m not sure in which circumstances you would want to use that; smartcard based decryption is so much cooler than that!

This mini-guide is for advanced users; I assume you understand all the commands involved. In the section Download is a link to the scripts that are used. I recommend reading the scripts to see what they are like.

Let’s assume you used LUKS to put a password on your root volume, for instance by using the Debian installer. We will install a LUKS “password” that is unlocked by an OpenPGP smartcard.

(If you also have other volumes protected by LUKS and want to unlock those with your smartcard, see the section Decrypt derived further down.)

So suppose your /etc/crypttab looks something like this:

mobidisk-crtest_crypt UUID=2b0c0898-a92d-48ac-a2b7-2dd9419121bd none luks

This is what the Debian wheezy installer created for me when I created a logical volume named crtest on the volume group mobidisk, and used that for an encrypted root volume. The UUID, by the way, can be found with:

# cryptsetup luksDump /dev/mobidisk/crtest

Copy the included scripts to their proper place: cryptgnupg_sc in /etc/initramfs-tools/hooks/ and decrypt_gnupg_sc in /lib/cryptsetup/scripts/. These scripts have been derived from their non-_sc-counterparts.

Have root get your public key. For example:

# gpg --recv-key {YOURKEYID}

The next step is to create some random material to be used as a LUKS key. I’m simply adapting the instructions from README.gnupg from the cryptsetup package to our use.

We create a GnuPG-encrypted file with both a public key and a password, so you can still enter that password when you don’t have your smartcard (or your smartcard is broken, and the initramfs demands you insert that specific card, which it will). NOTE that it is only possible to enter a different LUKS password on boot by specifying a custom cryptopts boot argument! By default, only the GnuPG way stays working. That’s what the emergency password on the GnuPG-encrypted file is for.

You need to enter both an existing password for the LUKS root volume and a new password for the GnuPG-encrypted file.

# mkdir -m 700 /etc/keys
# dd if=/dev/random bs=1 count=256 | gpg -o /etc/keys/cryptkey.gpg \
        -r {YOURKEY} -ec
# cd /root
# mkfifo -m 700 keyfifo
# gpg -d /etc/keys/cryptkey.gpg >keyfifo

Open a second root terminal and enter:

# cd /root
# cryptsetup luksAddKey /dev/mobidisk/crtest keyfifo

Continue on one of the two terminals (other can be closed):

# rm keyfifo
# gpg --export-options export-minimal --export {YOURKEYID} | gpg \
        --no-default-keyring --keyring /etc/keys/pubring.gpg \
        --secret-keyring /etc/keys/secring.gpg --import
# gpg --no-default-keyring --keyring /etc/keys/pubring.gpg \
        --secret-keyring /etc/keys/secring.gpg --card-status

Adapt /etc/crypttab in the following way (all on one line):

mobidisk-crtest_crypt UUID=2b0c0898-a92d-48ac-a2b7-2dd9419121bd
/etc/keys/cryptkey.gpg luks,keyscript=decrypt_gnupg_sc

And finally:

# update-initramfs -u
update-initramfs: Generating /boot/initrd.img-3.13-1-686-pae
WARNING: GnuPG key /etc/keys/cryptkey-new.gpg is copied to initramfs
WARNING: GnuPG secret keyring /etc/keys/secring.gpg is copied to initramfs

If your output isn’t comparable, look for a mistake.

You can now unlock your root volume with your smartcard or the password you entered when you created cryptkey.gpg. If not, you’ve now locked yourself out of your system. Good luck!

It is possible to have the initramfs ignore your /etc/crypttab (the copy included in the initramfs) by entering a custom cryptopts= boot argument. For instance, if the default command line were:

BOOT_IMAGE=/vmlinuz-3.13-1-686-pae root=/dev/mapper/mobidisk-crtest_crypt ro
initrd=/install/initrd.gz quiet

You could add:

cryptopts=target=mobidisk-crtest_crypt,source=/dev/mapper/mobidisk-crtest,luks

Now you will be prompted for any LUKS password, and decrypt_gnupg_sc is never invoked.

Decrypt derived

README.initramfs in the cryptsetup package discusses the decrypt_derived script, but it only uses this to unlock a plain dm-crypt volume, not a LUKS volume. If you used the Debian installer to create multiple LUKS volumes, it would be nice if they were all unlocked on boot without having to enter the smartcard PIN multiple times. decrypt_derived takes the master key from an already unlocked volume (that is, the cryptographic key actually used to encrypt data on the volume, not one of the LUKS passwords), and uses that as key for the volume to unlock. With LUKS, it’s possible to use the master key as a LUKS “password”.

I’m assuming the /etc/crypttab is as follows:

mobidisk-crtest_crypt [...]
mobidisk-crdata_crypt UUID=2fa1fd9d-e169-4d55-a59e-a0bd0553444f none luks

Enter the following command. You will be prompted for an existing LUKS password for crdata.

# cryptsetup luksAddKey /dev/mobidisk/crdata \
    <(/lib/cryptsetup/scripts/decrypt_derived mobidisk-crtest_crypt)

Change /etc/crypttab to:

mobidisk-crtest_crypt [...]
mobidisk-crdata_crypt UUID=2fa1fd9d-e169-4d55-a59e-a0bd0553444f \
    mobidisk-crtest_crypt luks,keyscript=decrypt_derived

The entry for crdata is one line; escape-style continuation is not supported for crypttab, and only used here for readability.

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