Cryptsetup for Debian

Table of Contents

1. Introduction into Cryptsetup for Debian

Cryptsetup is a command-line interface for configuring encrypted block devices via dm-crypt, a kernel device-mapper target. For documentation about the cryptsetup tool, see manpage of cryptsetup(8) and the frequently asked questions at /usr/share/doc/cryptsetup/FAQ.gz.

The Debian cryptsetup package provides the initscript /etc/init.d/cryptdisks and a configuration file /etc/crypttab for automatically configuring encrypted devices at boot time. The applications cryptdisks_start and cryptdisks_stop are provided to process crypttab configured devices manually. See the manpages of crypttab(5), cryptdisks_start(8) and cryptdisks_stop(8) for more information.

The luksformat script provides a simple interface for creating an encrypted device that follows the LUKS standard and for putting a file system onto the encrypted device. See man luksformat(8) for more information.

If you wish to perform a Debian installation to an encrypted root, you might be interested in using a version of Debian Installer with partman-crypto, which will install the system and setup cryptsetup and initramfs-tools.

For instructions about how to encrypt your root filesystem and integrate cryptsetup into initramfs on a running system, see /usr/share/doc/cryptsetup-initramfs/README.initramfs.gz.

2. Encrypted swap partition(s)

An encrypted swap partition prevents spying on plaintext secrets (passwords) that may be written to disk when memory is swapped to disk.

To encrypt your swap partitions, you’ll first have to deactivate your swap:

swapoff -a

You’ll have to add an entry for every swap partition in /etc/crypttab. Be sure to place the source device (here /dev/sde9) with your swap devices:

# <target name> <source device> <key file>      <options>
cswap1          /dev/sde9       /dev/urandom    swap,cipher=aes-xts-plain64,size=256,hash=sha1

Now you need to change the swap devices in /etc/fstab to the encrypted swap device names (/dev/mapper/cswap1 in this example).

# <file system> <mount point>   <type>  <options>     <dump>  <pass>
/dev/sde9        none           swap    sw            0       0

becomes

# <file system> <mount point>   <type>  <options>     <dump>  <pass>
/dev/mapper/cswap1  none        swap    sw            0       0

Then, you need to start the cryptsetup swap devices and reactivate swap:

cryptdisks_start cswap1
swapon -a

And finally, if /dev/sde9 was previously used as resume device, you should disable it (the new swap partition is mapped with a non-persistent key hence can’t be used for resuming after suspend to disk). With initramfs-tools 0.130 and later, this can be done with

echo "RESUME=none" >/etc/initramfs-tools/conf.d/resume
update-initramfs -u

That’s it! You have a crypted swap device. Note that /dev/urandom provides only pseudo-random entropy. So if you’re paranoid rather use /dev/random as source for random data. Be aware though that /dev/random might not provide enough random bytes for your key, causing your system to hang at boot, waiting for more entropy. Moving mouse and keyboard typing might help in this case.

Read the crypttab(5) manpage for more information, for example options to use a different encryption algorithm than the default.

3. Insecure mode/owner for keys

Any key that is stored somewhere to be used with cryptsetup should have the mode 400 (-r--------) and root as owner/group. chown root.root keyfile and chmod 400 keyfile will do the trick for you.

If a key is stored on a vfat filesystem (very common for removable media), chmod and chown will not work. The vfat filesystem (and several others too) does not support file permissions and ownership. Instead, you should use the uid, gid and umask options in /etc/fstab to ensure secure permissions for the key.

As an example, assume that /dev/sdg8 is the removable media containing keyfiles on a vfat filesystem and that it is going to be mounted on /media/flash0. The configuration in /etc/fstab should then be something like this:

# <file system> <mount point>   <type>  <options>               <dump>  <pass>
/dev/sdg8       /media/flash0   vfat    uid=0,gid=0,umask=277   0       0

If you are using udev, it might be a good idea to use the /dev/disk/by-label links instead of /dev/sdg8 as the link will work no matter in which order the media is inserted and detected.

4. Cryptsetup and udev

As a workaround for some yet-to-be-fixed race condition in kernel, device-mapper or udev, cryptsetup currently runs udevsettle.

This leads to problems if you invoke cryptsetup as part of a udev rule. udevsettle waits until queued kernel/udev events are processed and the “run programs” have finished. Due to cryptsetup itself being a “run program” in this case, this ends in a deadlock.

Therefore cryptsetup should be detached directly after invocation in this case, so that it runs asynchronously.

5. Useful keyscripts: askpass and passdev

The cryptsetup package ships with several keyscripts. Keyscripts may be configured in /etc/crypttab in order to provide the key required to unlock the device. The shipped keyscripts are located at /lib/cryptsetup/scripts.

Some keyscripts have an own README file at /usr/share/doc/cryptsetup/.

Two special keyscripts, worth being mentioned here, are askpass and passdev.

Askpass is located at /lib/cryptsetup/askpass. It’s a simple helper program that supports different methods (console, fifo, splashy, …) to prompt for a passphrase, and prints the result to stdout. The syntax is:

/lib/cryptsetup/askpass PROMPT

Passdev will wait for a given device to appear, mount it read-only, read the key, and unmount the device. See /usr/share/doc/cryptsetup-initramfs/README.initramfs.gz for more information about passdev.

6. The check option

The check option in crypttab allows one to configure checks to be run against the target device after cryptsetup has been invoked. The default check blkid can check for any known filesystem type, as it uses blkid from util-linux. you can check for a particular filesystem by giving for example checkargs=ext4 or checkargs=swap as an option in /etc/crypttab.

Please send us your checks, if you write new ones. If they are generally useful, we will include them in the package.

See man crypttab(5) for more information about the checksystem.

7. Cryptsetup and Splashy

Splashy support in cryptsetup is currently somehow limited. Splashy is known to freeze at the password dialog for encrypted non-root filesystems. Only the password dialog for the encrypted root filesystem works.

It seems like splashy freezes for any input dialog in initscripts while input dialogs at initramfs stage seem to work. This leads to the assumption that the bug is somewhere in splashy and neither in cryptsetups initscripts nor in askpass.

8. Remotely unlock encrypted rootfs

Thanks to Chris debian@x.ray.net it’s possible to install a dropbear SSH server into the initramfs, connect to this SSH server during execution of initramfs early in the boot process, and unlock encrypted devices - even the root device - before the boot process continues. (Note that in order to force an arbitrary device to be processed at initramfs stage you might need to set the initramfs option in its crypttab entry; see crypttab(5) for details.)

This way it is possible to use an encrypted root filesystem on headless systems where no physical access is available during boot process.

Dropbear 0.52-1 or later is required for this to work. (Since 2015.68-1 the functionality has its own binary package dropbear-initramfs.) Consult /usr/share/doc/dropbear-initramfs/README.initramfs from the dropbear-initramfs package for information how to install and configure the dropbear SSH server into the initramfs.

You can then unlock the disk remotely via SSH with

ssh -tF ~/.luks/ssh.conf root@remote.system.com cryptroot-unlock

Or, using a local gpg-encrypted key file:

gpg --decrypt ~/.luks/remote.key.gpg | ssh -TF ~/.luks/ssh.conf root@remote.system.com cryptroot-unlock

When its standard input is a TTY, cryptroot-unlock keeps prompting for passphrases until there are no more devices to unlock; otherwise you’ll need to invoke it as many times as there are devices to unlock.

That’s it. Now that all required encrypted devices are unlocked, the remote system should continue with the boot process.

You can also use the following authorized_keys(5) options in /etc/dropbear-initramfs/authorized_keys to restrict access and avoid users poking around:

no-port-forwarding,no-agent-forwarding,no-X11-forwarding,command="/bin/cryptroot-unlock" ssh-rsa ...

(Be sure to rebuild the initrd afterwards: update-initramfs -u -k all)

9. Backup the LUKS header

WARNING: This information might be outdated. Please read the cryptsetup FAQ at /usr/share/doc/cryptsetup/FAQ.gz for up-to-date information on how to backup the LUKS header.

The LUKS header is located at the beginning of every LUKS encrypted device. It stores information such as used cipher, hash, etc. But most importantly, the header contains eight keyslots, which do keep an encrypted version of the LUKS masterkey. the data on an encrypted LUKS partition is encrypted with this masterkey. thus, there’s no way to restore the data once the masterkey is lost. For that reason, one might want to backup the LUKS header in order to prevent accidental data loss.

On the other hand keeping a backup of the LUKS header isn’t recommended for security reasons. The reason is, that LUKS was designed with key revocation in mind. Once the LUKS header is copied to a backup, revoking a (possibly compromised) passphrase or keyfile from the keyslot isn’t enough anymore. the revoked passphrase/keyfile can easily be reactived by writing back the header backup to the device.

Beginning with version 1.1.0, cryptsetup has support for the commands luksHeaderBackup and luksHeaderRestore. If you want to store a backup of your LUKS header with the mentioned drawbacks in mind, do the following:

Prepare a ramdisk to store the backup temporarely. You should do that in order to prevent any hardware caching functions or filesystem jounals to copy the backup around to places you cannot control. If you want to store the backup permanently, write it to a read-only medium like CD immediately from ramdisk, without your burning program writing an intermediate image to some temp dir.

To actually backup the header, use the following command:

cryptsetup luksHeaderBackup <luks-device> --header-backup-file <destination-on-ramdisk>

That’s it. But once again, keep in mind all the security implications when doing LUKS header backups. In general it’s better to backup the data from encrypted LUKS devices to another encrypted LUKS device. That way you can manage the keyslots for both original and backup device independently.

10. Changing the boot order of cryptdisks init scripts

In order to support non-standard setups, it might be necessary to change the order of init scripts in the boot process. Cryptsetup already installs two init scripts, cryptdisks-early and cryptdisks, in order to support some complex setups. For example, both “lvm on luks” and “luks on lvm” are supported that way.

If your system isn’t supported by the default order of init scripts in the boot process, you need to change the boot process on your own. In some cases it might be enough to change the LSB dependency headers at initscripts, see /etc/init.d/README for more information about that. For more complex setups, more intrusive changes are required. For example, adding a third cryptdisks init script might help. See the log of bugreport #576646 and discussion on debian-devel for further information.

11. Unlocking LUKS devices from GRUB

GRUB has been able to unlock LUKS1 devices since early in Jessie’s release cycle. This feature removes the need for a separate cleartext /boot partition, hence enables “real” full disk encryption. However cryptsetup >=2.1 uses LUKS version 2 by default, which GRUB 2.02 doesn’t support. In other words, as of Buster it is not possible to unlock from GRUB new LUKS devices formatted with the default parameters.

Neither Jessie nor Stretch’s installers natively support unlocking from GRUB, hence users already had to implement various workarounds to enable it. Former workarounds won’t work anymore with LUKS2. Integration between LUKS and GRUB is documented at https://cryptsetup-team.pages.debian.net/cryptsetup/encrypted-boot.html, including recipes to enable the feature starting from the usual “encrypted LVM” partitioning method of the Debian Installer – both with LUKS1 (pre-Buster) and LUKS2 (Buster and later) devices.

12. Credits

People who contributed to the Debian cryptsetup package:

– Jonas Meurer jonas@freesources.org, Sun, 09 Jun 2019 15:01:09 +0200