Difference between revisions of "Dm-crypt"

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The third and fourth point are covered in the later sections. Since the Arch installation media comes with all the tools required for system encryption, you can follow the [[Installation Guide]] or the [[Beginners' Guide]] after the encrypted partitions are set up. You will have to adjust the system configuration to be able to boot from your LUKS-volumes though, which is also covered in setup examples in the later sections.
The third and fourth point are covered in the later sections. Since the Arch installation media comes with all the tools required for system encryption, you can follow the [[Installation Guide]] or the [[Beginners' Guide]] after the encrypted partitions are set up. You will have to adjust the system configuration to be able to boot from your LUKS-volumes though, which is also covered in setup examples in the later sections.
== Encrypting a system partition ==
{{Note | This install example covers the a full system encryption with dmcrypt+LUKS in a simple partition layout. For other partition layouts, e.g. LVM, please see the other examples.}}
Most of the installation of an encrypted system partition can be carried out normally. However, there are a few areas where it is important to make certain selections. These are marked below.
=== Prepare hard drive for Arch Install Scripts ===
This assumes you want to install an encrypted system with the Arch Install Scripts, have created partitions for {{ic|/}} (e.g. {{ic|/dev/sdaX}}) and {{ic|/boot}} ({{ic|/dev/sdaY}}) at least, following the [[Installation_Guide|Installation Guide]] and deciding against [[LUKS#Encrypting a LVM setup|using LVM]]. Prior to creating the partitions you have done a [[LUKS#Secure_erasure_of_the_hard_disk_drive|preparation]] of the disk for encryption according to your necessities (the necessary tools are on the installation-ISO).
First check, if the blockdevice mapper {{ic|dm_mod}} is loaded with
# lsmod | grep mod
If one wants to use the default LUKS-cipher algorithm, there is no need to specify one for the luksFormat. You may want to check the [[LUKS#Using_LUKS_to_Format_Partitions_with_a_Passphrase|defaults]] used by the cryptsetup version at time of installation and decide yourself. With defaults a dm-crypt/LUKS blockdevice for the crypted root can be created 
# cryptsetup -y -v luksFormat /dev/sdaX
# cryptsetup open /dev/sdaX cryptroot
formatted with your desired filesystem
# mkfs -t ext4 /dev/mapper/cryptroot
and mounted
# mount -t ext4 /dev/mapper/cryptroot /mnt
At this point, just before installing the base system, it might be useful to check the mapping works as intended:
# umount /mnt
# cryptsetup close cryptroot
and mount it again to check.
If you created a separate {{ic|/home}} partition, the steps have to be adapted and repeated for that. In [[LUKS#Encrypting_the_home_partition]] the creation is described and in [[LUKS#Crypttab]] how to activate the automount during boot. Note that each blockdevice requires its own passphrase. This may be inconvenient, because it results in a separate passphrase to be input during boot. An alternative is to use a key-file stored in the system partition to unlock the separate partition via {{ic|crypttab}}. How that is done may be derived from the [[Dm-crypt_with_LUKS#Encrypting_.2Fhome_after_reboot|LVM example]] below. If you leave unpartitioned disk space, the necessary configuration for the {{ic|/home}} partition may be done later too along with migrating data.
What you do have to setup is a non-encrypted {{ic|/boot}} partition, which is needed for a crypted root. For a standard [[EFI|MBR/non-EFI]] {{ic|/boot}} partition that may be achieved by formatting
# mkfs -t ext2 /dev/sdaY
creating a mount-point for installation
# mkdir /mnt/boot
and mounting it
# mount -t ext2 /dev/sdaY /mnt/boot
That is basically what is necessary at this point before installing the base system with the [[Installation_Guide#Mount_the_partitions|Arch Install Scripts]]. Take care to install the bootloader to {{ic|/mnt/boot}} with the {{ic|pacstrap}} script. Additional configuration steps must be followed before booting the installed system.
=== Configure initramfs ===
One important point is to add the hooks relevant for your particular install in the correct order to {{ic|/etc/mkinitcpio.conf}}. The one you ''have'' to add when encrypting the root filesystem is {{ic|encrypt}}. A recommended hook for LUKS encrypted blockdevices is {{ic|shutdown}} to ensure controlled unmounting during system shutdown. Others needed, e.g. {{ic|keymap}}, should be clear from other manual steps you follow during the installation and further details in the following. For detailed information about initramfs configuration and available Hooks refer to [[Mkinitcpio#HOOKS]].
{{Note|The {{ic|encrypt}} hook is only needed if your '''root''' partition is a ''LUKS'' partition (or a LUKS partition that needs to be mounted ''before'' root). The {{ic|encrypt}} hook is not needed for any other encrypted partitions (swap, for example). System initialization scripts ({{ic|/etc/rc.sysinit}} and {{ic|/etc/crypttab}} among others) take care of those.}}
It is important that the {{ic|encrypt}} hook comes ''before'' the {{ic|filesystems}} hook (in case you are using '''LVM on LUKS''', the order should be: {{ic|encrypt lvm2 filesystems}}), so make sure that your {{ic|HOOKS}} array looks something like this:
{{hc|etc/mkinitcpio.conf|HOOKS<nowiki>=</nowiki>"(base udev) ... '''encrypt''' ... filesystems ..."}}
If you need support for foreign keymaps for your encryption password, you have to specify the hook {{ic|keymap}} as well before {{ic|encrypt}}.
If you have a USB keyboard, you will need the {{ic|keyboard}} hook. Without it, no USB keyboard will work in early userspace. If you still have this problem after adding {{ic|keyboard}}, try {{ic|usbinput}}, though this is deprecated.
In the same file, you may want to add to "MODULES" '''dm_mod''' and the filesystem types used, e.g: {{ic|MODULES<nowiki>=</nowiki>"dm_mod ext4"}}
After you are done don't forget:
mkinitcpio -p linux
=== Kernel parameter configuration of the bootloader ===
In order to enable booting an encrypted root partition, it is passed to the {{ic|encrypt}} hook with [[kernel parameters]] to be set up in the bootloader.
The main parameter is '''cryptdevice''', with the following syntax:
; <device>: The path to the raw encrypted device. Usage of [[Persistent block device naming]] is advisable.
; <dmname>: The name given to the device after decryption, will be available as {{ic|/dev/mapper/<dmname>}}. (<dmname> '''MUST NOT''' be set to a name already used for LVM partitions!)
So if the encrypted root device in the example is {{ic|/dev/sda2}} and the decrypted one should be mapped to {{ic|/dev/mapper/cryptroot}}, the kernel parameter would be:
This will make the system prompt for the passphrase to unlock the root device on a cold boot.
Depending on the setup other parameters are required as well:
cryptdevice=<device>:<dmname> root=<device> resume=<device> cryptkey=<device>:<fstype>:<path>
; root=<device>: The device file of the actual (decrypted) root filesystem. If the filesystem is formatted directly on the decrypted device file this will be {{ic|/dev/mapper/<dmname>}}. If LVM is used, the device must be addressed like {{ic|/dev/mapper/<volgroup>-<pvol>}} or {{ic|/dev/<volgroup>/<pvol>}}.
; resume=<device>: The device file of the decrypted (swap) filesystem used for suspend2disk.
; cryptkey=<nowiki><device>:<fstype>:<path></nowiki>: Required for reading a [[#Storing_the_Key_File|keyfile]] from a filesystem.
The syntax for the optional '''cryptkey''' parameter is:
; <device>: The raw block device where the key exists.
; <fstype>: The filesystem type of <device> (or auto).
; <path>: The absolute path of the keyfile within the device.
Examples on configuration for [[GRUB#Root encryption|GRUB]] and [[Syslinux#Basic Config|Syslinux]] are available on the respective pages.
=== Fstab ===
Further, double-check the {{ic|gen[[fstab]]}} scripts result for your {{ic|/dev/mapper/cryptroot}} and other mounts.
== Encrypting the home partition ==
This example covers encryption of a home partition or comparable other partition containing user data. For full system encryption see [[LUKS#Encrypting a system partition|Encrypting a system partition]].
You can either have a single user's home directory on a partition, or create a common partition for all user's home partitions.
Prepare the hard disk drive by performing a [[LUKS#Secure_erasure_of_the_hard_disk_drive|secure erasure]]. Create at least one partition, that will be used as home partition, with a [[Partitioning|partitioning tool]] of your choice. Note that the secure erasure can also be done to that single partition only.
Setup the LUKS and dm-crypt magic with
# cryptsetup  [OPTION...] luksFormat <device>
as described in the section [[LUKS#Mapping_Physical_Partitions_to_LUKS|Mapping Physical Partitions to LUKS]].
In the above command replace {{ic|<device>}} with the previously created home partition.
To gain access to the encrypted partition, unlock it with the device mapper, using
# cryptsetup open <device> <name>
It is recommended to read the full explanation about this command [[LUKS#Using_LUKS_to_Format_Partitions_with_a_Passphrase|here]].
After unlocking the partition, it will be available at {{ic|/dev/mapper/<name>}}. Now create a [[File_Systems|filesystem]] of your choice with
# mkfs.fstype /dev/mapper/<name>
Mount the filesystem to {{ic|/home}}, or if it should be accessible to only one user to {{ic|/home/username}}.
Before shutting down the filesystem needs to unmounted and, in this order, the LUKS partition needs to be closed with
# cryptsetup close <name>
=== Automated unlocking and mounting ===
There are two different solutions for automating the process of unlocking the home partition and mounting its filesystem. Using crypttab, unlocking happens at boot time, and with Pam mount it happens on user login.
==== Crypttab ====
This is the recommended solution if you want to use one common partition for all user's home partitions.
To make systemd open and mount the encrypted partition on boot, two files need to be edited, {{ic|/etc/crypttab}} and {{ic|/etc/fstab}}. The {{ic|/etc/crypttab}} file describes encrypted block devices that are set up during system boot by {{ic|systemd-cryptsetup-generator}} automatically.
For example, to open the encrypted LUKS partition on the device {{ic|/dev/sdx1}} with the name {{ic|home}}, add this line:
{{hc|/etc/crypttab|home /dev/sdx1 none luks}}
The option {{ic|none}} will trigger a prompt during boot to type the passphrase for unlocking the partition. A keyfile can also be set up and referenced instead. This results in an automatic unlocking, if the keyfile is accessible during boot. Since LUKS offers the option to have multiple keys, the chosen option can also be changed later.
See {{ic|man crypttab (5)}} and read the file {{ic|/etc/crypttab}} for more information.
Edit {{ic|/etc/fstab}} and add an entry for the previously created path in {{ic|/dev/mapper}}. Following above example the fstab entry looks like this:
{{hc|/etc/fstab|/dev/mapper/home /home <filesystem> defaults 0 2}}
==== Pam mount ====
This is the recommended solution if you want to have a single user's home directory on a partition.
To automount user homes on login see [[Pam mount]].
== Encrypting a loopback filesystem ==
A loop device enables to map a blockdevice to a file with the standard util-linux tool {{ic|losetup}}. The file can then contain a filesystem, which can be used quite like any other filesystem. A lot of users know [[Truecrypt]] as a tool to create encrypted containers. Just about the same functionality can be achieved with a loopback filesystem encrypted with LUKS and is shown in the following example.
=== Preparation and mapping ===
First, start by creating an encrypted container!
dd if=/dev/urandom of=/bigsecret bs=1M count=10
This will create the file {{ic|bigsecret}} with a size of 10 megabytes.
losetup /dev/loop0 /bigsecret
This will create the device node {{ic|/dev/loop0}}, so that we can mount/use our container.
{{Note|If it gives you the error {{ic|/dev/loop0: No such file or directory}}, you need to first load the kernel module with {{ic|modprobe loop}}. These days (Kernel 3.2) loop devices are created on demand. Ask for a new loop device with {{ic|losetup -f}}.}}
cryptsetup luksFormat /dev/loop0
This will ask you for a password for your new container file.
{{Note|If you get an error like {{ic|Command failed: Failed to setup dm-crypt key mapping. Check kernel for support for the aes-cbc-essiv:sha256 cipher spec and verify that /dev/loop0 contains at least 133 sectors|}}, then run {{ic|modprobe dm-mod}}.}}
cryptsetup open --type luks /dev/loop0 secret
The encrypted container is now available through the device file {{ic|/dev/mapper/secret}}.
Now we are able to create a partition in the container:
mkfs.ext2 /dev/mapper/secret
and mount it...
mkdir /mnt/secret
mount -t ext2 /dev/mapper/secret /mnt/secret
We can now use the container as if it was a normal partition!
To unmount the container:
umount /mnt/secret
cryptsetup luksClose secret
losetup -d /dev/loop0 # free the loopdevice.
so, if you want to mount the container again, you just apply the following commands:
losetup /dev/loop0 /bigsecret
cryptsetup open --type luks /dev/loop0 secret
mount -t ext2 /dev/mapper/secret /mnt/secret
=== Encrypt using a key-file ===
Let us first generate a 2048 byte random keyfile:
dd if=/dev/urandom of=keyfile bs=1k count=2
We can now format our container using this key
cryptsetup luksFormat /dev/loop0 keyfile
or our partition :
cryptsetup luksFormat /dev/hda2 keyfile
Once formatted, we can now open the LUKS device using the key:
cryptsetup -d keyfile open --type luks /dev/loop0 container
You can now like before format the device {{ic|/dev/mapper/container}} with your favorite filesystem and then mount it just as easily.
The keyfile is now the only key to your file. I personally advise encrypting your keyfile using your private GPG key and storing an off-site secured copy of the file.
=== Resizing the loopback filesystem ===
First we should unmount the encrypted container:
umount /mnt/secret
cryptsetup luksClose secret
losetup -d /dev/loop0 # free the loopdevice.
After this we need to expand our container file with the size of the data we want to add:
dd if=/dev/urandom bs=1M count=1024 | cat - >> /bigsecret
Be careful to really use TWO {{ic|>}}, or you will override your current container!
You could use {{ic|/dev/zero}} instead of {{ic|/dev/urandom}} to significantly speed up the process, but with {{ic|/dev/zero}} your encrypted filesystems will ''not be as secure''. (A better option to create random data quicker than {{ic|/dev/urandom}} is {{ic|frandom}} [https://aur.archlinux.org/packages.php?ID=9869], available from the [[AUR]]).
A faster (almost instant) method than dd is {{ic|truncate}} , but its use has the same security implications as using /dev/zero. The size passed to truncate is the final size to make the file, so don't use a value less than that of the current file or you will lose data. e.g. to increase a 20G file by 10G: truncate -s 30G filename.
Now we have to map the container to the loop device:
losetup /dev/loop0 /bigsecret
cryptsetup open --type luks /dev/loop0 secret
After this we will resize the encrypted part of the container to the maximum size of the container file:
cryptsetup resize secret
Finally, we can resize the filesystem. Here is an example for ext2/3/4:
e2fsck -f /dev/mapper/secret # Just doing a filesystem check, because it's a bad idea to resize a broken fs
resize2fs /dev/mapper/secret
You can now mount your container again:
mount /dev/mapper/secret /mnt/secret
== Encrypting a LVM setup ==
It is easy to use encryption with [[LVM]]. This section describes specific aspects of setting up an encrypted LVM setup and has a mini-howto for users with experiences of using both, cryptsetup and LVM. If you are looking for verbose installation instructions of such a setup, you might want to read [[Encrypted_LVM]] first.
If you do not know how to set up LVM, then read [[Installing with Software RAID or LVM]]. Using LVM is particularly helpful when a system with multiple partitions is planned. While there are a number of alternatives for unlocking multiple partitions with the same passphrase/key, the default Arch {{ic|mkinitcpio}} hooks do not implement non-standard ways. A combination of the {{ic|encrypt}} and {{ic|lvm2}} hooks, however, enables to setup a system with numerous logical volumes as partitions while using one passphrase/key to unlock them.
'''LVM on LUKS'''
The straight-forward method is to set up LVM on top of the encrypted partition instead of the other way round. Technically the LVM is setup inside one big encrypted blockdevice. Hence, the LVM is not transparent until the blockdevice is unlocked and the underlying volume structure is scanned and mounted during boot.
The most important thing in setting LVM on '''top''' of encryption is to [[#Configure initramfs|configure the initramfs]] for running '''both''' the {{ic|encrypt}} hook '''and''' the {{ic|lvm2}} hook (and those two before the {{ic|filesystems}} hook). In the past, it was necessary to ensure the correct ordering of these hooks in {{ic|/etc/mkinitcpio.conf}} but the order no longer matters with the current implementation of {{ic|lvm2}}.
'''LUKS on LVM'''
To use encryption on top of LVM, the LVM volumes are set up first and then used as the base for the encrypted partitions. This way around a mixture of encrypted and non-encrypted volumes/partitions is possible as well.
For encrypted partitions inside an LVM, the LVM-hook has to run first, before the respective encrypted logical volumes can be unlocked. So for this add the {{ic|encrypt}} hook in {{ic|/etc/mkinitcpio.conf}} '''after''' the {{ic|lvm2}} hook, if you chose to set up encrypted partitions on '''top''' of LVM.
Both these options are described in more detail in [[Encrypted_LVM]]. The following represents the historic guide from before the verbose page.
=== LVM and dm-crypt manually (short version) ===
The following short example creates a LUKS on LVM setup and mixes in the use of a key-file for the /home partition and temporary crypt volumes for /tmp and /swap. The latter is considered desirable from a security perspective, because no potentially sensitive temporary data survives the reboot, when the encryption is re-initialised. If you are experienced with LVM, you will be able to ignore/replace LVM- and other specifics according to your plan. For {{ic|cryptsetup}} options, please see [[Dm-crypt_with_LUKS#Configuring_LUKS|above]].
==== Partitioning scheme ====
{{ic|/dev/sda1}} -> {{ic|/boot}}
{{ic|/dev/sda2}} -> LVM
==== Setting up the encrypted system ====
cryptsetup -d /dev/random -c aes-xts-plain -s 512 create lvm /dev/sda2
dd if=/dev/urandom of=/dev/mapper/lvm
cryptsetup remove lvm
lvm pvcreate /dev/sda2
lvm vgcreate lvm /dev/sda2
lvm lvcreate -L 10G -n root lvm
lvm lvcreate -L 500M -n swap lvm
lvm lvcreate -L 500M -n tmp lvm
lvm lvcreate -l 100%FREE -n home lvm
cryptsetup luksFormat -c aes-xts-plain -s 512 /dev/lvm/root
cryptsetup open --type luks /dev/lvm/root root
mkreiserfs /dev/mapper/root
mount /dev/mapper/root /mnt
dd if=/dev/zero of=/dev/sda1 bs=1M
mkreiserfs /dev/sda1
mkdir /mnt/boot
mount /dev/sda1 /mnt/boot
mkdir -p -m 700 /mnt/etc/luks-keys
dd if=/dev/random of=/mnt/etc/luks-keys/home bs=1 count=256
==== Install Arch Linux ====
Now after setup of the encrypted LVM partitioning, it would be time to install: [[Installation_Guide#Mount_the_partitions|Arch Install Scripts]].
==== Configuration ====
===== /etc/mkinitcpio.conf =====
Put {{ic|lvm2}} and {{ic|encrypt}} (in that order) before {{ic|filesystems}} in the {{ic|HOOKS}} array. Again, note that this is setting encryption on '''top''' of LVM.
If you want install the system on a usb stick, you need to put {{ic|usb}} just after {{ic|udev}}.
===== Boot options for LUKS-LVM =====
For the above example, change the kernel options for the root-device auto-configured in the bootloader installation from {{ic|root<nowiki>=</nowiki>/dev/hda3}} to
cryptdevice=/dev/lvm/root:root root=/dev/mapper/root
More general, the kernel command line for LUKS <-> LVM is constructed like this:
root=/dev/mapper/<volume-group>-<logical-volume> cryptdevice=/dev/<luks-part>:<volume-group>
For example:
root=/dev/mapper/vg-arch cryptdevice=/dev/sda4:vg
Or like this:
cryptdevice=/dev/<volume-group>/<logical-volume>:root root=/dev/mapper/root
If you want install the system on a usb stick, you need to add {{ic|lvmdelay<nowiki>=</nowiki>/dev/mapper/lvm-root}}
===== Filesystem mounts system =====
/dev/mapper/root        /      reiserfs        defaults        0      1
/dev/sda1              /boot  reiserfs        defaults        0      2
/dev/mapper/tmp        /tmp    tmpfs          defaults        0      0
/dev/mapper/swap        none    swap            sw              0      0}}
swap /dev/lvm/swap SWAP -c aes-xts-plain -h whirlpool -s 512
tmp /dev/lvm/tmp /dev/urandom -c aes-xts-plain -s 512}}
===== Encrypting /home after reboot =====
cryptsetup luksFormat -c aes-xts-plain -s 512 /dev/lvm/home /etc/luks-keys/home
cryptsetup open --type luks -d /etc/luks-keys/home /dev/lvm/home home
mkreiserfs /dev/mapper/home
mount /dev/mapper/home /home
===== Filesystem mounts home =====
home /dev/lvm/home  /etc/luks-keys/home}}
/dev/mapper/home        /home  reiserfs        defaults        0      0}}
== Resources ==
== Resources ==
* [http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions cryptsetup FAQ] - The main and foremost help resource, directly from the developers.
* [http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions cryptsetup FAQ] - The main and foremost help resource, directly from the developers.
* [http://www.freeotfe.org/ FreeOTFE] - Supports unlocking LUKS encrypted volumes in Microsoft Windows.
* [http://www.freeotfe.org/ FreeOTFE] - Supports unlocking LUKS encrypted volumes in Microsoft Windows.

Revision as of 13:36, 18 November 2013

Tango-document-new.pngThis article is a stub.Tango-document-new.png

Notes: This article is currently under heavy restructuring: for its latest stable revision see Dm-crypt with LUKS (Discuss in Talk:Dm-crypt#)

Merge-arrows-2.pngThis article or section is a candidate for merging with Plain dm-crypt without LUKS.Merge-arrows-2.png

Notes: Assess the possibility of merging the common content between the two articles in order to avoid duplication. (Discuss in Talk:Plain dm-crypt without LUKS#Merge)

This article focuses on how to set up full system encryption on Arch Linux, using dm-crypt with LUKS.

dm-crypt is the standard device-mapper encryption functionality provided by the Linux kernel. It can be used directly by those who like to have full control over all aspects of partition and key management.

LUKS is an additional convenience layer which stores all of the needed setup information for dm-crypt on the disk itself and abstracts partition and key management in an attempt to improve ease of use.

For more details on how dm-crypt+LUKS compares to other disk encryption solution, see Disk Encryption#Comparison table.


Warning: Encrypting a disk or partition will erase everything currently on that disk or partition:
  • Please make appropriate data backups prior to starting

Also be aware that encrypting a system might not only make the life of laptop thieves more miserable, but also yours if you don't plan ahead on

  • how to make secure backups of the encrypted system/-setup/data and
  • how to access the encrypted system manually for maintenance.
Keeping those points in mind while deciding on how to use encryption may help to decide on method and tools as well.

Initial Setup

Overview and Preparation

The installation of a LUKS-encrypted system is largely the same as installing an unencrypted system. Routine creation of an encrypted system follows these general steps:

  • Secure erasure of the hard disk drive(s)
  • Partitioning and setup of encryption (LVM optional)
  • Routine package selection and installation
  • System configuration to handle the encryption

This page covers the first two points in a general way for different configuration options available with LUKS.

The third and fourth point are covered in the later sections. Since the Arch installation media comes with all the tools required for system encryption, you can follow the Installation Guide or the Beginners' Guide after the encrypted partitions are set up. You will have to adjust the system configuration to be able to boot from your LUKS-volumes though, which is also covered in setup examples in the later sections.


  • cryptsetup FAQ - The main and foremost help resource, directly from the developers.
  • FreeOTFE - Supports unlocking LUKS encrypted volumes in Microsoft Windows.