dm-crypt/Encrypting a non-root file system
The following are examples of encrypting a secondary, i.e. non-root, filesystem with dm-crypt.
Encrypting a secondary filesystem usually protects only sensitive data, while leaving the operating system and program files unencrypted. This is useful for encrypting an external medium, such as a USB drive, so that it can be moved to different computers securely. One might also choose to encrypt sets of data separately according to who has access to it.
Because dm-crypt is a block-level encryption layer, it only encrypts full devices, full partitions and loop devices. To encrypt individual files requires a filesystem-level encryption layer, such as eCryptfs or EncFS. See Disk encryption for general information about securing private data.
This example covers the encryption of the
/home partition, but it can be applied to any other comparable non-root partition containing user data.
/homedirectory on a partition, or create a common partition for all user's
First make sure the partition is empty (has no file system attached to it). Delete the partition and create an empty one if it has a file system. Then prepare the partition by securely erasing it, see Dm-crypt/Drive preparation#Secure erasure of the hard disk drive.
Create the partition which will contain the encrypted container.
Then setup the LUKS header with:
# cryptsetup options luksFormat device
device with the previously created partition. See Dm-crypt/Device encryption#Encryption options for LUKS mode for details like the available
To gain access to the encrypted partition, unlock it with the device mapper, using:
# cryptsetup open device name
After unlocking the partition, it will be available at
/dev/mapper/name. Now create a file system of your choice with:
# mkfs.fstype /dev/mapper/name
Mount the file system to
/home, or if it should be accessible to only one user to
/home/username, see #Manual mounting and unmounting.
Manual mounting and unmounting
To mount the partition:
# cryptsetup open device name # mount -t fstype /dev/mapper/name /mnt/home
To unmount it:
# umount /mnt/home # cryptsetup close name
Automated unlocking and mounting
There are three different solutions for automating the process of unlocking the partition and mounting its filesystem.
At boot time
/etc/crypttab configuration file, unlocking happens at boot time by systemd's automatic parsing. This is the recommended solution if you want to use one common partition for all user's home partitions or automatically mount another encrypted block device.
See Dm-crypt/System configuration#crypttab for references and Dm-crypt/System configuration#Mounting at boot time for an example set up.
On user login
Using pam_exec it is possible to unlock (cryptsetup open) the partition on user login: this is the recommended solution if you want to have a single user's home directory on a partition. See dm-crypt/Mounting at login.
Unlocking on user login is also possible with pam_mount.
There are two methods for using a loop device as an encrypted container, one using
losetup directly and one without.
Using losetup directly can be avoided completely by doing the following :
# dd if=/dev/urandom of=bigsecret.img bs=100M count=1 iflag=fullblock # cryptsetup luksFormat bigsecret.img
Creating a file smaller than the LUKS2 header (16 MiB) will give a
Requested offset is beyond real size of device bigsecret.img error when trying to open the device.
Manual mounting and unmounting procedure is equivalent to #Manual mounting and unmounting.
A loop device enables to map a blockdevice to a file with the standard util-linux tool
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.
First, start by creating an encrypted container, using an appropriate random number generator:
# dd if=/dev/urandom of=bigsecret.img bs=100M count=1 iflag=fullblock
This will create the file
bigsecret.img with a size of 100 mebibytes.
Next create the device node
/dev/loop0, so that we can mount/use our container:
# losetup /dev/loop0 bigsecret.img
/dev/loop0: No such file or directory, you need to first load the kernel module with
modprobe loop. These days (Kernel 3.2) loop devices are created on demand. Ask for a new loop device with
# losetup -f.
From now on the procedure is the same as for #Partition, except for the fact that the container is already randomised and will not need another secure erasure.
Manual mounting and unmounting
To unmount the container:
# umount /mnt/secret # cryptsetup close secret # losetup -d /dev/loop0
To mount the container again:
# losetup /dev/loop0 bigsecret.img # cryptsetup open /dev/loop0 secret # mount -t ext4 /dev/mapper/secret /mnt/secret