dm-crypt/Encrypting a non-root file system - ArchWiki

# dm-crypt/Encrypting a non-root file system

From ArchWiki
< Dm-crypt

The following are examples of encrypting a secondary, i.e. non-root, filesystem with dm-crypt.

## Overview

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.

## Partition

This example covers the encryption of the `/home` partition, but it can be applied to any other comparable non-root partition containing user data.

Tip: You can either have a single user's `/home` directory on a partition, or create a common partition for all user's `/home` partitions.

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.

Then setup the LUKS header with:

    # cryptsetup options luksFormat device

Replace `device` with the previously created partition. See Dm-crypt/Device encryption#Encryption options for LUKS mode for details like the available `options`.

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.

Tip: Unmount and mount once to verify that the mapping is working as intended.

### Manual mounting and unmounting

To mount the partition:

    # cryptsetup --type luks open device name
    # mount -t fstype /dev/mapper/name /mnt/home

To unmount it:

    # umount /mnt/home
    # cryptsetup close name

Tip: GVFS can also mount encrypted partitions. One can use a file manager with gvfs support (e.g. Thunar) to mount the partition, and a password dialog will pop-up. For other desktops, zulucryptAUR also provides a GUI.

### Automated unlocking and mounting

There are three different solutions for automating the process of unlocking the partition and mounting its filesystem.

#### At boot time

Using the `/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.

## Loop device

Tip: It should be noted that using losetup directly can be avoided completely by doing an alternative method:

    # dd if=/dev/urandom of=key.img bs=20M count=1
    # cryptsetup --align-payload=1 --hash=sha512 --cipher=serpent-xts-plain64 --key-size=512 -i 30000 luksFormat key.img
    # cryptsetup open key.img lukskey

Having too small of a file will get you `Requested offset is beyond real size of device /dev/loop0` error, but as a rough reference creating a 4MiB file will encrypt it successfully. [1]

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 bs=1M count=10

This will create the file `bigsecret` with a size of 10 megabytes.

Note: To avoid having to resize the container later on, make sure to make it larger than the total size of the files to be encrypted, in order to at least also host the associated metadata needed by the internal file system. If you are going to use LUKS mode, its metadata header requires one to two megabytes alone.

Next create the device node `/dev/loop0`, so that we can mount/use our container:

    # losetup /dev/loop0 /bigsecret

Note: If it gives you the error `/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.

Tip: Containers with dm-crypt can be very flexible. Have a look at the features and documentation of Tomb. It provides a dm-crypt script wrapper for fast and flexible handling.

### 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
    # cryptsetup --type luks open /dev/loop0 secret
    # mount -t ext4 /dev/mapper/secret /mnt/secret

Categories:

- Encryption
- File systems