Automount LUKS Encrypted Device in Linux

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In this tutorial, you will learn how to automount LUKS encrypted device in Linux on system startup. Unless you configure the device to automount, it usually doesn’t by default. However, if you enabled device encryption with LUKS during system install, the automount is usually setup and the device automatically mounts once you supply the correct drive encryption passphrase.

Please note that security wise, automounting an encrypted device might not be a good practise, IMO.

Automount LUKS Encrypted Device in Linux

In our previous tutorial, we learnt how to encrypt a disk partition with LUKS in Linux. We will be using the same device to demonstrate how to automount LUKS Encrypted Device in Linux.

Below command lists the block device that we will use to demonstrate the auto-mounting procedure.

lsblk
NAME                                          MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
sda                                             8:0    0   15G  0 disk  
├─sda1                                          8:1    0   13G  0 part  /
├─sda2                                          8:2    0    1K  0 part  
└─sda5                                          8:5    0    2G  0 part  [SWAP]
sdb                                             8:16   0    4G  0 disk  
└─sdb1                                          8:17   0    4G  0 part  
  └─luks-242c24d8-ac65-413d-b3a2-eb7f2f0993b0 254:0    0    4G  0 crypt

Create LUKS Key File

With LUKS encryption, you can unlock the device by interactively supplying the passphrase or automatically specifying a key file containing the passphrase to unlock the drive.

To automount LUKS encrypted device in Linux, then you need to use the key file containing the passphrase.

To create the LUKS key file, you use the dd command as follows.

dd if=/dev/random of=/etc/.crypt-me bs=32 count=1

So, we use the /etc/.crypt-me file as our LUKS key file, can be a different file for you. The command above fills random data on the key file as evident by the command below;

xxd /etc/.crypt-me
00000000: 62cc f2b2 b431 fdb5 d908 8cfd b6c5 b27d  b....1.........}
00000010: f38b 877a 6575 279c 3c20 5b36 a5fa ce7d  ...zeu'.< [6...}

Add a Passphrase to LUKS Key File

Once you have created a LUKS key file, you need to add a new passphrase to the file using the cryptsetup utility:

cryptsetup luksAddKey <device> <path-to-key-file>
cryptsetup luksAddKey /dev/sdb1 /etc/.crypt-me

You will be prompted to enter any existing passphrase.

If you specified the existing passphrase using the key file as well, then use the command below;

cryptsetup luksAddKey <device> <path-to-key-file> --key-file <path-to-existing-passphrase-key-file>
cryptsetup luksAddKey /dev/sdb1 /etc/.crypt-me --key-file ~/luks-key

For now, the device has two key slots used, as per out setup. To confirm, print the device details.

LUKS header information
Version:       	2
Epoch:         	4
Metadata area: 	16384 [bytes]
Keyslots area: 	16744448 [bytes]
UUID:          	242c24d8-ac65-413d-b3a2-eb7f2f0993b0
Label:         	(no label)
Subsystem:     	(no subsystem)
Flags:       	(no flags)

Data segments:
  0: crypt
	offset: 16777216 [bytes]
	length: (whole device)
	cipher: aes-xts-plain64
	sector: 512 [bytes]

Keyslots:
  0: luks2
	Key:        512 bits
	Priority:   normal
	Cipher:     aes-xts-plain64
	Cipher key: 512 bits
	PBKDF:      argon2i
	Time cost:  4
	Memory:     1003317
	Threads:    2
	Salt:       b3 c8 b0 69 db 38 cb bd 1c 58 d0 a2 8a b8 92 12 
	            05 47 ca dd c7 3d dd 94 c0 f7 51 04 12 fb 3a 56 
	AF stripes: 4000
	AF hash:    sha256
	Area offset:32768 [bytes]
	Area length:258048 [bytes]
	Digest ID:  0
  1: luks2
	Key:        512 bits
	Priority:   normal
	Cipher:     aes-xts-plain64
	Cipher key: 512 bits
	PBKDF:      argon2i
	Time cost:  4
	Memory:     984615
	Threads:    2
	Salt:       17 9c 29 fc 61 a2 a4 b0 8b 10 42 6d 51 a0 5b 37 
	            77 18 ef db 05 40 79 71 79 88 0a b1 85 41 ee 41 
	AF stripes: 4000
	AF hash:    sha256
	Area offset:290816 [bytes]
	Area length:258048 [bytes]
	Digest ID:  0
Tokens:
Digests:
  0: pbkdf2
	Hash:       sha256
	Iterations: 133338
	Salt:       e1 9b 70 5e 87 25 46 d6 08 20 43 60 6c ae 2c 06 
	            42 fa 61 32 f0 fc ca 5f 10 f9 3d 63 dd 22 a4 96 
	Digest:     e9 62 ab 83 4c 3c 81 88 52 08 42 9b 47 c2 e1 b6 
	            d5 8a 59 88 5c 17 02 54 c4 89 36 7e 5f e0 f5 ec

Verify that you can unlock the disk with the key file created using the command;

cryptsetup luksOpen <device> <name> --key-file <path-to-key-file>

If the drive is already opened, then close if first;

cryptsetup -v luksClose luks-242c24d8-ac65-413d-b3a2-eb7f2f0993b0 

Next, verify the new key file can unlock the LUKS drive;

cryptsetup -v luksOpen /dev/sdb1 luks-242c24d8-ac65-413d-b3a2-eb7f2f0993b0 --key-file /etc/.crypt-me

Sample output;

Key slot 1 unlocked.
Command successful.

Automount LUKS Encrypted Device in Linux on System Startup

Update crypttab file with device information

Next, you need to add an entry to /etc/crypttab describing the information about the LUKS encrypted device that you need to automount.

An entry in /etc/crypttab should look like;

<target name> <source device> <key-file> <options>

Where:

  • target name: describes the mapped device name. For example, if your device mapping is /dev/mapper/name, then name is the required target.
  • source device: describes either the block special device or file that contains the encrypted data. This is specified using UUID=<uuid>, or LABEL=<label>, PARTUUID=<partuuid> or PARTLABEL=<partlabel>.

You can obtain the UUID, PARTUUID using the blkid command. For example:

blkid /dev/sdb1
/dev/sdb1: UUID="242c24d8-ac65-413d-b3a2-eb7f2f0993b0" TYPE="crypto_LUKS" PARTUUID="629e6177-01"

To obtain the LABEL, use lsblk command;

lsblk -f /dev/sdb1
NAME                                        FSTYPE      LABEL UUID                                 FSAVAIL FSUSE% MOUNTPOINT
sdb1                                        crypto_LUKS       242c24d8-ac65-413d-b3a2-eb7f2f0993b0                
└─luks-242c24d8-ac65-413d-b3a2-eb7f2f0993b0 ext4              e940b45b-dbc8-4c40-aaa5-9acf9fcb2119

Also, you can obtain the UUID using the command below;

cryptsetup luksDump /dev/sdb1 | grep "UUID"
  • key file: describes the file to use as a key for decrypting the data of the source device. Note that the passphrase must not be followed by a newline character.
  • options: describes the cryptsetup options associated with the encryption process. At minimum, the field should contain either the string luks respectively tcrypt or the cipher, hash and size options. Options are in the format: key=value [,key=value …].

Consult man crypttab for more information.

Therefore, this is how our device entry looks on /etc/crypttab file.

luks-242c24d8-ac65-413d-b3a2-eb7f2f0993b0 UUID="242c24d8-ac65-413d-b3a2-eb7f2f0993b0" /etc/.crypt-me luks

Update fstab file with Device information

Next, you need to update the /etc/fstab file with device information as well to define how to mount the LUKS device.

The entry in the /etc/fstab file should take the format;

<file system> <mount point>   <type>  <options>       <dump>  <pass>
/dev/mapper/luks-242c24d8-ac65-413d-b3a2-eb7f2f0993b0 /mnt/luks-242c24d8 ext4 defaults 0 0

Make the changes accordingly.

Ensure the mount point exists.

Verify the mounting using the mount command before you can reboot your system. If all is well, you should see “successfully mounted” for your LUKS device.

mount -av
/                        : ignored
/mnt/luks-242c24d8       : successfully mounted

You can now reboot your system to confirm the same.

systemctl reboot

Once the reboot is done, check the mounting;

lsblk
NAME                                          MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
sda                                             8:0    0   15G  0 disk  
├─sda1                                          8:1    0   13G  0 part  /
├─sda2                                          8:2    0    1K  0 part  
└─sda5                                          8:5    0    2G  0 part  [SWAP]
sdb                                             8:16   0    4G  0 disk  
└─sdb1                                          8:17   0    4G  0 part  
  └─luks-242c24d8-ac65-413d-b3a2-eb7f2f0993b0 254:0    0    4G  0 crypt /mnt/luks-242c24d8

Or use df command.

df -hT
Filesystem     Type      Size  Used Avail Use% Mounted on
udev           devtmpfs  984M     0  984M   0% /dev
tmpfs          tmpfs     200M  3.1M  197M   2% /run
/dev/sda1      ext4       13G  3.6G  8.5G  30% /
/dev/dm-0      ext4      3.9G   16M  3.7G   1% /mnt/luks-242c24d8

That concludes our guide on how to automount LUKS encrypted device in Linux on system startup.

Other tutorials;

How to Use VeraCrypt on Command Line to Encrypt Drives on Ubuntu 18.04

How to Encrypt Files and Folders with eCryptFS on Ubuntu 18.04

Install and Setup VeraCrypt on Ubuntu 20.04

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