Encrypted external drive with LUKS

 

 

Introduction

Since TrueCrypt is no longer under active development, I’ve decided to wipe my external drives and re-encrypt them with LUKS and dm-crypt. These are standard Linux tools that are available by default in most distributions. I’ve used LUKS and dm-crypt in the past, when I installed Arch Linux on my laptop. Here, I’ll be performing a similar procedure, but without LVM.

Note – TrueCrypt is still considered safe, and an audit of its encryption mechanisms was recently completed. If you’d still like to use TrueCrypt, I have a guide for that, below.

Creating the encrypted drive

Step 1 – Identify your disk

Make sure you know which disk you’re working with by using lsblk.

sudo lsblk

Here, you can see the physical disk called /dev/sdb, which is my 1TB external drive. Do not assume that your drive will be /dev/sdb.

NAME                      MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                         8:0    0 298.1G  0 disk  
├─sda1                      8:1    0  1007K  0 part  
├─sda2                      8:2    0   128M  0 part  /boot
└─sda3                      8:3    0   298G  0 part  
  └─VolGroup00            254:0    0   298G  0 crypt 
    ├─VolGroup00-lvolswap 254:1    0     8G  0 lvm   [SWAP]
    ├─VolGroup00-lvolroot 254:2    0    25G  0 lvm   /
    └─VolGroup00-lvolhome 254:3    0   265G  0 lvm   /home
sdb                         8:16   0 931.5G  0 disk

Step 2 – Securely wipe your disk

You should always wipe your disk before doing anything. Unfortunately, depending on the size of the disk, this could take a long time. If your drive is already encrypted, you could simply wipe the header and your data would be safe. Since I’m paranoid, I always choose to nuke the entire drive. You could use badblocks to do a destructive write test (as I did here), or use dd, as shown below. If you’re using a SSD, your techniques will have to be a little different.

sudo dd if=/dev/zero of=/dev/sdb iflag=nocache oflag=direct bs=4096

Please don’t copy/paste this command directly, as you could risk destroying your current system. I’m not responsible for anything you break 🙂

Step 3 – Create partition

Start by creating a partition using fdisk.

sudo fdisk /dev/sdb

Use the n option to create a new partition, then press p for a primary partition. Press 1 to  edit the first partition, then use the defaults for first and last sectors (this will fill up the entire drive). Finally, press w to write your changes.

Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-1953458175, default 2048): 
Last sector, +sectors or +size{K,M,G,T,P} (2048-1953458175, default 1953458175): 

Created a new partition 1 of type 'Linux' and of size 931.5 GiB.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

Running lsblk again, we can see our new partition, /dev/sdb1.

NAME                      MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                         8:0    0 298.1G  0 disk  
├─sda1                      8:1    0  1007K  0 part  
├─sda2                      8:2    0   128M  0 part  /boot
└─sda3                      8:3    0   298G  0 part  
  └─VolGroup00            254:0    0   298G  0 crypt 
    ├─VolGroup00-lvolswap 254:1    0     8G  0 lvm   [SWAP]
    ├─VolGroup00-lvolroot 254:2    0    25G  0 lvm   /
    └─VolGroup00-lvolhome 254:3    0   265G  0 lvm   /home
sdb                         8:16   0 931.5G  0 disk  
└─sdb1                      8:17   0 931.5G  0 part

Step 4 – Encrypt partition

Here, we’ll setup encryption on our new partition.

sudo cryptsetup -v -y -c aes-xts-plain64 -s 512 -h sha512 -i 5000 --use-random luksFormat /dev/sdb1

-v = verbose
-y = verify passphrase, ask twice, and complain if they don’t match
-c = specify the cipher used
-s = specify the key size used
-h = specify the hash used
-i = number of milliseconds to spend passphrase processing (if using anything more than sha1, must be great than 1000)
–use-random = which random number generator to use
luksFormat = to initialize the partition and set a passphrase
/dev/sdb1 = the partition to encrypt

Seriously, read the man page and the FAQ on cryptsetup before you do anything here.

To see the configuration of the header, use the command below.

sudo cryptsetup luksDump /dev/sdb1

Step 4.5 – Backup the LUKS header

The LUKS header stores metadata about the LUKS device, as well as the master key, key files, etc… Specifically, from the cryptsetup FAQ and specification:

A LUKS partition starts with the LUKS partition header (phdr) and is followed by key material. After the key material, the bulk data is located, which is encrypted by the master key. The phdr contains information about the used cipher, cipher mode, the key length, a uuid and a master key checksum.
Also, the phdr contains information about the key slots. Every key slot is associated with a key material section after the phdr.  When a key slot is active, the key slot stores an encrypted copy of the master key in its key material section. This encrypted copy is locked by a user password. Supplying this user password unlocks the decryption for the key material, which stores the master key. The master key in turn unlocks the bulk data. For a key slot, all parameters how to decrypt its key material with a given user password are stored in the phdr (f.e. salt, iteration depth).

That paragraph is probably easier explained with an image.

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If you don’t understand already, it’s important to not damage the LUKS header. It’s best practice to backup the header as soon as you create the LUKS device, because you’re probably lazy and won’t do it later. If you damage the header at all (accidental formatting, wrong device when using dd, bad block, etc…), you’ll need this backup header to recover the data. The header file itself needs encrypted and should be stored in safe place (not on the same drive, obviously).

sudo cryptsetup luksHeaderBackup --header-backup-file /path/to/file.img /dev/sdb1

Note – TrueCrypt (brilliant piece of software) uses a backup header at the end of the drive as well, to mitigate the risk of a single header.

Step 5 – Unlock the LUKS device

Open the LUKS container using the passphrase you just set. This will mount the device at /dev/mapper/volume01.

sudo cryptsetup luksOpen /dev/sdb1 volume01

Step 6 – Create filesystem

I’m going to be using ext4. Again, specify the name of device you set in the step above.

sudo mkfs.ext4 /dev/mapper/volume01

Step 7 – Mount device

Here, create a mount point, then mount the device to that location.

sudo mkdir -p /mnt/drive01
sudo mount /dev/mapper/volume01 /mnt/drive01

Step 8 – Unmount and close container

Now, unmount the drive and close the LUKS container.

sudo umount /mnt/drive01
sudo cryptsetup luksClose /dev/mapper/volume01

 

Mounting the encrypted drive

Every time you want to use this drive, you’ll need to open the LUKS container, mount the drive, do your work, unmount the drive, then close the LUKS container. You can do this manually, or a file manager to auto-mount it.

Manual

Below are the steps to manually unlock and use the drive.

sudo cryptsetup luksOpen /dev/sdb1 volume01
sudo mount /dev/mapper/volume01 /mnt/drive01
##DO YOUR WORK HERE##
sudo umount /mnt/drive01
sudo cryptsetup luksClose /dev/mapper/volume01

Automatic

Modern file managers like Thunar and Nautilus have support for unlocking and mounting LUKS devices automatically. The only downside to this setup is that you can’t use keyfiles in addition to passphrases.

Install and configure Thunar

If you don’t have Thunar, you’ll need it, along with a couple other packages.

sudo pacman -S thunar thunar-volman gvfs gvfs-afc

Open Thunar and navigate to Edit–>Preferences–>Advanced and ensure that Enable Volume Management is checked.

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Click on Configure, and ensure that Mount removable drives when hot-plugged and Mount removable media when inserted are both checked. Click on Close to save your changes.

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Mount drive

Plug in your external drive and open Thunar. In the top-left, you should see your drive under DEVICES.

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When you click on the drive, you’ll be prompted for the passphrase you set earlier.

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To unmount the drive, right-click on it and select Unmount.

 

-Logan

2 thoughts on “Encrypted external drive with LUKS

  1. Leaving the LUKS header on the disk is not necesary and is also a risk.

    Better use a two-factor… internal or external HDD encrypted with LUKS without headers, then another USB stick to hold the mount… so you plug the HDD (if external) and connect the USB stick, mount it, disconnect the USB stick and put it on a safe place.

    If an attacker come to you, just pull out power and the disk is safe… master key of LUKS is not on the disk… is on the USB that is on a safe place.

    But ensure not to have with you such USB stick after LUKS opened.

    Search for: How to mount LUKS without header.

    And also seach how to create a LUKS without header using 100% RAW disk space.

    It is much more complicated, but much more secure.

    And, of course, never ever on your live use only one LUKS layer.

    And never ever use AES, specially if you have Intel CPU with AES hardware instructions… i had worked on a project to break AES based on Hardware versus Software AES instructions, INTEL chips has some BUGS (i preffer to call them Back Doors) that let decode encrypted data by hardware AES… also software AES encrypted is broken, but that takes just a little more time.

    AMD and ARM has better hardware AES (or better said, less Back Door exploit).

    I am not allowed to tell exactly how it goes, but think it is based on comparing what hardware does versus software, detecting differences, etc.; i can not tell more, i risk my live for telling this.

    US military and Goverment are in plan to migrate to another way of cyphring and do not allow anyone involved to talk about theese ‘back door’.

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