Encrypt a USB drive in linux and automatically mount it on startup using a keyfile and dm_crypt

The easiest way of doing this is to use dm_crypt‘s “cryptsetup” on your USB drive, create a keyfile then set the options in “/etc/fstab” and “/etc/crypttab”. By using a keyfile you can get the drive to automatically mount without having to type in your encryption password. I was doing this on a bare install of CentOS 6.3 but the steps should be similar on other distros with “cryptsetup” installed.

I needed to back up some important (and confidential) files to a USB portable drive that I wanted to encrypt with full disk encryption. You can do this in a variety of ways but the method here was the easiest I found. More information can be found at Brad’s Blog and HowtoForge.

Encrypting and mounting your USB drive

First you need to physically plug in your USB drive to the machine and then unmount it if it automatically mounts. I performed all the commands here using the root user. In my case, when I plugged in the USB drive it was found as “/dev/sdb” and automatically mounted by CentOS. To unmount:

umount /dev/sdb

Now the USB drive needs to be formatted using “cryptsetup” and the “luksFormat” command:

cryptsetup luksFormat /dev/sdb

The tool will give you a warning about overwriting data, which you need to confirm by typing an uppercase “YES”. You then type in and confirm your LUKS passphrase, which will be used to unlock the drive in future. This passphrase is also used later when creating the keyfile.

Now you can create a device mapper for the drive using “cryptsetup” and the “luksOpen” command. I called my mapper “secretvol” in this example so the drive will be mapped to “/dev/mapper/secretvol”. You will be prompted for the passphrase:

cryptsetup luksOpen /dev/sdb secretvol

Now before you can mount your newly mapped device you need to format the file system (I used ext3):

mkfs.ext3 /dev/mapper/secretvol

Now you can mount the USB drive. Make sure you have created the mount point (in my case “/mnt/encrypteddrive”) first then mount it with:

mkdir /mnt/encrypteddrive
mount /dev/mapper/secretvol /mnt/encrypteddrive

To test this all works properly reboot your machine before unlocking and mounting your USB drive manually (requiring entry of the passphrase):

cryptsetup luksOpen /dev/sdb secretvol
mount /dev/mapper/secretvol /mnt/encrypteddrive

To unmount and lock the drive by closing the device mapper with the “luksClose” command:

umount /dev/mapper/secretvol
cryptsetup luksClose secretvol

Creating a keyfile to avoid entering your passphrase manually

A keyfile is good as it means you can unlock your USB drive without having to manually type the passphrase. To create a keyfile “/root/keyfile” for your device using “cryptsetup” and the “luksAddKey” command enter the following (you will need to enter your passphrase). The first command creates a random 4096 byte file, the second makes it read only to root and the third stores your passphrase in the keyfile using “luksAddKey”:

dd if=/dev/urandom of=/root/keyfile bs=1024 count=4
chmod 0400 /root/keyfile
cryptsetup luksAddKey /dev/sdb /root/keyfile

Now you can unlock your previously created drive without manually entering the passphrase using:

cryptsetup luksOpen –key-file /root/keyfile /dev/sdb secretvol

And mount with:

mount /dev/mapper/secretvol /mnt/encrypteddrive

Automatically unlock and mount your encrypted USB drive at system startup

Now that you have a keyfile you can set up your linux install to automatically unlock and mount the USB drive by editing a couple of files.

Edit your “/etc/crypttab” file:

nano /etc/crypttab

Add the line below to add the “/dev/mapper/secretvol” device:

secretvol /dev/sdb /root/keyfile luks

NOTE: You can also use the UUID of your drive in “/etc/crypttab” to make sure that the right disk as detected by the kernel is used. In cases where you may be adding or removing disks this is really important as you may have “sdb” or “sdc” or “sdX” depending on what order the disks are detected by your linux install. To find the right UUID type:

ls -l /dev/disk/by-uuid

Which in my case told me that my UUID for “sdb” (my USB drive) was “6858274d-2370-4377-9426-d786c3e7a410″. The line in “/etc/crypttab” that you should use in this case to add “/dev/mapper/secretvol” is:

secretvol /dev/disk/by-uuid/6858274d-2370-4377-9426-d786c3e7a410 /root/keyfile luks

Now edit your “/etc/fstab” file:

nano /etc/fstab

Add the line below to automatically mount the device to “/mnt/encrypteddrive”:

/dev/mapper/secretvol /mnt/encrypteddrive ext3 defaults 0 2

Now to test this, reboot your machine and navigate to “/mnt/encrypteddrive” where your USB drive will be mounted automatically for you. Easy!

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