HOWTO: Automatically Unlock LUKS Encrypted Drives With A Keyfile

Author: Stephan Jau
Revision: v1.0
Last Change: July 3 2008

Introduction

Well, I have written so far two tutorials with LUKS/dm_crypt involved. First one was how to enable encryption on Feisty Fawn (wasn't included back then by default) and the other one was how to reboot/unlock through a remote connection.

This howto was then written because of a question in the second howto. The problem there was how to unlock multiple devices in the intial ramdisk remotely. I suggested instead to use a keyfile for automatic unlocking. The keyfile should be stored in the normally encrypted root partition - so you still have to unlock that one. During boot process it will then be used to unlock all the other devices.

Of course one could also use an encrypted LVM so that all space is encrypted but only one password is required. I have not seen a usage for LVM so far and it seems, that others do not also but prefer to have individual encrypted devices. That way you could easily move an encrypted harddisk to another computer. I'm not sure if that works with LVM encrypted drives also because I have no experience with it.

One might ask how secure is this setup? I'd say pretty safe. During this howto the keyfile will be made read-only to root. So, if someone can access the keyfile you have more serious problems anyway on your computer.

Step 1: Create a random keyfile

sudo dd if=/dev/urandom of=/root/keyfile bs=1024 count=4

This will create a file with random content with the size of 4096 bits (better than a 20/30 character password....). You can use any file to act as keyfile but I think a 4kb file with random content is good suited.

Step 2: Make the keyfile read-only to root

sudo chmod 0400 /root/keyfile

That will make the keyfile readable only by root. If someone get access to this keyfile, then you have a bigger problem on your computer anyway.

Alternatively chown your desired keyfile to root:root and move it into the /root folder

Step 3: Add the keyfile to LUKS

LUKS/dm_crypt enabled devices may hold up to 10 different keyfiles/passwords. So, next to having the already setup password we're going to add this keyfile as additional authorization method.

sudo cryptsetup luksAddKey /dev/sdX /root/keyfile

sdX is of course your LUKS device.

First you'll be prompted to enter an (existing) password to unlock the drive. If everything works well, you should get an output like this:

Enter any LUKS passphrase:
key slot 0 unlocked.
Command successful.

Step 4: Create a mapper

LUKS devices need to create a mapper that can then be referenced in the fstab. Open /etc/crypttab

sudo nano /etc/crypttab

and add then a line like this:

sdX_crypt      /dev/sdX  /root/keyfile  luks

or you can use the UUID of the device:

sdX_crypt      /dev/disk/by-uuid/247ad289-dbe5-4419-9965-e3cd30f0b080  /root/keyfile  luks

sdX_crypt is the name of the mapper that is being created. You can use here any name e.g. "music" or "movies" or "sfdsfawe" ....

Save and close the file by issuing ctrl-x, enter, enter. Ctrl-x closes nano but first it asks to save the file [yes = enter] and what the name shall be [same name = enter].

What we have done there actually is telling that /root/keyfile shall be used instead of password entry to unlock the drive.

Step 5: Mount the device in fstab

Now, we have an unlocked device (well, not yet but when the system is being booted up) and we just need to mount it now. Open /etc/fstab:

sudo nano /etc/fstab

and add a new entry like:

/dev/mapper/sdX_crypt  /media/sdX     ext3    defaults        0       2

Make sure you have the correct mapper name that you added in step 4. Also make sure that the mount point/folder exists. After having added it, save again the file and close it (ctrl-x, enter, enter).