Backtrack 4 – Bootable USB Thumb Drive with “Full” Disk Encryption

This is a step-by-step guide showing how to create a encrypted bootable Backtrack 4 USB thumb drive.

Before we get started, here are a few housekeeping items:

• There is a PDF version of this article available here.
• I also made a video of the process. It is here.
• Finally, if you want to be notified of updates to this page, subscribe to my RSS feed here.

I put quotes around full in the title because technically the whole disk isn't encrypted. We use LVM and the native encryption routines included in Ubuntu 8.10 to encrypt all partitions except for a small boot partition that never contains any data.

This is a fairly involved process, but I have done my best to document each detail. Please let me know if I missed anything or you have any questions. I can be reached via the contact form on the 'About' page of this website or via the comments below.

I strongly recommend you read through this guide at least once before starting. I will be making a PDF available in the near future.

As in all my how-tos, user entered text is bold and comments are preceded by a # sign and generally not part of the output of a command.

Finally, a couple of posts from the Ubuntu Community Documentation site were instrumental in getting this working.

https://help.ubuntu.com/community/EncryptedFilesystemOnIntrepid

https://help.ubuntu.com/community/EncryptedFilesystemLVMHowto

WARNING: Before you start, please be aware that you can cause the system you are using to build this with to not boot correctly. During the install process below there is a warning about indicating where you want the boot loader to be installed. Be very careful at this point.

First we are going to need some stuff.

Tools and Supplies

1. A USB thumbdrive - minimum capacity 8GB
2. A Backtrack 4 DVD or an additional USB thumbdrive  (minimum 2GB, must be Backtrack 4)
3. Optional: UNetbootin - A tool to transfer an iso image to a USB drive.
4. Working internet connection once Backtrack 4 is booted.

Let's get started!

Let's grab a copy of the Backtrack 4 Pre Release ISO.

Description: Image Download
Name:: bt4-final.iso
Size: 1570 MB
MD5: af139d2a085978618dc53cabc67b9269

Now that we have the goods in hand, we can get to cooking. This tutorial is based on booting Backtrack 4 first. This means that you need some form of bootable Backtrack 4 media. This can be a virtual machine, DVD, or USB drive. Use your favorite method of creating a DVD or USB drive or you can use UNetBootin to create the thumb drive.  Below is a screenshot of using UnetBootin to install Backtrack 4 on a USB drive.

It is as simple as selecting the image we want to write to the USB drive, the drive to write it to, and then clicking the 'OK' button. Warning: Make sure you pick the correct destination drive. You don't want to shoot yourself in the foot.

Partitioning

The first step is the physical partitioning of the drive.

Boot up Backtrack 4 from your DVD or USB drive. We will need both networking and the graphical interface running. The following commands will get us there.

/etc/init.d/networking start
startx

We will also need to figure out which drive is our target drive.  The following command will show the drives available and you can determine from that which is the new USB drive. Open a terminal windows and execute the following.

dmesg | egrep hd.\|sd.

We need to physically partition the target drive as follows:

1. The first partition needs to be a primary partition, 100 MB in size set to type ext3. Also remember to make this partition active when you are creating it. Otherwise you might have some boot problems.
2. The rest of the drive should be configured as an extended partition and then a logical partition created on top of it.

Below are the steps to take to get the drive partitioned.  A '# blah blah' indicates a comment and is not part of the command and user typed commands are bolded. One note, we will need to delete any existing partitions on the drive. Final note, the cylinder numbers below are specific to my test machines/thumb drives, yours may be different.

fdisk /dev/sdb # use the appropriate drive letter for your system

# delete existing partitions. There may be more than one.

Command (m for help): d
Partition number (1-4): 1

# create the first partition

Command (m for help): n
Command action
e   extended
p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-1044, default 1): <enter>
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-1044, default 1044): +100M

#create the extended partition

Command (m for help): n
Command action
e   extended
p   primary partition (1-4)
e
Partition number (1-4): 2
First cylinder (15-1044, default 15): <enter>
Using default value 15
Last cylinder, +cylinders or +size{K,M,G} (15-1044, default 1044): <enter>
Using default value 1044

# Create the logical partition.

Command (m for help): n
Command action
l    logical (5 or over)
p   primary partition (1-4)
l
First cylinder (15-1044, default 15): <enter>
Using default value 15
Last cylinder, +cylinders or +size{K,M,G} (15-1044, default 1044): <enter>
Using default value 1044

# Setting the partition type for the first partition to ext3

Command (m for help): t
Partition number (1-4): 1
Hex code (type L to list codes): 83

# Setting the first partition active

Command (m for help): a
Partition number (1-4): 1

Command (m for help): w

It is now time to get a couple additional packages installed that we need for LVM and encryption. First we need to update the local repositories and then install lvm2 and hashalot. Output has been ommitted.

apt-get update
apt-get install hashalot lvm2

Our next step is to enable encryption on the logical partition we created above and make it available for use.

Before we do that though, there is an optional step we can take if we want to make sure no one can tell where our data is on the drive. It isn't really necessary since anything written will be encrypted, but if we want to be thorough and make sure no one can see where our data even sits on the drive, we can fill the logical partition with random data before enabling encryption on it. This will take some time, as much as a couple hours or more. Execute the following command:

dd if=/dev/urandom of=/dev/sdb5

The following commands will setup encryption services for the partition and open it for use. There are several ciphers that can be used, but the one indicated in the command is supposed to be the most secure and quickest for Ubuntu 8.10. Please note that the case of the command luksFormat is required.

cryptsetup -y --cipher aes-xts-plain --key-size 512 luksFormat /dev/sdb5

WARNING!
========
This will overwrite data on /dev/sdb5 irrevocably.

Are you sure? (Type uppercase yes): YES
Enter LUKS passphrase: (enter passphrase) [type passphrase]
Verify passphrase: (repeat passphrase) [type passphase]
Command successful.

cryptsetup  luksOpen /dev/sdb5 pvcrypt
Enter LUKS passphrase: [type passphrase]
key slot 0 unlocked.
Command successful.

Now that that's all done, we can create our root and swap partitions using LVM. Again, the commands below will do so. 7.3 GB was the largest I could make my root partition. Play around with it a little and you may be able to make it a bit larger or you may have to make it a bit smaller.

pvcreate /dev/mapper/pvcrypt
Physical "volume /dev/mapper/pvcrypt" successfully created
vgcreate vg /dev/mapper/pvcrypt
Volume group "vg" successfully created
lvcreate -n swap -L 512M vg
/dev/cdrom3: open failed: Read-only file system. # this is normal
Logical volume "swap" created.
lvcreate -n root -L 7.3G vg
/dev/cdrom3: open failed: Read-only file system. # this is normal
Logical volume "root" created.

The final step is to format the logical volumes we just created. I have not included the output below for brevity's sake.

mkswap /dev/mapper/vg-swap
mkfs.ext3 /dev/mapper/vg-root

Believe it or not, we are finally ready to start installing Backtrack. To do, click on the install.sh icon on the desktop. This will start the graphical installer.

WARNING: You must click on the advanced tab on the next page and select your USB drive as the target for installing the bootloader. You will break your system if you do not.

We have now installed the main distribution to our thumb drive. The next step is to configure the newly installed system to use LVM and open the encrypted partition.

However, before we do that we need to figure out the UUID of our encrypted volume. We want to do this so that we don't run into problems if the device name of the drive changes from machine to machine. The command vol_id will give us the information we need. So execute vol_id as below.

vol_id /dev/sdb5
ID_FS_USAGE=crypto
ID_FS_TYPE=crypto_LUKS
ID_FS_VERSION=2
ID_FS_UUID=09330b5a-5659-4efd-8e9d-0abc404c5162
ID_FS_UUID_ENC=09330b5a-5659-4efd-8e9d-0abc404c5162
ID_FS_LABEL=
ID_FS_LABEL_ENC=
ID_FS_LABEL_SAFE=

Make a note of the ID_FS_UUID value which is in italics above. We will need it later. Note: your output will be different than mine.

Now time to configure our newly installed system. The first thing we have to do is make the newly installed system active so we can make changes to it. We do that by mounting the partitions and chrooting to it.

mkdir /mnt/backtrack4
mount /dev/mapper/vg-root /mnt/backtrack4
mount /dev/sdb1 /mnt/backtrack4/boot
chroot /mnt/backtrack4
mount -t proc proc /proc
mount -t sysfs sys /sys

To make everything truly operational, we can mount /dev/pts, but every time I try I have problems unless I reboot first. That is a real pain, so I just don't mount /dev/pts. We will get a couple warnings/errors as we go along, but they do not affect our install.

The magic to making all this work is to rebuild the initrd image that is used to boot our system. We need to include some things, load some modules, and tell it to open the encrypted volume, but first we have to go through the whole process of installing software again. We have to do this because we are essentially right back where we started when we booted the live cd. Do the following again.

apt-get update
apt-get install hashalot lvm2

The next step is to configure how initramfs-tools will create our initrd file. We do this by added two scripts and editing the modules file. I have added the text of the scripts here, but also provided a command that will grab them from my website.

The first script we need to create is /etc/initramfs-tools/hooks/pvcrypt. This script will copy the needed files for the initrd image. Executing the following will get the script where it needs to be.

cd /etc/initramfs-tools/hooks
wget -O pvcrypt http://www.infosecramblings.com/hooks-pvcrypt

The contents of the script should look like this.

PREREQ=""

prereqs()
{
   echo "$PREREQ"
}

case $1 in
prereqs)
  prereqs
  exit 0
  ;;
esac

if [ ! -x /sbin/cryptsetup ]; then
  exit 0
fi

. /usr/share/initramfs-tools/hook-functions

mkdir -p ${DESTDIR}/etc/console-setup
cp /etc/console-setup/boottime.kmap.gz ${DESTDIR}/etc/console
copy_exec /bin/loadkeys /bin
copy_exec /bin/chvt /bin
copy_exec /sbin/cryptsetup /sbin
copy_exec /sbin/vol_id /sbin

The next script we need to create is /etc/initramfs-tools/scripts/local-top/pvcrypt. This script tells the system to open the encrypted volume and requests the passphrase. Executing the following will get the script where it needs to be.

cd /etc/initramfs-tools/scripts/local-top
wget -O pvcrypt http://www.infosecramblings.com/local-top-pvcrypt

Unlike the first script, you will need to edit this script to point to your encrypted volume. This is where the UUID we found earlier comes in. Replace the word UUID with the value you noted above.

PREREQ="udev"

prereqs()
{
        echo "$PREREQ"
}

case $1 in
# get pre-requisites
prereqs)
        prereqs
        exit 0
        ;;
esac

/bin/loadkeys -q /etc/console-setup/boottime.kmap.gz
modprobe -Qb dm_crypt
modprobe -Qb sha256
modprobe -Qb aes_i586
modprobe -Qb xts

# The following command will ensure that the kernel is aware of
# the partition before we attempt to open it with cryptsetup.
/sbin/udevadm settle

sleep 10

if grep -q splash /proc/cmdline; then
    /bin/chvt 1
fi
/sbin/cryptsetup luksOpen /dev/disk/by-uuid/UUID pvcrypt

Both scripts need to be executable.

chmod +x /etc/initramfs-tools/hooks/pvcrypt
chmod +x /etc/initramfs-tools/scripts/local-top/pvcrypt

The final change we need to make before rebuilding initrd is to edit the /etc/initramfs-tools/modules file and add a couple encryption modules. This will make sure they are copied into the initrd image. We can do this one of two ways. We can use our favorite editor and add the following lines to the bottom of the file and save it.

aes_i586
xts

or use a wget command like above.

cd /etc/initramfs-tools
wget -O modules http://www.infosecramblings.com/initramfs-modules

Either way, your /etc/initramfs-tools/modules file should look like this:

# List of modules that you want to include in your initramfs.
#
# Syntax:  module_name [args ...]
#
# You must run update-initramfs(8) to effect this change.
#
# Examples:
#
# raid1
# sd_mod
fbcon
vesafb
aes_i586
xts

Now it's time to rebuild our initrd image.

update-initramfs -u

If all goes well, you are now ready to cross your fingers and reboot. The system will start to boot then ask you for your LUKS passphrase. Type that bad boy in and, if all goes well, your system will boot.

If, however, you run into any problems, you don't have to start over. As long as your encrypted volume is built correctly and you have the correct LUKS passphrase, you can get back to the place you were with the Live CD. Simply boot with the original Live CD/USB drive and enter the following.

/etc/init.d/networking start
apt-get update
apt-get instal hashalot lvm2
cryptsetup luksOpen /dev/[your logical partition] pvcrypt
mkdir /mnt/backtrack4
mount /dev/mapper/vg-root /mnt/backtrack4
mount /dev/[boot partition] /mnt/backtrack4/boot
chroot /mnt/backtrack4
mount -t proc proc /proc
mount -t sysfs sys /sys
mount -t devpts devpts /dev/pts

You can now do any trouble shooting you need to do and try to reboot again. One note, if you want to check the UUID of your partition, do it before you chroot.

Once you have a booting system, you are ready to login. The default userid is root and the default password is toor. You are now ready to login and being playing. Don't forget to change the root password as soon as you login the first time.

That's it. You can make some final tweaks if you want like setting the network to start automatically and starting KDE at boot, but for all intents and purposes you have successfully installed Backtrack 4 to USB drive and don't have to worry about sensitive information being intercepted if it gets lost of stolen.

Good luck!

-Kevin

Backtrack 4 – Bootable USB Thumb Drive with “Full” Disk Encryption by Kevin Riggins is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.
Permissions beyond the scope of this license may be available at http://www.infosecramblings.com/about/.