How to Encrypt Your USB Flash Drives

Did you know that your modern Linux kernel comes with a built-in encryption framework? I am talking about dm-crypt (device-mapper crypt) and the user-friendly layer on top of it, called LUKS (Linux Unified Key Setup). I just encrypted all of my USB flash drives two weeks ago using the dm-crypt + LUKS method and I am very happy with the results.

The process itself is very simple.

# 1. Find the correct device.

lsblk

# 2. Wipe the device with random data. I prefer to target the disk by its UUID
# because using the /dev/sdX convention is not very reliable (the letters can
# change between boots/hotmounts). NOTE: You might be interested in
# http://frandom.sourceforge.net/ if your device is over 16 GiB or so, because
# using /dev/urandom can be very slow. If using Arch Linux, you can get it from
# the AUR: https://aur.archlinux.org/packages/frandom/.

dd if=/dev/urandom of=/dev/disk/by-uuid/XXX bs=4096

# 3. Create the partition on the device.

cfdisk /dev/disk/by-uuid/XXX

# 4. Encrypt the partition and make it LUKS-compatible. See the manpage for
# cryptsetup(8).
#   -c: cipher type to use
#   -y: LUKS will ask you to input the passphrase; using -y will ask you twice
#       and complain if the two do not match.
#   -s: Key size in bits; the larget the merrier, but limited by the cipher/mode used.

cryptsetup -c aes-xts-plain -y -s 512 luksFormat /dev/disk/by-uuid/XXX

# 5. Open the partition with LUKS.

cryptsetup luksOpen /dev/disk/by-uuid/XXX mycrypteddev

# The partition is now available from /dev/mapper/mycrypteddev as a "regular"
# partition, since LUKS is now handling all block device encryption between the
# user and the device.

# 6. Set up a filesystem on the partition.

mkfs.ext4 /dev/mapper/mycrypteddev

# 7. Close the partition with LUKS.

cryptsetup luksClose /dev/mapper/mycrypteddev

# Encryption setup complete! Now every time you want to access the partition,
# you must first open it with LUKS and then mount it. Then when you're done, do
# the reverse: unmount and close it with LUKS.

# To mount and open with LUKS:

cryptsetup luksOpen /dev/disk/by-uuid/XXX mycrypteddev
mount -t ext4 /dev/mapper/mycrypteddev /mnt/mount_point

# To unmount and close with LUKS:

umount /mnt/mount_point
cryptsetup luksClose mycrypteddev

The mount/open and unmount/close steps necessary for using the device is laborious. That’s why you should write a bash script to run them. I’ve written the following bash script called cmount.sh to access my 3 USB drives this way:

#!/bin/zsh
# LICENSE: PUBLIC DOMAIN
# mount/unmount encrypted flash drives

mp=$3
uuid=""

case $2 in
    "0")
        uuid="11e102cd-dea1-46a8-ae9b-b3f74b536e64" # my red USB drive
        ;;
    "1")
        uuid="cf169437-b937-4a39-86cb-7ca82bd9fe94" # my green one
        ;;
    "2")
        uuid="57a0b7d5-d2a6-47e0-a0e3-adf69501d0cd" # my blue one
        ;;
    *)
        ;;
esac

if [[ $uuid == "" ]]; then
    echo "No predefined device specified."
    exit 0
fi

case $1 in
    "m")
        echo "Authorizing encrypted partition /dev/mapper/$mp..."
        sudo cryptsetup luksOpen /dev/disk/by-uuid/$uuid $mp
        echo -n "Mounting partition on /mnt/$mp..."
        sudo mount -t ext4 /dev/mapper/$mp /mnt/$mp && echo "done."
        ;;
    "u")
        echo -n "Unmounting /mnt/$mp..."
        sudo umount /mnt/$mp && echo "done."
        echo -n "Closing encrypted partition /dev/mapper/$mp..."
        sudo cryptsetup luksClose $mp && echo "done."
        ;;
    *)
        ;;
esac

To mount the green USB to /mnt/ef0 (“ef0” is just an arbitrary folder name):

./cmount.sh m 1 ef0

Then to unmount:

./cmount.sh u 1 ef0

Simple, eh? Go forth and encrypt all of your USB drives, so that when they get lost, they can’t be read by curious strangers. You can use the above steps to create and encrypt multiple partitions in the same device, or to only encrypt one partition while leaving other partitions unencrypted (i.e., steps 4 through 7 are partition-specific). The choice is yours. I prefer partition-level (aka “block device”) encryption over file/folder encryption because I don’t have to mentally think every time “hey, do I want to encrypt this?” for every file/folder I create.

If you want to look into encrypting your hard drives and swap partitions, read through this disk encryption page, and particularly, this section. There are many “levels” of encryption, and you should consider the many options available to you.

Advertisements