Featured image of post Preparing a LUKS key file image for usb flash drive on Linux

Preparing a LUKS key file image for usb flash drive on Linux

Tutorial on how to prepare a key image for decrypting LUKS partition on boot.

A few days ago, I was preparing a key drive to unlock the encrypted partition on boot, the process of doing that manually is quite tedious, so I decided to write a script to automate the process.



mkdir -p /tmp/sandbox/storage
cd /tmp/sandbox

dd if=/dev/zero of=$keyName bs=1M count=512

devLocation=$(losetup -Pf --show $keyName)

(echo n; echo p; echo 1; echo "\n"; echo "\n"; echo t; echo c; echo w) | fdisk $devLocation

mkfs.vfat ${devLocation}p1
mount ${devLocation}p1 storage

dd bs=512 count=4 if=/dev/random of=storage/key.bin iflag=fullblock

umount ${devLocation}p1

DEVUUID=$(blkid ${devLocation}p1 -s UUID | cut -d '"' -f 2)

losetup -d $devLocation

mv $keyName "${keyName%.*}-${DEVUUID}.${keyName##*.}"


doas /usr/local/libexec/script/makeKeyImg "$@"

If you use sudo replace doas with sudo . Once you run the script it should create a image file with the key.bin inside on a vfat partition, the image file will be located in /tmp/sandbox , the file name should be something like key-ABCD-1234.img (Yes, that the UUID after first hyphen) and run the following to write image to usb-storage.

dd if=key-ABCD-1234.img of=/dev/sdx bs=1M

Now the usb-storage device is ready for decrypting the partition, all you have to do is to configure luks to accept the key, there is already a tutorial on Arch Wiki on how to do that and can be found at https://wiki.archlinux.org/title/Dm-crypt/Device_encryption#Configuring_LUKS_to_make_use_of_the_keyfile


You can prevent the drive from being auto-mounted on boot up and on insert, just add the following line to /etc/fstab quite easy.

UUID=ABCD-1234  /root/key   vfat    ro,noauto,umask=0377    0 0

The ro means read-only, noauto prevent auto-mounting and umask restrict the permission making it read only for root user and no access for other users and groups.

I also recommend making an encrypted backup copy of the image file.

Have fun 🤩

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