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Arch Linux: Encrypting your Hard Drive

By Colin on 2023-10-04

This is a guide for encrypting your hard drive with LUKS after you've already completed the Arch installation. In my own case, I had been using my system for a year before doing the encryption. All of the information presented here is available on the Arch Wiki, but here the execution steps are filtered and ordered to be more immediately reproducable.

This guide assumes that you:

  • have /boot on a separate partition
  • use GRUB as your boot manager
  • have a USB drive to flash an Arch ISO onto
  • can read this guide on a device other than the one you're encrypting

If these aren't the case, then you shouldn't follow this guide.


While several places in the Wiki make statements like:

Depending on your situation, you may have to XYZ...

for my case, these were the main steps:

  1. Back up important files.
  2. Flash an Arch image onto a USB.
  3. Boot from the USB.
  4. Resize the filesystem.
  5. Encrypt the partition.
  6. Configure the kernel and GRUB.


Backing up Important Files

It's up to you how to do this, but consider:

  • Uploading all unfinished code to a Git forge
  • Keeping all your config files in a dotfiles repo
  • Using Syncthing to copy files automatically between machines

Remember that on-disk backups won't save you if you destroy the partition in a later step.

Flashing an Arch Image

First, download the latest Arch ISO. Then plug in your USB drive and find out what it's called:

ls -l /dev/disk/by-id/usb-*
lrwxrwxrwx 1 root root 9 Oct  4 19:15 /dev/disk/by-id/usb-TOSHIBA_TransMemory_0022CFF6B8A6C311EC7DAD98-0:0 -> ../../sdb

This USB here doesn't have partitions, but yours likely will (e.g. sdb1). Make sure it's not mounted. You can confirm this as follows:

sda      8:0    0 476.9G  0 disk
└─sda1   8:1    0 476.9G  0 part /
sdb      8:16   1  28.9G  0 disk

Now become root and copy the image:

cat ~/Downloads/archlinux-2023.09.01-x86_64.iso > /dev/disk/by-id/usb-TOSHIBA_TransMemory_0022CFF6B8A6C311EC7DAD98-0:0

It is critical that you copy the ISO directly to the root of the USB, not to one of its partitions. The image already has a file system and a partition table, so it needs to be inserted right at the beginning to overwrite whatever was already there.

Once the cat is complete, run sync for good measure and remove the drive.

Booting from the USB

It's time to boot the USB drive that you just flashed. If your USB is plugged in when you start your machine, you should be able to enter a boot or BIOS menu and boot from the USB. Do so, and eventually systemd will start and automatically drop you into a root shell.

This should look familiar, as you probably did this to install Arch in the first place.

If you use an alternate keyboard scheme like Colemak, set it now:

loadkeys colemak

Let's check the disks on this machine:

loop0         7:0    0 682.6M  1 loop /run/archiso/airootfs
sda           8:0    1  14.4G  0 disk
└─sda1        8:1    1   789M  0 part
└─sda2        8:2    1    15M  0 part
nvme0n1     259:0    0 476.9G  0 disk
└─nvme0n1p1 259:1    0   260M  0 part
└─nvme0n1p2 259:2    0 476.7G  0 part

In my case, the sda entries belong to the boot USB, and the nvme0n1 entries are the hard drive. nvme0n1p1 is normally mounted to /boot, so it's the p2 partition we'd be working on. For all example commands below that include nvme0n1p2, replace it with the partition on your own machine.

Resizing the Filesystem

Warning: Do not mount your filesystem!

Before you continue, the dm_crypt kernel module must be loaded.

modprobe dm_crypt
lsmod | grep crypt
dm_crypt        65536  0
encrypted_keys  28672  1 dm_crypt
... etc. ...

Since you booted Arch from a USB and we just saw from lsblk that the hard drive isn't mounted, you can now check its filesystem:

e2fsck -f /dev/nvme0n1p2

And then resize it to add space for the LUKS header. Note that this does not shrink the partition, and won't result in any data loss.

resize2fs -p -M /dev/nvme0n1p2

-M will shrink the filesystem as much as it can. The Wiki suggests doing some clever math instead and only shrinking as much as you need, but I used -M as-is. It took about an hour on a 500gb machine.

Encrypting the Partition

Reminder: The dm_crypt kernel module must be loaded for this step to work!

The following command will encrypt your partition with sane defaults:

cryptsetup reencrypt --encrypt --reduce-device-size 32M /dev/nvme0n1p2

It will ask you to set a passphrase to use for decrypting during the normal boot process. This doesn't need to be your usual password. The encryption will take some time.

Once complete, decrypt the partition and expand the filesystem back to its original size:

cryptsetup open /dev/nvme0n1p2 recrypt
resize2fs /dev/mapper/recrypt

The name recrypt here can be a name of your choosing. It refers to the partition after decryption.

Alright, your partition should now have its encryption set up! Don't shut down yet though, there's some final configuration to do.

Configuring the Kernel and GRUB

Assuming everything above went well, it's time to mount your decrypted partition and configure some boot parameters before shutting down. Without this step, GRUB and the kernel won't actually know what to decrypt during boot.

First, let's get into your filesystem:

mount /dev/mapper/recrypt /mnt
mount /dev/nvme0n1p1 /mnt/boot
arch-chroot /mnt

You should now be inside your normal system as root. Now edit /etc/mkinitcpio.conf. The HOOKS array should look like:

HOOKS=(base udev autodetect modconf kms keyboard keymap consolefont block encrypt filesystems fsck)

You need udev, keymap, consolefont, and most importantly encrypt. Generate a new kernel image:

mkinitcpio -P

Now edit /etc/default/grub. To its GRUB_CMDLINE_LINUX_DEFAULT string, add:


The mycrypt name can be anything. Like above, this is the name given to the partition after decryption.

Finally, regenerate your GRUB configuration:

grub-mkconfig -o /boot/grub/grub.cfg

Now restart your machine and boot normally. If everything was successful, you should be prompted for the passphrase you set previously. Doing so decrypts your drive, and then you'll be brought to the usual login prompt.

A last lsblk will confirm that you're mounted to the decrypted partition:

nvme0n1     259:0    0 476.9G  0 disk
└─nvme0n1p1 259:1    0   260M  0 part  /boot
└─nvme0n1p2 259:2    0 476.7G  0 part
  └─mycrypt 254:0    0 476.7G  0 crypt /

Well done.


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