Auto-mount LUKS

1. You'll have to know a valid unlock password for the partition to mount (duh)
2. All placeholders in this text are enclosed in brackets - your need to replace all of it, e.g. dd if=[name] where the name is bob becomes dd if=bob, not "dd if=[bob]"!
3. The placeholder [device] is the device path of your partition, e.g. something like /dev/sdb2 or /dev/nvme1n1p2
4. The placeholder [name] is the friendly name for the unencrypted mapper, becoming /dev/mapper/[name] - use something like saveDrive here for example.
5. The placeholders [user] and [group] are the user and group to mount the unlocked partition for - typically they are both your current username

Create a new key file

Let's first create a new key file of random data and make it readable for root only - you can of course change the location of the file but you'll have to use the other location in the following steps, too.

openssl rand -out /root/mykeyfile.key 4096
chmod 600 /root/mykeyfile.key
chown root:root /root/mykeyfile.key

Add key file to your LUKS device

Now add the keyfile as an alternative key (one of 8 key slots) to decrypt the device

cryptsetup luksAddKey /dev/[device] /root/mykeyfile.key

You'll be asked for your already installed unlock password here.

Registering in crypttab

Edit or create the file /etc/crypttab and add a line for your partition:

[name] [device] /root/mykeyfile.key luks

You can use your dev path if that doesn't change - in my case, it's random in which order the devices are initalized. So solve that, you can use the partition's UUID instead:

blkid [device]

Then take the UUID, not the PARTUUID and insert it here instead of [uuid]:

[name]  UUID=[uuid] /root/mykeyfile.key luks

Register default mount options

Edit the file /etc/fstab and add a line for the new mapper

/dev/mapper/[name]  /media/[user]/[name]  ext4  defaults   0   2

This will mount the unlocked device to /media/[user]/[name] - you can change that of course.

Auto-create the mount location

Create a new file /etc/tmpfiles.d/[name] and add the following line:

d /media/[user]/[name] 0755 [user] [group]
systemd-tmpfiles --create