[arch-general] mkinitcpio hook for custom root decryption with systemd boot

[Riccardo Paolo Bestetti]

I have root encryption set up on my system and I currently boot with the sd-encrypt and sd-lvm hooks.

I would like to change my current crypto setup in a way that would require more step to unlock the root than just typing in a passphares. For this reason, sd-encrypt clearly cannot serve my use case.

For this reason, I would like to write a custom hook to mount the root volume. Now, systemd boot doesn't have a concept of runtime hooks. Thus, I need to make a systemd unit that gets pulled in by cryptsetup.target in the place of systemd-cryptsetup at .service. (Basically, I need to replace the whole systemd-cryptsetup-generator and systemd-cryptsetup logic.)

However, I really have no idea on how to achieve this. Should I write a custom mkinitcpio hook which completely bypasses sd-crypt/cryptsetup.target and instead starts a different unit with my own decryption logic? Or is there a way to hook into cryptsetup.target and instruct it to pull in my logic instead of systemd-cryptsetup*?

Of course, the other possibility is to just stop using a systemd boot and instead setting up a busybox early userspace. Then it's just a matter of writing a shell script. However, since I'm already using systemd for everything - from the bootloader to userspace - I don't think it makes much sense to do that.

Any help/guidance/suggestion/criticism is highly appreciated.

Riccardo Paolo Bestetti