Two files can guide you to where that key exists:
- /etc/default/grub (delineated by colon-separated options)
- /etc/crypttab (requires root user to read the contents)
The crypttab is actually unnecessary for the system partition, since that is handled by the initramfs and bootloader. Where the crypttab is actually used is for any other encrypted device/container aside from the system partition.
Yet, distros still automatically make an entry for the system partition in the crypttab (perhaps for the end-user’s own reference?)
I always comment out the entry for the system partition in my crypttab after a successful installation.
Don’t believe me? Just look at the big bold warning in every crypttab. They wrote it for a reason. Yet distros still add the system partition entry after installation. Not sure why. I always comment it out.
# NOTE: Do not list your root (/) partition here, it must be set up
# beforehand by the initramfs (/etc/mkinitcpio.conf).