Grub disk decryption with key file on another device

[Vivien Kraus]

Dear Guix,

I have a fully encrypted disk: it has an EFI partition where I install
grub-efi-bootloader and the rest is a single-partition luks device
(including the grub configuration /boot).

I also have another disk that I can partition however I want.

I would like the other disk to store a key file to decrypt the main
disk.

Is this possible?

I see in the manual that grub can be configured to load a CPIO archive
containing the key, but from what I understand, the cpio archive must
be in the root file system, which is encrypted in my case, so it does
not appear to solve the issue. Or can I somehow instruct grub to load
it from another disk?

Best regards,

Vivien