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From: James Chase <mr.jdchase@gmail.com>
Date: Wed, 8 Jan 2020 17:13:56 -0500
Message-ID: <CAFosHnHD90XOnV=T7L7Y1UmVusk9DmeyROnuLHquMPtoRXeOVQ@mail.gmail.com>
Subject: Decrypt LVM containing primary OS from USB on boot
To: ~alpine/users@lists.alpinelinux.org
Content-Type: text/plain; charset="UTF-8"

So I've got a home server with Alpine installed operating as a
hypervisor for various VM's. I've got full disk encryption set up, and
I've added a key to a vfat USB drive that will sometimes be used to
boot when I'm traveling. I've also added the key via cryptsetup
luksAddKey /mnt/usb etc. I've added some stuff to mkinitfs.conf in the
hopes that it'll aid in mounting the USB prior/early on in the boot
process, which now looks like this:
features="ata base ide scsi fat vfat usb virtio ext4 lvm cryptsetup"

It boots fine with a passphrase but I'm not sure how I would go about
getting cryptsetup to use the key on /mnt/boot or /dev/sdb1 before the
OS even boots. I've read about people using initramfs (slackware has
their mkinitrd command with a flag -K to do this assuming you have the
right kernel modules set up). I did find this:
# mkinitfs -L | grep cryptkey
cryptkey

I have no idea how to use that or if it's what I'm looking for,
however. I'm also open to editing crypttab, but I can't figure out
what the syntax would be for the key field on a USB drive... Current
crypttab looks like this:
lvmcrypt UUI=*my UUID* none luks

Not sure what I'd put in the "none" slot to somehow denote the USB and
a file on that USB. Any help or resources would be appreciated!