And if you want to use an external usb disk that's shared with FreeBSD, you can use https://www.freshports.org/sysutils/fusefs-lkl/ to fuse-mount with the original linux kernel filesystem code. Once lkl is updated to 4.7 or 4.8, ext4's encryption support should allow a shared, external usb drive that work with Linux and FreeBSD, but I don't know if lkl will also support the keyring on FreeBSD.