luksFormat data leak

Release: Ubuntu 17.10
Package version: 2:1.7.3-4ubuntu1
Package architecture: amd64

When initializing a disk with "cryptsetup luksFormat <device>", cryptsetup creates a luks header including uninitialized areas.

Try this:
--
DISK=sdx
# Store 2MiB of a repeating pattern on disk
python -c 'print("\x00\x11\x22\x33\x44\x55\x66\x77\x88\x99\xaa\xbb\xcc\xdd\xee\xff" * (2 << 16))' > /dev/$DISK
cryptsetup luksFormat /dev/$DISK
cryptsetup luksHeaderBackup --header-backup-file luks_header.bin /dev/$DISK
hd luks_header.bin
--

You'll observe the following output:
[...]
00020400 00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff |.."3DUfw........|
*
00100400 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00101000

So even if you open the luks device and wipe /dev/mapper/$DISK_crypt completely, this piece of data, possibly containing plaintext data from of a prior filesystem, will stay forever. Or, probably, until all key slots get used.

To make it even worse, it gets stored in backups of the luks header, so you can't overwrite it safely and restore the header later.

I'd expect cryptsetup luksFormat to fill unused fields of the luks header with constant or random data.

It would be nice if there was an easy way to safely wipe unused areas of the luks header on an already formatted disk.

Regards,
Andreas