[bug#38826] doc: Mention no LUKS2 for luks-device-mapping

[Danny Milosavljevic <dannym@scratchpost.org>]

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Hi,

On Mon, 30 Dec 2019 21:47:01 -0600
David Trudgian <dave@trudgian.net> wrote:

> I spent a bit of time trying to mount some existing LUKS2 devices on
> boot in a Guix system. They worked to open and mount manually in a
> booted system, but not on boot with luks-device-mapping. Eventually
> worked out LUKS2 is not supported by the code that inspects the
> superblock directly for the (LUKS1) UUID.
> 
> A mention LUKS2 is not supported in the docs might be nice.

I agree.

But better yet would be to implement LUKS2 in the uuid code.
Since you have such a device could you find where the magic number /
uuid parts in it are?

Both references [1] and [2] say that the magic number is 6 bytes and the
uuid is at offset 168 Byte, length 40 Byte.  Endianness is also big endian
in both, so I have no idea where the problem comes from.  The code should
work for both.

[1] LUKS1 on-disk format: https://gitlab.com/cryptsetup/cryptsetup/-/wikis/LUKS-standard/on-disk-format.pdf
[2] LUKS2 on-disk format: https://habd.as/post/external-backup-drive-encryption/assets/luks2_doc_wip.pdf

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