Re: [dm-crypt] Variable data offset for a LUKS volume that uses a detached header.

From: Milan Broz <gmazyland@gmail.com>
To: ".. ink .." <mhogomchungu@gmail.com>,
	"dm-crypt@saout.de" <dm-crypt@saout.de>
Subject: Re: [dm-crypt] Variable data offset for a LUKS volume that uses a detached header.
Date: Tue, 22 Dec 2015 14:44:08 +0100	[thread overview]
Message-ID: <567953A8.9020903@gmail.com> (raw)
In-Reply-To: <CAFnMBaQHdK-nnUy=LKpQjUb0dZbdpFkn06CTfo=icNdgiHn+Bg@mail.gmail.com>

On 12/22/2015 01:00 PM, .. ink .. wrote:
> Happy holidays everybody,
> 
> I am trying to create a LUKS volume with a detached header that has
> data offset at sector 256 and things fail with
>  an error that reads:
> 
> "Data offset for detached LUKS header must be either 0 or higher than
> header size (4036 sectors)."
> 
> Is there a technical reason why this is so and is it possible to allow
> other data offsets?

The reason was that it allows header to copy back to the LUKS device.
(Or create a fake header :)

TBH this is the first time I see someone want to use it for Veracrypt
device, I can fix the code to allow it but not sure if this is really
worth to do it.

You can probably write a script that will modify final mapping using
dmsetup (but that's quite a hack). Also simple program should be able
to reformat header to downgrade the header (keep keys and cipher chain).

But wouldn't be better just to use old TrueCrypt modes here from
the beginning?

Milan

> Reason for doing this is an attempt at having a detached LUKS header
> capable of unlocking a VeraCrypt volume since
> unlocking a VeraCrypt volume takes too long and its annoying.
> 
> Everything seems to be working except for the data offset since i cant
> set it to 256 sectors through the library API.
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