[dm-crypt] Size of LUKS header and how to overwrite

Arno Wagner arno at wagner.name
Wed Feb 10 23:07:57 CET 2016


Just added Item 2.20 to the FAQ for this.

Regards,
Arno

On Wed, Feb 10, 2016 at 22:50:14 CET, Arno Wagner wrote:
> 
> I should probably update the FAQ with the information about the
> "payload offset" field in a separate item. Maybe "How to overwrite
> only the LUKS header".
> 
> Keep in mind that much of the statements in the FAQ are still 
> in their first incarnation and quite a few things may lack 
> a simplified description, as I like to give the full story.
> Hence I am grateful if anybody points out such shortcommings.
> 
> Regards and thanks,
> Arno
> 
> 
> On Wed, Feb 10, 2016 at 21:29:07 CET, Sven Eschenberg wrote:
> > Ah, I see. Well the FAQ gives alternatives and more information on
> > the size of the header for different keylengths and so on. But as
> > the header+keymaterial can hardly be bigger than the offset of the
> > encrypted payload on the backing device a dd with
> > count=payload-offset and bs=512 indeed does the trick.
> > 
> > Of course you'd have to check the payload offset of that particular
> > container and not mix any numbers etc. .
> > 
> > Regards
> > 
> > -Sven
> > 
> > 
> > Am 10.02.2016 um 21:13 schrieb Michael Kjörling:
> > >On 10 Feb 2016 21:07 +0100, from sven at whgl.uni-frankfurt.de (Sven Eschenberg):
> > >>Yes, it will overwrite the header and potential free space after the
> > >>header up to the first sector of encrypted data.
> > >>
> > >>Does this seem so weird?
> > >
> > >No, but given the somewhat roundabout way this was described in the
> > >FAQ at least last I looked, this seems like a much easier way to
> > >describe it...
> > >
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> 
> -- 
> Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
> GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
> ----
> A good decision is based on knowledge and not on numbers. -- Plato
> 
> If it's in the news, don't worry about it.  The very definition of 
> "news" is "something that hardly ever happens." -- Bruce Schneier
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-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
A good decision is based on knowledge and not on numbers. -- Plato

If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier


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