[dm-crypt] Some questions/clarifications around the LUKS spec

Sven Eschenberg sven at whgl.uni-frankfurt.de
Mon Mar 14 22:24:25 CET 2016

Am 14.03.2016 um 20:54 schrieb Milan Broz:
> On 03/14/2016 07:58 PM, Sven Eschenberg wrote:
> ...
>> On a sidenote: When cryptsetup switched it's defaults to aes-xts it
>> stopped being compliant to the (original) LUKS specs. No doubt.
> ...
>> Again cryptsetup gave up on the specs, as there was a severe need for
>> these changes. It really had been wise to ditch the original on disk
>> format at that time and create a new revised (sub)version of the on disk
>> format. It did not happen though and currently the reference
>> implementation is the 'de facto' specification, while the original spec
>> is only the 'de jure' specification. But then again, 'industry' and
>> 'practice' define standards, don't they? *SCNR*
> Sorry? Spec is updated, it mentions XTS mode, and disk format remains still the same,
> despite I did last update in 2011. (Simply because there was no need to update
> it.) I'll mention more in another reply.

Updating a spec needs more than just mentioning something. Esp. changes 
may not be incompatible to previous revisions. If changes are 
incompatible, a new version is needed (instead of a simple revision). A 
change to the list of valid values as well as the change in offset 
calculation to meet alignment requirements are indeed incompatible to 
the original specification for the v1 header, like it or not. Thus, by 
introducing these changes, a new version of the on disk format was 
introduced and this should have been reflected by reversioning the 
header as well. Having multiple possible specs for the same 
magic+version is something one really should not go for.

> Cryptsetup never gave up specs, it is still compatible to the document.
> Milan

Interesting *raising eyebrow*


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