Re: [dm-crypt] Why is it necessary to "wipe" an authenticated luks2 device when creating it?

Arno Wagner
2019-09-26T14:31:29Z

On Thu, Sep 26, 2019 at 09:41:39 CEST, Milan Broz wrote:
> On 25/09/2019 21:40, .. ink .. wrote:
> > I just added an ability to create an authenticated luks2 device in
> > zuluCrypt[1] and i am
> > wondering why these volumes need to be wiped when created. I made it work by
> > looking at how cryptsetup does it but i don't understand why because i
> > have so far
> > failed to find any documentation about it.
>
> I think it is explained in the referenced paper, we should add a FAQ about
> it.
>
> Initial wipe recalculates integrity tags - so you can read the device afterward.
>
> If you skip initialization (wipe), integrity tags for all sectors is
> incorrect and read will return integrity failure (EILSEQ errno).
>
> In theory, it is not a problem ("do not read what you did not write").
>
> But it reality it cases many programs to fail because it can access device
> through page cache. If the *write* is not aligned to a page, page cache tries
> to first read content, then update content, and write it back to the device.
>
> But as said above, all read fails because integrity tags are not
> initialized, thus even page-unaligned writes can fail. (I have seen this
> problem even in programs like mkfs, where it is apparent bug.)

This is a specific problem with anything authenticated: Even non-data
needs to be authenticated, because there is no way to distinguish
it from data on that level. Hence expecting to have to do a full
"initialization" pass at the start on authenticated storage is
perfectly reasonable.

I can add an FAQ secion on authenticated encryption fpr this.
Do you have a link to the reference paper?

Regards,
Arno

--
Arno Wagner, Dr. sc. techn., Dipl. Inform., Email: arno@wagner.name
GnuPG: ID: CB5D9718 FP: 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718
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