Re: [PATCH 01/04] Adding cipher mode context information to crypto_tfm

[Fruhwirth Clemens <clemens@endorphin.org>]

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On Wed, 2005-02-09 at 19:30 -0500, James Morris wrote:
> On Wed, 9 Feb 2005, Fruhwirth Clemens wrote:
> 
> > I can't code for the case of two. Because, first, that's the idea of
> > generic in the name "generic scatterwalk", second, I need at least 3
> > scatterlists in parallel for LRW.
> 
> Can you explain why you need a third scatterlist for the LRW tweak?

Because a tweak is different from an IV. There can be an arbitrary
number of tweaks. For instance, EME takes 1 tweak per 512 bytes. If you
have a 4k page to encrypt, you have to process 8 tweaks of whatever
size. 
 Therefore, you need 3 scatterlists: src, dst and the running along
tweak.

However, I don't want to limit the discussion to the specific needs of
LRW or EME. I wanted to write something nice and generic for other
people to use, thus scatterwalk_walk. 

There must be a solution to get an arbitrary number of kmappings in
softirq problem. If it's possible for 2 pages, I can't see a reason why
this ain't possible for more. The use of scratch buffers and constant
switching of kmap_atomic mapping is just ridiculously stupid.

> My understanding is that the tweak value is calculated from the disk
> position of the plaintext block and and the secondary key.

That's only partially correct. The tweak value _is_ the location on
disk. The value which is XORed twice is computed from the tweak and the
secondary key. In LRW, you need a tweak value per block. So, if you
encode 256 blocks, you need 256 tweaks. That's what the additional
scatterlist is for.

> It would be useful to see the original patch (which seems to be
> unavailable now), with dm-crypt integration, to see how the entire
> mechanism works beyond the test vectors.

Frankly, I don't see a point escalating this discussion. It must be
possible to process more than 2 mappings in softirq context. If not, I
feel emotionally offended.
-- 
Fruhwirth Clemens <clemens@endorphin.org>  http://clemens.endorphin.org

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