[dm-devel] [RFC PATCH v2] md/dm-crypt - reuse eboiv skcipher for IV generation

[Eric Biggers]

On Wed, Aug 07, 2019 at 04:14:22PM +0000, Pascal Van Leeuwen wrote:
> > > > In your case, we are not dealing with known plaintext attacks,
> > > >
> > > Since this is XTS, which is used for disk encryption, I would argue
> > > we do! For the tweak encryption, the sector number is known plaintext,
> > > same as for EBOIV. Also, you may be able to control data being written
> > > to the disk encrypted, either directly or indirectly.
> > > OK, part of the data into the CTS encryption will be previous ciphertext,
> > > but that may be just 1 byte with the rest being the known plaintext.
> > >
> > 
> > The tweak encryption uses a dedicated key, so leaking it does not have
> > the same impact as it does in the EBOIV case. 
> >
> Well ... yes and no. The spec defines them as seperately controllable -
> deviating from the original XEX definition - but in most practicle use cases 
> I've seen, the same key is used for both, as having 2 keys just increases 
> key  storage requirements and does not actually improve effective security 
> (of the algorithm itself, implementation peculiarities like this one aside 
> :-), as  XEX has been proven secure using a single key. And the security 
> proof for XTS actually builds on that while using 2 keys deviates from it.
> 

This is a common misconception.  Actually, XTS needs 2 distinct keys to be a
CCA-secure tweakable block cipher, due to another subtle difference from XEX:
XEX (by which I really mean "XEX[E,2]") builds the sequence of masks starting
with x^1, while XTS starts with x^0.  If only 1 key is used, the inclusion of
the 0th power in XTS allows the attack described in Section 6 of the XEX paper
(https://web.cs.ucdavis.edu/~rogaway/papers/offsets.pdf).

Of course, it's debatable what this means *in practice* to the usual XTS use
cases like disk encryption, for which CCA security may not be critical...  But
technically, single-key XTS isn't secure under as strong an attack model as XEX.

- Eric