> -----Original Message----- > From: Milan Broz <gmazyland@xxxxxxxxx> > Sent: Thursday, August 8, 2019 2:53 PM > To: Pascal Van Leeuwen <pvanleeuwen@xxxxxxxxxxxxxx>; Eric Biggers <ebiggers@xxxxxxxxxx> > Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>; linux-crypto@xxxxxxxxxxxxxxx; > herbert@xxxxxxxxxxxxxxxxxxx; agk@xxxxxxxxxx; snitzer@xxxxxxxxxx; dm-devel@xxxxxxxxxx > Subject: Re: [RFC PATCH v2] md/dm-crypt - reuse eboiv skcipher for IV generation > > On 08/08/2019 11:31, Pascal Van Leeuwen wrote: > >> -----Original Message----- > >> From: Eric Biggers <ebiggers@xxxxxxxxxx> > >> Sent: Thursday, August 8, 2019 10:31 AM > >> To: Pascal Van Leeuwen <pvanleeuwen@xxxxxxxxxxxxxx> > >> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>; linux-crypto@xxxxxxxxxxxxxxx; > >> herbert@xxxxxxxxxxxxxxxxxxx; agk@xxxxxxxxxx; snitzer@xxxxxxxxxx; dm-devel@xxxxxxxxxx; > >> gmazyland@xxxxxxxxx > >> Subject: Re: [RFC PATCH v2] md/dm-crypt - reuse eboiv skcipher for IV generation > >> > >> 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). > >> > > Interesting ... I'm not a cryptographer, just a humble HW engineer specialized > > in implementing crypto. I'm basing my views mostly on the Liskov/Minematsu > > "Comments on XTS", who assert that using 2 keys in XTS was misguided. > > (and I never saw any follow-on comments asserting that this view was wrong ...) > > On not avoiding j=0 in the XTS spec they actually comment: > > "This difference is significant in security, but has no impact on effectiveness > > for practical applications.", which I read as "not relevant for normal use". > > > > In any case, it's frequently *used* with both keys being equal for performance > > and key storage reasons. > > There is already check in kernel for XTS "weak" keys (tweak and encryption keys must not be > the same). > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/crypto/xts.h# > n27 > > For now it applies only in FIPS mode... (and if I see correctly it is duplicated in all > drivers). > I never had any need to look into FIPS for XTS before, but this actually appears to be accurate. FIPS indeed *requires this*. Much to my surprise, I might add. Still looking for some actual rationale that goes beyond suggestion and innuendo (and is not too heavy on the math ;-) though. > Milan Regards, Pascal van Leeuwen Silicon IP Architect, Multi-Protocol Engines @ Verimatrix www.insidesecure.com
