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

[Milan Broz <gmazyland@gmail.com>]

On 08/08/2019 11:31, Pascal Van Leeuwen wrote:
>> -----Original Message-----
>> From: Eric Biggers <ebiggers@kernel.org>
>> Sent: Thursday, August 8, 2019 10:31 AM
>> To: Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>; linux-crypto@vger.kernel.org;
>> herbert@gondor.apana.org.au; agk@redhat.com; snitzer@redhat.com; dm-devel@redhat.com;
>> gmazyland@gmail.com
>> 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).

Milan