On Fri, Aug 09, 2019 at 09:17:23AM +0000, Pascal Van Leeuwen wrote: > <trimmed to: list due to being somewhat off-topic> > > -----Original Message----- > > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > Sent: Thursday, August 8, 2019 7:15 PM > > To: Pascal Van Leeuwen <pvanleeuwen@xxxxxxxxxxxxxx> > > Cc: Milan Broz <gmazyland@xxxxxxxxx>; 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 Thu, Aug 08, 2019 at 01:23:10PM +0000, Pascal Van Leeuwen wrote: > > > > -----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". > > > > See page 6 of "Comments on XTS": > > > > Note that j = 0 must be excluded, as f(0, v) = v for any v, which > > implies ρ = 1. Moreover, if j = 0 was allowed, a simple attack based on > > this fact existed, as pointed out by [6] and [3]. Hence if XEX is used, > > one must be careful to avoid j being 0. > > > Ok, I missed that part. Something to do with being surrounded by far too > much math :-P > > I did figure out by myself that forcing the ciphertext to 0 for the first > block and being able to observe the plaintext coming out would give you > S ^ E(S) if both keys are equal due do D(0 ^ E(x)) being x. > I guess that's the f(0,v) = v in the above. > Which would give you E(S) as S is usually known. (But this doesn't have to > be the case! S *can* be made a secret within the XTS specification!) > Which in turn would give you all tweaks E(S) * alpha(j), reducing the > encryption /for that sector only/ to just basic ECB. > > Still, that does not constitute a full attack on the sector at hand (which > is not so relevant, since it was leaking plaintext, so you can assume it > does not contain any sensitive data!), let alone any other sector on the > disk or even the key. At least, I have not seen that demonstrated yet. > > So it may be bad in the general cryptographic sense, but I still doubt it > has very significant practicle implications if you assume the system is > not leaking any plaintext from any sensitive areas of the disk. > > Still, FIPS seems to consider it a risk so who am I to doubt that ;-) > > > The part you quoted is only talking about XTS *as specified*, i.e. with 2 keys. > > > Ok, that makes sense actually. Would have been better if they mentioned > that that statement only only holds if the keys are not equal ... (which, > BTW, is not a requirement mentioned anywhere in the XTS specification) > > > > > > > > > > > In any case, it's frequently *used* with both keys being equal for performance > > > > > and key storage reasons. > > > > It's broken, so it's broken. Doesn't matter who is using it. > > > Well, it does kind of matter for people that still want to read their disk > - and possibly continue to use it - encrypted with the "broken" version :-) > > And "broken" is a relative term anyway. As long as you can't get to the key, > decrypt random sectors or manipulate random bits, it may be secure enough > for its purpose. > > > > > > > > > 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. > > > > As I said, the attack is explained in the original XEX paper. Basically the > > adversary can submit a chosen ciphertext query for the first block of sector 0 > > to leak the first "mask" of that sector, then submit a chosen plaintext or > > ciphertext query for the reminder of the sector such that they can predict the > > output with 100% certainty. (The standard security model for tweakable block > > ciphers says the output must appear random.) > > > Yes, but that only affects a sector that was leaking plaintext to begin > with. I'm not impressed until you either recover the key or can decrypt > or manipulate *other* sectors on the disk. > There's no proof that other attacks don't exist. If you're going to advocate for using it regardless, then you need to choose a different (weaker) attack model, then formally prove that the construction is secure under that model. Or show where someone else has done so. - Eric
