RE: [dm-devel] xts fuzz testing and lack of ciphertext stealing support

[Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>]

> -----Original Message-----
> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Sent: Sunday, July 21, 2019 11:50 AM
> To: Milan Broz <gmazyland@gmail.com>
> Cc: Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>; Herbert Xu <herbert@gondor.apana.org.au>; dm-devel@redhat.com; linux-
> crypto@vger.kernel.org; Horia Geanta <horia.geanta@nxp.com>
> Subject: Re: [dm-devel] xts fuzz testing and lack of ciphertext stealing support
> 
> On Sat, 20 Jul 2019 at 10:35, Milan Broz <gmazyland@gmail.com> wrote:
> >
> > On 20/07/2019 08:58, Eric Biggers wrote:
> > > On Thu, Jul 18, 2019 at 01:19:41PM +0200, Milan Broz wrote:
> > >> Also, I would like to avoid another "just because it is nicer" module dependence (XTS->XEX->ECB).
> > >> Last time (when XTS was reimplemented using ECB) we have many reports with initramfs
> > >> missing ECB module preventing boot from AES-XTS encrypted root after kernel upgrade...
> > >> Just saying. (Despite the last time it was keyring what broke encrypted boot ;-)
> > >>
> > >
> > > Can't the "missing modules in initramfs" issue be solved by using a
> > > MODULE_SOFTDEP()?  Actually, why isn't that being used for xts -> ecb already?
> > >
> > > (There was also a bug where CONFIG_CRYPTO_XTS didn't select CONFIG_CRYPTO_ECB,
> > > but that was simply a bug, which was fixed.)
> >
> > Sure, and it is solved now. (Some systems with a hardcoded list of modules
> > have to be manually updated etc., but that is just bad design).
> > It can be done properly from the beginning.
> >
> > I just want to say that that switching to XEX looks like wasting time to me
> > for no additional benefit.
> >
> > Fully implementing XTS does make much more sense for me, even though it is long-term
> > the effort and the only user, for now, would be testmgr.
> >
> > So, there are no users because it does not work. It makes no sense
> > to implement it, because there are no users... (sorry, sounds like catch 22 :)
> >
> > (Maybe someone can use it for keyslot encryption for keys not aligned to
> > block size, dunno. Actually, some filesystem encryption could have use for it.)
> >
> > > Or "xts" and "xex" could go in the same kernel module xts.ko, which would make
> > > this a non-issue.
> >
> > If it is not available for users, I really see no reason to introduce XEX when
> > it is just XTS with full blocks.
> >
> > If it is visible to users, it needs some work in userspace - XEX (as XTS) need two keys,
> > people are already confused enough that 256bit key in AES-XTS means AES-128...
> > So the examples, hints, man pages need to be updated, at least.
> >
> 
> OK, consider me persuaded. We are already exposing xts(...) to
> userland, and since we already implement a proper subset of true XTS,
> it will be simply a matter of making sure that the existing XTS
> implementations don't regress in performance on the non-CTS code
> paths.
> 
> It would be useful, though, to have some generic helper functions,
> e.g., like the one we have for CBC, or the one I recently proposed for
> CTS, so that existing implementations (such as the bit sliced AES) can
> easily be augmented with a CTS code path (but performance may not be
> optimal in those cases). For the ARM implementations based on AES
> instructions, it should be reasonably straight forward to implement it
> close to optimally by reusing some of the code I added for CBC-CTS
> (but I won't get around to doing that for a while). If there are any
> volunteers for looking into the generic or x86/AES-NI implementations,
> please come forward :-) Also, if any of the publications that were
> quoted in this thread have suitable test vectors, that would be good
> to know.

Unfortunately, these algorithm & protocol specifications tend to be very frugal when it
comes to providing test vectors, barely scratching the surface of any corner cases, but
at least there is one non-multiple-of-16 vector in the original IEEE P1619 / D16 
specification in Annex B Test Vectors (last vector, "XTS-AES-128 applied for a data unit 
that is not a multiple of 16 bytes")

Besides that, I'd be happy to generate some testvectors from our defacto-standard
implementation ;-)

Regards,
Pascal van Leeuwen
Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
www.insidesecure.com