> -----Original Message----- > From: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > Sent: Sunday, July 21, 2019 11:50 AM > To: Milan Broz <gmazyland@xxxxxxxxx> > Cc: Pascal Van Leeuwen <pvanleeuwen@xxxxxxxxxxxxxx>; Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>; dm-devel@xxxxxxxxxx; linux- > crypto@xxxxxxxxxxxxxxx; Horia Geanta <horia.geanta@xxxxxxx> > Subject: Re: [dm-devel] xts fuzz testing and lack of ciphertext stealing support > > On Sat, 20 Jul 2019 at 10:35, Milan Broz <gmazyland@xxxxxxxxx> 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
