On Tue, Oct 23, 2018 at 07:40:34AM -0300, Ard Biesheuvel wrote: > On 20 October 2018 at 15:12, Eric Biggers <ebiggers@xxxxxxxxxx> wrote: > > Hi Ard, > > > > On Sat, Oct 20, 2018 at 12:17:58PM +0800, Ard Biesheuvel wrote: > >> On 16 October 2018 at 01:54, Eric Biggers <ebiggers@xxxxxxxxxx> wrote: > >> > From: Eric Biggers <ebiggers@xxxxxxxxxx> > >> > > >> > Add support for the Adiantum encryption mode. Adiantum was designed by > >> > Paul Crowley and is specified by our paper: > >> > > >> > Adiantum: length-preserving encryption for entry-level processors > >> > (https://eprint.iacr.org/2018/720.pdf) > >> > > >> > See our paper for full details; this patch only provides an overview. > >> > > >> > Adiantum is a tweakable, length-preserving encryption mode designed for > >> > fast and secure disk encryption, especially on CPUs without dedicated > >> > crypto instructions. Adiantum encrypts each sector using the XChaCha12 > >> > stream cipher, two passes of an ε-almost-∆-universal (εA∆U) hash > >> > function, and an invocation of the AES-256 block cipher on a single > >> > 16-byte block. On CPUs without AES instructions, Adiantum is much > >> > faster than AES-XTS; for example, on ARM Cortex-A7, on 4096-byte sectors > >> > Adiantum encryption is about 4 times faster than AES-256-XTS encryption, > >> > and decryption about 5 times faster. > >> > > >> > Adiantum is a specialization of the more general HBSH construction. Our > >> > earlier proposal, HPolyC, was also a HBSH specialization, but it used a > >> > different εA∆U hash function, one based on Poly1305 only. Adiantum's > >> > εA∆U hash function, which is based primarily on the "NH" hash function > >> > like that used in UMAC (RFC4418), is about twice as fast as HPolyC's; > >> > consequently, Adiantum is about 20% faster than HPolyC. > >> > > >> > This speed comes with no loss of security: Adiantum is provably just as > >> > secure as HPolyC, in fact slightly *more* secure. Like HPolyC, > >> > Adiantum's security is reducible to that of XChaCha12 and AES-256, > >> > subject to a security bound. XChaCha12 itself has a security reduction > >> > to ChaCha12. Therefore, one need not "trust" Adiantum; one need only > >> > trust ChaCha12 and AES-256. Note that the εA∆U hash function is only > >> > used for its proven combinatorical properties so cannot be "broken". > >> > > >> > >> So what happens if the part of the input covered by the block cipher > >> is identical between different generations of the same disk block > >> (whose sector count is used as the 'outer' IV). How are we not in the > >> same boat as before when using stream ciphers for disk encryption? > >> > > > > This is the point of the hash step. The value encrypted with the block cipher > > to produce the intermediate value C_M (used as the stream cipher nonce) is > > H(T, P_L) + P_R. (T is the tweak a.k.a the IV, P_L is the plaintext except the > > last 16 bytes, P_R is the last 16 bytes.) A collision in this value occurs iff: > > > > H(T1, P1_L) + P1_R = H(T2, P2_L) + P2_R > > i.e. > > H(T1, P1_L) - H(T2, P2_L) = P2_R - P1_R > > > > If (T1, P1_L) = (T2, P2_L) then P1_R != P2_R so the equation has no solutions > > (since we don't consider queries where the whole input is the same; those > > unavoidably produce the same ciphertext). Otherwise (T1, P1_L) != (T2, P2_L), > > and since the hash function H is ε-almost-∆-universal over integers mod 2^128, > > the equation is true for at most a very small proportion 'ε' of hash keys. > > But, the hash key is chosen at random and is unknown to the attacker. > > > > The same applies in the other direction, for chosen ciphertext attacks. > > > > Basically, it's very difficult for an attacker to cause the intermediate value > > C_M to be reused, and the outputs will appear random until they do. > > > > Of course, all this is explained much more precisely and comprehensively in our > > paper. See section 5, "Security reduction". > > > > Thanks for the explanation. I saw that the result of the AES > encryption was used as the XChaCha nonce, but I failed to spot that > the result of the nhpoly1305 pass is added/subtracted to/from that > particular block first. > > In any case, this looks good to me: as far as I can tell, the code > implements the algorithm as described in the paper, and the plumbing > into the crypto API looks correct to me as well. > > Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > > Whether the paper is correct is a different matter: it looks > convincing to me but IANAC. > > The only request I have is to add a speed test to tcrypt as well so we > can easily benchmark it. I'll add an Adiantum testing mode to tcrypt, though I have to admit that I really dislike tcrypt, so I've actually been using a custom patch instead (https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/commit/?h=cryptobench). FWIW, I mostly dislike tcrypt's lack of flexibility. Every benchmark setting has to be coded explicitly in tcrypt, with a mode number to select it. It's often missing what I want, or what I want is included but comes with a bunch of unwanted tests too. Also tcrypt has to be built as a loadable module, so it can't be used with CONFIG_MODULES=n. And the benchmark results only go directly to the kernel log, where they aren't easily retrievable from a script. The patch I've been using just exposes a file /proc/cryptobench (note: it maybe should be made into a char device instead) to which you can write commands like: algtype=skcipher algname=adiantum(xchacha12,aes) keysize=32 bufsize=4096 niter=1000 Then userspace reads back the result: SUCCESS algname=adiantum(xchacha12,aes) driver_name=adiantum(xchacha12-software,aes-aesni,nhpoly1305-generic) measurement=0x30a5abd5776e0af enc_time=44831104 dec_time=38303077 It's then pretty straightforward to wrap this API in a userspace script that does all the benchmarks you need, and formats the results nicely. For correctness testing there's also an option 'sgl_fuzz' that randomizes the scatterlist division. Currently the patch is missing some features such as AEAD support, but at some point I'd like to get it into a state where it can be included upstream. - Eric
