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.
