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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
	<linux-crypto@vger.kernel.org>,
	linux-fscrypt@vger.kernel.org,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Paul Crowley <paulcrowley@google.com>,
	Greg Kaiser <gkaiser@google.com>,
	Michael Halcrow <mhalcrow@google.com>,
	"Jason A . Donenfeld" <Jason@zx2c4.com>,
	Samuel Neves <samuel.c.p.neves@gmail.com>,
	Tomer Ashur <tomer.ashur@esat.kuleuven.be>
Subject: Re: [RFC PATCH v2 11/12] crypto: adiantum - add Adiantum support
Date: Tue, 23 Oct 2018 07:40:34 -0300	[thread overview]
Message-ID: <CAKv+Gu9zBKK7ughbBAKkHdfgX2KrBXkF6MhH3xrSdLegrjjYJQ@mail.gmail.com> (raw)
In-Reply-To: <20181020071206.GE876@sol.localdomain>

On 20 October 2018 at 15:12, Eric Biggers <ebiggers@kernel.org> 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@kernel.org> wrote:
>> > From: Eric Biggers <ebiggers@google.com>
>> >
>> > 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@linaro.org>

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.

  reply	other threads:[~2018-10-23 10:40 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-15 17:54 [RFC PATCH v2 00/12] crypto: " Eric Biggers
2018-10-15 17:54 ` [RFC PATCH v2 01/12] crypto: chacha20-generic - add HChaCha20 library function Eric Biggers
2018-10-19 14:13   ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 02/12] crypto: chacha20-generic - add XChaCha20 support Eric Biggers
2018-10-19 14:24   ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 03/12] crypto: chacha20-generic - refactor to allow varying number of rounds Eric Biggers
2018-10-19 14:25   ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 04/12] crypto: chacha - add XChaCha12 support Eric Biggers
2018-10-19 14:34   ` Ard Biesheuvel
2018-10-19 18:28     ` Eric Biggers
2018-10-15 17:54 ` [RFC PATCH v2 05/12] crypto: arm/chacha20 - add XChaCha20 support Eric Biggers
2018-10-20  2:29   ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 06/12] crypto: arm/chacha20 - refactor to allow varying number of rounds Eric Biggers
2018-10-20  3:35   ` Ard Biesheuvel
2018-10-20  5:26     ` Eric Biggers
2018-10-15 17:54 ` [RFC PATCH v2 07/12] crypto: arm/chacha - add XChaCha12 support Eric Biggers
2018-10-20  3:36   ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 08/12] crypto: poly1305 - add Poly1305 core API Eric Biggers
2018-10-20  3:45   ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 09/12] crypto: nhpoly1305 - add NHPoly1305 support Eric Biggers
2018-10-20  4:00   ` Ard Biesheuvel
2018-10-20  5:38     ` Eric Biggers
2018-10-20 15:06       ` Ard Biesheuvel
2018-10-22 18:42         ` Eric Biggers
2018-10-22 22:25           ` Ard Biesheuvel
2018-10-22 22:40             ` Eric Biggers
2018-10-22 22:43               ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 10/12] crypto: arm/nhpoly1305 - add NEON-accelerated NHPoly1305 Eric Biggers
2018-10-20  4:12   ` Ard Biesheuvel
2018-10-20  5:51     ` Eric Biggers
2018-10-20 15:00       ` Ard Biesheuvel
2018-10-15 17:54 ` [RFC PATCH v2 11/12] crypto: adiantum - add Adiantum support Eric Biggers
2018-10-20  4:17   ` Ard Biesheuvel
2018-10-20  7:12     ` Eric Biggers
2018-10-23 10:40       ` Ard Biesheuvel [this message]
2018-10-24 22:06         ` Eric Biggers
2018-10-30  8:17           ` Herbert Xu
2018-10-15 17:54 ` [RFC PATCH v2 12/12] fscrypt: " Eric Biggers
2018-10-19 15:58 ` [RFC PATCH v2 00/12] crypto: " Jason A. Donenfeld
2018-10-19 18:19   ` Paul Crowley
2018-10-20  3:24     ` Ard Biesheuvel
2018-10-20  5:22       ` Eric Biggers
     [not found]     ` <2395454e-a0dc-408f-4138-9d15ab5f20b8@esat.kuleuven.be>
2018-10-22 11:20       ` Tomer Ashur
2018-10-19 19:04   ` Eric Biggers
2018-10-20 10:26     ` Milan Broz
2018-10-20 13:47       ` Jason A. Donenfeld
2018-11-16 21:52       ` Eric Biggers
2018-11-17 10:29         ` Milan Broz
2018-11-19 19:28           ` Eric Biggers
2018-11-19 20:05             ` Milan Broz
2018-11-19 20:30               ` Jason A. Donenfeld
2018-10-21 22:23     ` Eric Biggers
2018-10-21 22:51       ` Jason A. Donenfeld
2018-10-22 17:17         ` Paul Crowley

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