Re: [PATCH v9 3/7] md: dm-crypt: switch to ESSIV crypto API template

From: Milan Broz <gmazyland@gmail.com>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
	<linux-crypto@vger.kernel.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Eric Biggers <ebiggers@google.com>,
	device-mapper development <dm-devel@redhat.com>,
	linux-fscrypt@vger.kernel.org,
	Gilad Ben-Yossef <gilad@benyossef.com>
Subject: Re: [PATCH v9 3/7] md: dm-crypt: switch to ESSIV crypto API template
Date: Mon, 12 Aug 2019 09:44:47 +0200	[thread overview]
Message-ID: <82a87cae-8eb7-828c-35c3-fb39a9abe692@gmail.com> (raw)
In-Reply-To: <CAKv+Gu-ZPPR5xQSR6T4o+8yJvsHY2a3xXZ5zsM_aGS3frVChgQ@mail.gmail.com>

On 12/08/2019 08:54, Ard Biesheuvel wrote:
> On Mon, 12 Aug 2019 at 09:33, Milan Broz <gmazyland@gmail.com> wrote:
>> Try for example
>> # cryptsetup luksFormat /dev/sdc -c aes-cbc-essiv:sha256 --integrity hmac-sha256 -q -i1
>>
>> It should produce Crypto API string
>>   authenc(hmac(sha256),essiv(cbc(aes),sha256))
>> while it produces
>>   essiv(authenc(hmac(sha256),cbc(aes)),sha256)
>> (and fails).
>>
> 
> No. I don't know why it fails, but the latter is actually the correct
> string. The essiv template is instantiated either as a skcipher or as
> an aead, and it encapsulates the entire transformation. (This is
> necessary considering that the IV is passed via the AAD and so the
> ESSIV handling needs to touch that as well)

Hm. Constructing these strings seems to be more confusing than dmcrypt mode combinations :-)

But you are right, I actually tried the former string (authenc(hmac(sha256),essiv(cbc(aes),sha256)))
and it worked, but I guess the authenticated IV (AAD) was actually the input to IV (plain sector number)
not the output of ESSIV? Do I understand it correctly now?

Milan