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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Mon, 17 Jun 2019 19:29:27 +0200
Message-ID: <CAKv+Gu-37P+_4=Men92wR7S7LQS7U-4L2-ZaPdEN18TWAa3QaQ@mail.gmail.com>
Subject: Re: [RFC PATCH 0/3] crypto: switch to shash for ESSIV generation
To:     Milan Broz <gmazyland@gmail.com>
Cc:     Gilad Ben-Yossef <gilad@benyossef.com>,
        Eric Biggers <ebiggers@kernel.org>,
        device-mapper development <dm-devel@redhat.com>,
        linux-fscrypt@vger.kernel.org,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>
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On Mon, 17 Jun 2019 at 19:05, Milan Broz <gmazyland@gmail.com> wrote:
>
> On 17/06/2019 16:39, Ard Biesheuvel wrote:
> >>
> >> In other words, if you add some additional limit, we are breaking backward compatibility.
> >> (Despite the configuration is "wrong" from the security point of view.)
> >>
> >
> > Yes, but breaking backward compatibility only happens if you break
> > something that is actually being *used*. So sure,
> > xts(aes)-essiv:sha256 makes no sense but people use it anyway. But is
> > that also true for, say, gcm(aes)-essiv:sha256 ?
>
> These should not be used.  The only way when ESSIV can combine with AEAD mode
> is when you combine length-preserving mode with additional integrity tag, for example
>
>   # cryptsetup luksFormat -c aes-cbc-essiv:sha256 --integrity hmac-sha256 /dev/sdb
>
> it will produce this dm-crypt cipher spec:
>   capi:authenc(hmac(sha256),cbc(aes))-essiv:sha256
>
> the authenc(hmac(sha256),cbc(aes)) is direct crypto API cipher composition, the essiv:sha256
> IV is processed inside dm-crypt as IV.
>
> So if authenc() composition is problem, then yes, I am afraid these can be used in reality.
>
> But for things like gcm(aes)-essiv:sha256 (IOW real AEAD mode with ESSIV) - these are
> not supported by cryptsetup (we support only random IV in this case), so these should
> not be used anywhere.
>

OK, understood. Unfortunately, that means that the essiv template
should be dynamically instantiated as either a aead or a skcipher
depending on the context, but perhaps this is not a big deal in
reality, I will check.

One final question before I can proceed with my v2: in
crypt_ctr_blkdev_cipher(), do you think we could change the code to
look at the cipher string rather than the name of the actual cipher?
In practice, I don't think they can be different, but in order to be
able to instantiate
'essiv(authenc(hmac(sha256),cbc(aes)),sha256,aes)', the cipher part
needs to be parsed before the TFM(s) are instantiated.