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Subject: Re: [RFC PATCH 0/3] crypto: switch to shash for ESSIV generation
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Milan Broz <gmazyland@gmail.com>
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        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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From:   Milan Broz <gmazyland@gmail.com>
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Date:   Mon, 17 Jun 2019 19:05:51 +0200
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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.

Milan