[dm-devel] [RFC PATCH 0/3] crypto: switch to shash for ESSIV generation

[Milan Broz]

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