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

[Milan Broz]

On 17/06/2019 19:29, Ard Biesheuvel wrote:
> On Mon, 17 Jun 2019 at 19:05, Milan Broz <gmazyland at 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.

You mean to replace crypto_tfm_alg_name() with the cipher string
from the device-mapper table constructor?

I hope I am not missing anything, but it should be ok. It just could
fail later (in tfm init).
The constructor is de-facto atomic step for device-mapper, I think
it does not matter when it fails, the effect of failure is the same
for userspace.

Milan