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

[Milan Broz]

On 12/08/2019 09:50, Ard Biesheuvel wrote:
> On Mon, 12 Aug 2019 at 10:44, Milan Broz <gmazyland at gmail.com> wrote:
>>
>> On 12/08/2019 08:54, Ard Biesheuvel wrote:
>>> On Mon, 12 Aug 2019 at 09:33, Milan Broz <gmazyland at 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?
>>
> 
> Indeed. The former string instantiates the skcipher version of the
> ESSIV template, and so the AAD handling is omitted, and we end up
> using the plain IV in the authentication rather than the encrypted IV.
> 
> So when using the latter string, does it produce any error messages
> when it fails?

The error is
table: 253:1: crypt: Error decoding and setting key

and it is failing in crypt_setkey() int this  crypto_aead_setkey();

And it is because it now wrongly calculates MAC key length.
(We have two keys here - one for length-preserving CBC-ESSIV encryption
and one for HMAC.)

This super-ugly hotfix helps here... I guess it can be done better :-)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index e9a0093c88ee..7b06d975a2e1 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -2342,6 +2342,9 @@ static int crypt_ctr_auth_cipher(struct crypt_config *cc, char *cipher_api)
        char *start, *end, *mac_alg = NULL;
        struct crypto_ahash *mac;
 
+       if (strstarts(cipher_api, "essiv(authenc("))
+               cipher_api += strlen("essiv(");
+
        if (!strstarts(cipher_api, "authenc("))
                return 0;
 
Milan