Re: [PATCH v2 6/7] crypto: omap-aes: Add support for GCM mode

[Lokesh Vutla <lokeshvutla@ti.com>]

On Wednesday 08 July 2015 01:23 PM, Herbert Xu wrote:
> On Wed, Jul 08, 2015 at 03:48:05PM +0800, Herbert Xu wrote:
>> On Wed, Jul 08, 2015 at 12:29:47PM +0530, Lokesh Vutla wrote:
>>>
>>>>> +	if (req->assoclen + req->cryptlen == 0) {
>>>>> +		scatterwalk_map_and_copy(ctx->auth_tag, req->dst, 0, authlen,
>>>>> +					 1);
>>>>> +		return 0;
>>>>> +	}
>>>>
>>>> How can this be right? Did you enable the selftest?
>>> Why not? Self tests are passed for this case.
>>>
>>> As per the equation given in GCM spec[1], we can see that
>>> if assoclen and cryptlen is 0, then output of GCM  is just E(K, Y0)
>>> where Y0 = IV||(0^31)1
>>> I have E(K, Y0) calculated in previous step. And copying it
>>> to destination if assoclen and cryptlen is 0.
>>>
>>> Correct me if I am wrong.
>>
>> It should be E(K, Y0) ^ GHASH(0).  So unless GHASH(0) == 0, your
>> code doesn't work.
> 
> OK, GHASH(0) is indeed zero so I guess your code does work after
> all.
Sorry. I did not see this message and replied on the other thread.

Thanks and regards,
Lokesh
> 
> Cheers,
>