From: Lokesh Vutla <lokeshvutla@ti.com>
Subject: Re: [PATCH v2 6/7] crypto: omap-aes: Add support for GCM mode
Date: Wed, 8 Jul 2015 14:00:57 +0530
Message-ID: <559CDFC1.4040402@ti.com>
References: <1436283109-13318-1-git-send-email-lokeshvutla@ti.com> <1436283109-13318-7-git-send-email-lokeshvutla@ti.com> <20150708041838.GA12744@gondor.apana.org.au> <559CCA63.4080609@ti.com> <20150708074805.GA18991@gondor.apana.org.au> <20150708075307.GA19381@gondor.apana.org.au>
Mime-Version: 1.0
Content-Type: text/plain; charset="windows-1252"
Content-Transfer-Encoding: 7bit
Cc: <linux-crypto@vger.kernel.org>, <davem@davemloft.net>,
	<linux-omap@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<t-kristo@ti.com>, <nsekhar@ti.com>
To: Herbert Xu <herbert@gondor.apana.org.au>
In-Reply-To: <20150708075307.GA19381@gondor.apana.org.au>
Sender: linux-crypto-owner@vger.kernel.org

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,
>