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

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Date:	Wed, 8 Jul 2015 13:45:42 +0530
From:	Lokesh Vutla <lokeshvutla@...com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	<linux-crypto@...r.kernel.org>, <davem@...emloft.net>,
	<linux-omap@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<t-kristo@...com>, <nsekhar@...com>
Subject: Re: [PATCH v2 6/7] crypto: omap-aes: Add support for GCM mode

On Wednesday 08 July 2015 01:18 PM, 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.
Yes, thats right. I have considered that.
So, we need GHASH(H, {}, {}).
As per the spec, 
GHASH(H, A, C) = X(m + n + 1). 
					m = n = 0 in our case.

X0 = 0
X1 = (X(m  + n) ^ (len(A) || len(C)) . H
X1 = 0 . H		(GF(128) Multiplication)
X1 = 0	

The same thing is given in the Test case 1 of the spec. GHASH(H, {}, {}) = 0

Thanks and regards,
Lokesh

> 
> Cheers,
> 

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