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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