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

On Wednesday 08 July 2015 01:23 PM, Herbert Xu wrote:

On Wed, Jul 08, 2015 at 03:48:05PM +0800, Herbert Xu wrote:

quoted

On Wed, Jul 08, 2015 at 12:29:47PM +0530, Lokesh Vutla wrote:

quoted

quoted

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+	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,