From: Lokesh Vutla Subject: Re: [PATCH v2 6/7] crypto: omap-aes: Add support for GCM mode Date: Wed, 8 Jul 2015 13:45:42 +0530 Message-ID: <559CDC2E.7060505@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> Mime-Version: 1.0 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Cc: , , , , , To: Herbert Xu Return-path: Received: from arroyo.ext.ti.com ([192.94.94.40]:49812 "EHLO arroyo.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932997AbbGHISj (ORCPT ); Wed, 8 Jul 2015 04:18:39 -0400 In-Reply-To: <20150708074805.GA18991@gondor.apana.org.au> Sender: linux-crypto-owner@vger.kernel.org List-ID: 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, >