Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Content-Type: text/plain;
        charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 13.4 \(3608.120.23.2.4\))
Subject: Re: Why the auxiliary cipher in gss_krb5_crypto.c?
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <20201204154626.GA26255@fieldses.org>
Date:   Fri, 4 Dec 2020 11:05:24 -0500
Cc:     David Howells <dhowells@redhat.com>,
        CIFS <linux-cifs@vger.kernel.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "open list:NETWORKING [GENERAL]" <netdev@vger.kernel.org>,
        linux-kernel@vger.kernel.org,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        linux-crypto@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-afs@lists.infradead.org
Content-Transfer-Encoding: quoted-printable
Message-Id: <76331A46-235E-4A35-BA07-F4811FA29EB5@oracle.com>
References: <2F96670A-58DC-43A6-A20E-696803F0BFBA@oracle.com>
 <160518586534.2277919.14475638653680231924.stgit@warthog.procyon.org.uk>
 <118876.1607093975@warthog.procyon.org.uk>
 <20201204154626.GA26255@fieldses.org>
To:     Bruce Fields <bfields@fieldses.org>
Precedence: bulk


> On Dec 4, 2020, at 10:46 AM, Bruce Fields <bfields@fieldses.org> =
wrote:
>=20
> On Fri, Dec 04, 2020 at 02:59:35PM +0000, David Howells wrote:
>> Hi Chuck, Bruce,
>>=20
>> Why is gss_krb5_crypto.c using an auxiliary cipher?  For reference, =
the
>> gss_krb5_aes_encrypt() code looks like the attached.
>>=20
>>> =46rom what I can tell, in AES mode, the difference between the main =
cipher and
>> the auxiliary cipher is that the latter is "cbc(aes)" whereas the =
former is
>> "cts(cbc(aes))" - but they have the same key.
>>=20
>> Reading up on CTS, I'm guessing the reason it's like this is that CTS =
is the
>> same as the non-CTS, except for the last two blocks, but the non-CTS =
one is
>> more efficient.
>=20
> CTS is cipher-text stealing, isn't it?  I think it was Kevin Coffman
> that did that, and I don't remember the history.  I thought it was
> required by some spec or peer implementation (maybe Windows?) but I
> really don't remember.  It may predate git.  I'll dig around and see
> what I can find.

I can't add more here, this design comes from well before I started
working on this body of code (though, I worked near Kevin when he
implemented it).


--
Chuck Lever