Re: [RFC/RFT PATCH 0/2] crypto: add CTS output IVs for arm64 and testmgr

From: Ard Biesheuvel <ardb@kernel.org>
To: "Stephan Müller" <smueller@chronox.de>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>,
	Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Eric Biggers <ebiggers@kernel.org>
Subject: Re: [RFC/RFT PATCH 0/2] crypto: add CTS output IVs for arm64 and testmgr
Date: Sun, 24 May 2020 00:40:04 +0200	[thread overview]
Message-ID: <CAMj1kXG2SuWWyAkinAumW=uBVSdg6BUX0zVPWDX1pnxVW8RusA@mail.gmail.com> (raw)
In-Reply-To: <9730423.nUPlyArG6x@positron.chronox.de>

On Sat, 23 May 2020 at 20:52, Stephan Müller <smueller@chronox.de> wrote:
>
> Am Donnerstag, 21. Mai 2020, 15:23:41 CEST schrieb Ard Biesheuvel:
>
> Hi Ard,
>
> > On Thu, 21 May 2020 at 15:01, Gilad Ben-Yossef <gilad@benyossef.com> wrote:
> > > Hi Ard,
> > >
> > > Thank you for looping me in.
> > >
> > > On Wed, May 20, 2020 at 10:09 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > On Wed, 20 May 2020 at 09:01, Stephan Mueller <smueller@chronox.de>
> wrote:
> > > > > Am Mittwoch, 20. Mai 2020, 08:54:10 CEST schrieb Ard Biesheuvel:
> > > > >
> > > > > Hi Ard,
> > > > >
> > > > > > On Wed, 20 May 2020 at 08:47, Stephan Mueller <smueller@chronox.de>
> wrote:
> > > > ...
> > > >
> > > > > > > The state of all block chaining modes we currently have is defined
> > > > > > > with
> > > > > > > the
> > > > > > > IV. That is the reason why I mentioned it can be implemented
> > > > > > > stateless
> > > > > > > when I am able to get the IV output from the previous operation.
> > > > > >
> > > > > > But it is simply the same as the penultimate block of ciphertext. So
> > > > > > you can simply capture it after encrypt, or before decrypt. There is
> > > > > > really no need to rely on the CTS transformation to pass it back to
> > > > > > you via the buffer that is only specified to provide an input to the
> > > > > > CTS transform.
> > > > >
> > > > > Let me recheck that as I am not fully sure on that one. But if it can
> > > > > be
> > > > > handled that way, it would make life easier.
> > > >
> > > > Please refer to patch 2. The .iv_out test vectors were all simply
> > > > copied from the appropriate offset into the associated .ctext member.
> > >
> > > Not surprisingly since to the best of my understanding this behaviour
> > > is not strictly specified, ccree currently fails the IV output check
> > > with the 2nd version of the patch.
> >
> > That is what I suspected, hence the cc:
> > > If I understand you correctly, the expected output IV is simply the
> > > next to last block of the ciphertext?
> >
> > Yes. But this happens to work for the generic case because the CTS
> > driver itself requires the encapsulated CBC mode to return the output
> > IV, which is simply passed through back to the caller. CTS mode itself
> > does not specify any kind of output IV, so we should not rely on this
> > behavior.
>
> Note, the update to the spec based on your suggestion is already in a merge
> request:
>
> https://github.com/usnistgov/ACVP/issues/860
>
> Thanks for your input.
>

Thanks for the head's up. I've left a comment there, as the proposed
change is not equivalent to the unspecified current behavior.