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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Thu, 21 May 2020 15:23:41 +0200
Message-ID: <CAMj1kXFJJcg-YeSw+_FDfyOvjQTJ6w1YyKqWaxCWSjDhRLEDoA@mail.gmail.com>
Subject: Re: [RFC/RFT PATCH 0/2] crypto: add CTS output IVs for arm64 and testmgr
To:     Gilad Ben-Yossef <gilad@benyossef.com>
Cc:     Stephan Mueller <smueller@chronox.de>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Eric Biggers <ebiggers@kernel.org>
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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.