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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 19 May 2020 19:50:23 +0200
Message-ID: <CAMj1kXFfY+1LcJPGw7P8RPAN+Rts2ZJQwO6++WZSTFm3qbwaww@mail.gmail.com>
Subject: Re: ARM CE: CTS IV handling
To:     Stephan Mueller <smueller@chronox.de>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>
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On Tue, 19 May 2020 at 19:35, Stephan Mueller <smueller@chronox.de> wrote:
>
> Am Dienstag, 19. Mai 2020, 18:21:01 CEST schrieb Ard Biesheuvel:
>
> Hi Ard,
>
> >
> > To be honest, this looks like the API is being used incorrectly. Is
> > this a similar issue to the one Herbert spotted recently with the CTR
> > code?
> >
> > When you say 'leaving the TFM untouched' do you mean the skcipher
> > request? The TFM should not retain any per-request state in the first
> > place.
> >
> > The skcipher request struct is not meant to retain any state either -
> > the API simply does not support incremental encryption if the input is
> > not a multiple of the chunksize.
> >
> > Could you give some sample code on how you are using the API in this case?
>
> What I am doing technically is to allocate a new tfm and request at the
> beginning and then reuse the TFM and request. In that sense, I think I violate
> that constraint.
>
> But in order to implement such repetition, I can surely clear / allocate a new
> TFM. But in order to get that right, I need the resulting IV after the cipher
> operation.
>
> This IV that I get after the cipher operation completes is different between C
> and CE.
>

So is the expected output IV simply the last block of ciphertext that
was generated (as usual), but located before the truncated block in
the output?